What happens in a nerve cell when a thought is generated?

What happens in a nerve cell when a thought is generated?

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I know basic nerve physiology of impulse conduction and transmission, but I don't know what actually happens in a nerve cell when a thought is generated. When a external stimulus (like tactile stimulation, injury causing tissue damage) leads to stimulation of respective Na+Channels and action potential is generated. But what is the stimulus in the neurons of the Central Nervous System when we think about something? What exactly is the stimulus that cause the neuron to generate action potential and then following it, it is transmitted to different circuits and then we react accordingly?

This is a recurring question that many people are bothered with. I read an interesting answer on Quora. Basically, the author says that there is always on-going activity in the brain. Each thought is triggered by something, let it be conscious or unconscious. The brain is never silent, even in your deepest sleep with only slow-wave synchronized activity, there is still activity. Even when it seems that a certain thought pops up totally randomly, it is still triggered by certain inputs, let they be sensory or internal. Note that action potentials can be triggered either by signals from the outside world (sensory receptor neurons), or by signals from other cells in the brain and body.

Also note that most, if not all neurons show background activity ('neuronal noise'); they never stop firing even without inputs. When one or more of those background potentials impinge on a postsynaptic neuron, that neuron may be activated an fire an action potential. This may also explain some random events in the nervous system.

How Your Thoughts Change Your Brain, Cells, And Genes

Every minute of every day, your body is physically reacting, literally changing, in response to the thoughts that run through your mind.

It’s been proven over and over again that just thinking about something causes your brain to release neurotransmitters, chemical messengers that allow it to communicate with parts of itself and your nervous system. Neurotransmitters control virtually all of your body’s functions, from hormones to digestion to feeling happy, sad, or stressed.

Studies have shown that thoughts alone can improve vision, fitness, and strength. The placebo effect, as observed with fake operations and sham drugs, for example, works because of the power of thought. Expectancies and learned associations have been shown to change brain chemistry and circuitry which results in real physiological and cognitive outcomes, such as less fatigue, lower immune system reaction, elevated hormone levels, and reduced anxiety.

A sizable body of research exploring the nature of consciousness, carried on for more than thirty years in prestigious scientific institutions around the world, shows that thoughts are capable of affecting everything from the simplest machines to the most complex living beings. This evidence suggests that human thoughts and intentions are an actual physical “something” with astonishing power to change our world. Every thought we have is tangible energy with the power to transform. A thought is not only a thing a thought is a thing that influences other things. ( Read more )

Ask An Engineer

The human brain is composed of about 100 billion nerve cells (neurons) interconnected by trillions of connections, called synapses. On average, each connection transmits about one signal per second. Some specialized connections send up to 1,000 signals per second. “Somehow… that’s producing thought,” says Charles Jennings, director of neurotechnology at the MIT McGovern Institute for Brain Research.

Given the physical complexity of what’s happening inside your head, it’s not easy to trace a thought from beginning to end. “That’s a little like asking where the forest begins. Is it with the first leaf, or the tip of the first root?” says Jennings. Simpler, then to start by considering perceptions—“thoughts” that are directly triggered by external stimuli—a feather brushes your skin, you see these words on the computer screen, you hear a phone ring. Each of these events triggers a series of signals in the brain.

When you read these words, for example, the photons associated with the patterns of the letters hit your retina, and their energy triggers an electrical signal in the light-detecting cells there. That electrical signal propagates like a wave along the long threads called axons that are part of the connections between neurons. When the signal reaches the end of an axon, it causes the release of chemical neurotransmitters into the synapse, a chemical junction between the axon tip and target neurons. A target neuron responds with its own electrical signal, which, in turn, spreads to other neurons. Within a few hundred milliseconds, the signal has spread to billions of neurons in several dozen interconnected areas of your brain and you have perceived these words. (All that and you probably didn’t even break a sweat.)

The fact that you are then able to convert the perception of these shapes into symbols, language, and meaning is a whole other story—and a good indication of the complexity of neuroscience. Trying to imagine how trillions of connections and billions of simultaneous transmissions coalesce inside your brain to form a thought is a little like trying to look at the leaves, roots, snakes, birds, ticks, deer—and everything else in a forest—at the same moment.

With new brain imaging tools, however, researchers are making strides towards doing just that. A better understanding of where and how different types of thoughts arise in the brain—such as facial recognition, emotion, or language—may help researchers develop treatments for disorders such as autism or dyslexia.

But reaching that goal? “That’s a tall order,” said Evelina Fedorenko, a postdoctoral associate at the McGovern Institute. Working with Brain and Cognitive Sciences professor Nancy Kanwisher, Fedorenko is working to develop better tools for dissecting recordings of thoughts. Their recent work reveals a clearer picture of where the brain processes language, one of the defining activities that makes us human.

Thanks to Rugada Meghanath of Srikakulam, Andhra Pradesh, India, for this question.


Nearly all cell membranes in animals, plants and fungi maintain a voltage difference between the exterior and interior of the cell, called the membrane potential. A typical voltage across an animal cell membrane is −70 mV. This means that the interior of the cell has a negative voltage relative to the exterior. In most types of cells, the membrane potential usually stays fairly constant. Some types of cells, however, are electrically active in the sense that their voltages fluctuate over time. In some types of electrically active cells, including neurons and muscle cells, the voltage fluctuations frequently take the form of a rapid upward spike followed by a rapid fall. These up-and-down cycles are known as action potentials. In some types of neurons, the entire up-and-down cycle takes place in a few thousandths of a second. In muscle cells, a typical action potential lasts about a fifth of a second. In some other types of cells and plants, an action potential may last three seconds or more. [3]

The electrical properties of a cell are determined by the structure of the membrane that surrounds it. A cell membrane consists of a lipid bilayer of molecules in which larger protein molecules are embedded. The lipid bilayer is highly resistant to movement of electrically charged ions, so it functions as an insulator. The large membrane-embedded proteins, in contrast, provide channels through which ions can pass across the membrane. Action potentials are driven by channel proteins whose configuration switches between closed and open states as a function of the voltage difference between the interior and exterior of the cell. These voltage-sensitive proteins are known as voltage-gated ion channels.

Process in a typical neuron Edit

All cells in animal body tissues are electrically polarized – in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential. This electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels. In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites, axon, and cell body different electrical properties. As a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentials), whereas others are not. Recent studies have shown that the most excitable part of a neuron is the part after the axon hillock (the point where the axon leaves the cell body), which is called the initial segment, but the axon and cell body are also excitable in most cases. [4]

Each excitable patch of membrane has two important levels of membrane potential: the resting potential, which is the value the membrane potential maintains as long as nothing perturbs the cell, and a higher value called the threshold potential. At the axon hillock of a typical neuron, the resting potential is around –70 millivolts (mV) and the threshold potential is around –55 mV. Synaptic inputs to a neuron cause the membrane to depolarize or hyperpolarize that is, they cause the membrane potential to rise or fall. Action potentials are triggered when enough depolarization accumulates to bring the membrane potential up to threshold. When an action potential is triggered, the membrane potential abruptly shoots upward and then equally abruptly shoots back downward, often ending below the resting level, where it remains for some period of time. The shape of the action potential is stereotyped this means that the rise and fall usually have approximately the same amplitude and time course for all action potentials in a given cell. (Exceptions are discussed later in the article). In most neurons, the entire process takes place in about a thousandth of a second. Many types of neurons emit action potentials constantly at rates of up to 10–100 per second. However, some types are much quieter, and may go for minutes or longer without emitting any action potentials.

Action potentials result from the presence in a cell's membrane of special types of voltage-gated ion channels. [5] A voltage-gated ion channel is a transmembrane protein that has three key properties:

  1. It is capable of assuming more than one conformation.
  2. At least one of the conformations creates a channel through the membrane that is permeable to specific types of ions.
  3. The transition between conformations is influenced by the membrane potential.

Thus, a voltage-gated ion channel tends to be open for some values of the membrane potential, and closed for others. In most cases, however, the relationship between membrane potential and channel state is probabilistic and involves a time delay. Ion channels switch between conformations at unpredictable times: The membrane potential determines the rate of transitions and the probability per unit time of each type of transition.

Voltage-gated ion channels are capable of producing action potentials because they can give rise to positive feedback loops: The membrane potential controls the state of the ion channels, but the state of the ion channels controls the membrane potential. Thus, in some situations, a rise in the membrane potential can cause ion channels to open, thereby causing a further rise in the membrane potential. An action potential occurs when this positive feedback cycle (Hodgkin cycle) proceeds explosively. The time and amplitude trajectory of the action potential are determined by the biophysical properties of the voltage-gated ion channels that produce it. Several types of channels capable of producing the positive feedback necessary to generate an action potential do exist. Voltage-gated sodium channels are responsible for the fast action potentials involved in nerve conduction. Slower action potentials in muscle cells and some types of neurons are generated by voltage-gated calcium channels. Each of these types comes in multiple variants, with different voltage sensitivity and different temporal dynamics.

The most intensively studied type of voltage-dependent ion channels comprises the sodium channels involved in fast nerve conduction. These are sometimes known as Hodgkin-Huxley sodium channels because they were first characterized by Alan Hodgkin and Andrew Huxley in their Nobel Prize-winning studies of the biophysics of the action potential, but can more conveniently be referred to as NaV channels. (The "V" stands for "voltage".) An NaV channel has three possible states, known as deactivated, activated, and inactivated. The channel is permeable only to sodium ions when it is in the activated state. When the membrane potential is low, the channel spends most of its time in the deactivated (closed) state. If the membrane potential is raised above a certain level, the channel shows increased probability of transitioning to the activated (open) state. The higher the membrane potential the greater the probability of activation. Once a channel has activated, it will eventually transition to the inactivated (closed) state. It tends then to stay inactivated for some time, but, if the membrane potential becomes low again, the channel will eventually transition back to the deactivated state. During an action potential, most channels of this type go through a cycle deactivatedactivatedinactivateddeactivated. This is only the population average behavior, however – an individual channel can in principle make any transition at any time. However, the likelihood of a channel's transitioning from the inactivated state directly to the activated state is very low: A channel in the inactivated state is refractory until it has transitioned back to the deactivated state.

The outcome of all this is that the kinetics of the NaV channels are governed by a transition matrix whose rates are voltage-dependent in a complicated way. Since these channels themselves play a major role in determining the voltage, the global dynamics of the system can be quite difficult to work out. Hodgkin and Huxley approached the problem by developing a set of differential equations for the parameters that govern the ion channel states, known as the Hodgkin-Huxley equations. These equations have been extensively modified by later research, but form the starting point for most theoretical studies of action potential biophysics.

As the membrane potential is increased, sodium ion channels open, allowing the entry of sodium ions into the cell. This is followed by the opening of potassium ion channels that permit the exit of potassium ions from the cell. The inward flow of sodium ions increases the concentration of positively charged cations in the cell and causes depolarization, where the potential of the cell is higher than the cell's resting potential. The sodium channels close at the peak of the action potential, while potassium continues to leave the cell. The efflux of potassium ions decreases the membrane potential or hyperpolarizes the cell. For small voltage increases from rest, the potassium current exceeds the sodium current and the voltage returns to its normal resting value, typically −70 mV. [6] [7] [8] However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates. This results in a runaway condition whereby the positive feedback from the sodium current activates even more sodium channels. Thus, the cell fires, producing an action potential. [6] [9] [10] [note 1] The frequency at which a neuron elicits action potentials is often referred to as a firing rate or neural firing rate.

Currents produced by the opening of voltage-gated channels in the course of an action potential are typically significantly larger than the initial stimulating current. Thus, the amplitude, duration, and shape of the action potential are determined largely by the properties of the excitable membrane and not the amplitude or duration of the stimulus. This all-or-nothing property of the action potential sets it apart from graded potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, and synaptic potentials, which scale with the magnitude of the stimulus. A variety of action potential types exist in many cell types and cell compartments as determined by the types of voltage-gated channels, leak channels, channel distributions, ionic concentrations, membrane capacitance, temperature, and other factors.

The principal ions involved in an action potential are sodium and potassium cations sodium ions enter the cell, and potassium ions leave, restoring equilibrium. Relatively few ions need to cross the membrane for the membrane voltage to change drastically. The ions exchanged during an action potential, therefore, make a negligible change in the interior and exterior ionic concentrations. The few ions that do cross are pumped out again by the continuous action of the sodium–potassium pump, which, with other ion transporters, maintains the normal ratio of ion concentrations across the membrane. Calcium cations and chloride anions are involved in a few types of action potentials, such as the cardiac action potential and the action potential in the single-cell alga Acetabularia, respectively.

Although action potentials are generated locally on patches of excitable membrane, the resulting currents can trigger action potentials on neighboring stretches of membrane, precipitating a domino-like propagation. In contrast to passive spread of electric potentials (electrotonic potential), action potentials are generated anew along excitable stretches of membrane and propagate without decay. [11] Myelinated sections of axons are not excitable and do not produce action potentials and the signal is propagated passively as electrotonic potential. Regularly spaced unmyelinated patches, called the nodes of Ranvier, generate action potentials to boost the signal. Known as saltatory conduction, this type of signal propagation provides a favorable tradeoff of signal velocity and axon diameter. Depolarization of axon terminals, in general, triggers the release of neurotransmitter into the synaptic cleft. In addition, backpropagating action potentials have been recorded in the dendrites of pyramidal neurons, which are ubiquitous in the neocortex. [c] These are thought to have a role in spike-timing-dependent plasticity.

In the Hodgkin–Huxley membrane capacitance model, the speed of transmission of an action potential was undefined and it was assumed that adjacent areas became depolarized due to released ion interference with neighbouring channels. Measurements of ion diffusion and radii have since shown this not to be possible. [ citation needed ] Moreover, contradictory measurements of entropy changes and timing disputed the capacitance model as acting alone. [ citation needed ] Alternatively, Gilbert Ling's adsorption hypothesis, posits that the membrane potential and action potential of a living cell is due to the adsorption of mobile ions onto adsorption sites of cells. [12]

Maturation of the electrical properties of the action potential Edit

A neuron's ability to generate and propagate an action potential changes during development. How much the membrane potential of a neuron changes as the result of a current impulse is a function of the membrane input resistance. As a cell grows, more channels are added to the membrane, causing a decrease in input resistance. A mature neuron also undergoes shorter changes in membrane potential in response to synaptic currents. Neurons from a ferret lateral geniculate nucleus have a longer time constant and larger voltage deflection at P0 than they do at P30. [13] One consequence of the decreasing action potential duration is that the fidelity of the signal can be preserved in response to high frequency stimulation. Immature neurons are more prone to synaptic depression than potentiation after high frequency stimulation. [13]

In the early development of many organisms, the action potential is actually initially carried by calcium current rather than sodium current. The opening and closing kinetics of calcium channels during development are slower than those of the voltage-gated sodium channels that will carry the action potential in the mature neurons. The longer opening times for the calcium channels can lead to action potentials that are considerably slower than those of mature neurons. [13] Xenopus neurons initially have action potentials that take 60–90 ms. During development, this time decreases to 1 ms. There are two reasons for this drastic decrease. First, the inward current becomes primarily carried by sodium channels. [14] Second, the delayed rectifier, a potassium channel current, increases to 3.5 times its initial strength. [13]

In order for the transition from a calcium-dependent action potential to a sodium-dependent action potential to proceed new channels must be added to the membrane. If Xenopus neurons are grown in an environment with RNA synthesis or protein synthesis inhibitors that transition is prevented. [15] Even the electrical activity of the cell itself may play a role in channel expression. If action potentials in Xenopus myocytes are blocked, the typical increase in sodium and potassium current density is prevented or delayed. [16]

This maturation of electrical properties is seen across species. Xenopus sodium and potassium currents increase drastically after a neuron goes through its final phase of mitosis. The sodium current density of rat cortical neurons increases by 600% within the first two postnatal weeks. [13]

Anatomy of a neuron Edit

Several types of cells support an action potential, such as plant cells, muscle cells, and the specialized cells of the heart (in which occurs the cardiac action potential). However, the main excitable cell is the neuron, which also has the simplest mechanism for the action potential.

Neurons are electrically excitable cells composed, in general, of one or more dendrites, a single soma, a single axon and one or more axon terminals. Dendrites are cellular projections whose primary function is to receive synaptic signals. Their protrusions, known as dendritic spines, are designed to capture the neurotransmitters released by the presynaptic neuron. They have a high concentration of ligand-gated ion channels. These spines have a thin neck connecting a bulbous protrusion to the dendrite. This ensures that changes occurring inside the spine are less likely to affect the neighboring spines. The dendritic spine can, with rare exception (see LTP), act as an independent unit. The dendrites extend from the soma, which houses the nucleus, and many of the "normal" eukaryotic organelles. Unlike the spines, the surface of the soma is populated by voltage activated ion channels. These channels help transmit the signals generated by the dendrites. Emerging out from the soma is the axon hillock. This region is characterized by having a very high concentration of voltage-activated sodium channels. In general, it is considered to be the spike initiation zone for action potentials, [17] i.e. the trigger zone. Multiple signals generated at the spines, and transmitted by the soma all converge here. Immediately after the axon hillock is the axon. This is a thin tubular protrusion traveling away from the soma. The axon is insulated by a myelin sheath. Myelin is composed of either Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system), both of which are types of glial cells. Although glial cells are not involved with the transmission of electrical signals, they communicate and provide important biochemical support to neurons. [18] To be specific, myelin wraps multiple times around the axonal segment, forming a thick fatty layer that prevents ions from entering or escaping the axon. This insulation prevents significant signal decay as well as ensuring faster signal speed. This insulation, however, has the restriction that no channels can be present on the surface of the axon. There are, therefore, regularly spaced patches of membrane, which have no insulation. These nodes of Ranvier can be considered to be "mini axon hillocks", as their purpose is to boost the signal in order to prevent significant signal decay. At the furthest end, the axon loses its insulation and begins to branch into several axon terminals. These presynaptic terminals, or synaptic boutons, are a specialized area within the axon of the presynaptic cell that contains neurotransmitters enclosed in small membrane-bound spheres called synaptic vesicles.

Initiation Edit

Before considering the propagation of action potentials along axons and their termination at the synaptic knobs, it is helpful to consider the methods by which action potentials can be initiated at the axon hillock. The basic requirement is that the membrane voltage at the hillock be raised above the threshold for firing. [6] [7] [19] [20] There are several ways in which this depolarization can occur.

Dynamics Edit

Action potentials are most commonly initiated by excitatory postsynaptic potentials from a presynaptic neuron. [21] Typically, neurotransmitter molecules are released by the presynaptic neuron. These neurotransmitters then bind to receptors on the postsynaptic cell. This binding opens various types of ion channels. This opening has the further effect of changing the local permeability of the cell membrane and, thus, the membrane potential. If the binding increases the voltage (depolarizes the membrane), the synapse is excitatory. If, however, the binding decreases the voltage (hyperpolarizes the membrane), it is inhibitory. Whether the voltage is increased or decreased, the change propagates passively to nearby regions of the membrane (as described by the cable equation and its refinements). Typically, the voltage stimulus decays exponentially with the distance from the synapse and with time from the binding of the neurotransmitter. Some fraction of an excitatory voltage may reach the axon hillock and may (in rare cases) depolarize the membrane enough to provoke a new action potential. More typically, the excitatory potentials from several synapses must work together at nearly the same time to provoke a new action potential. Their joint efforts can be thwarted, however, by the counteracting inhibitory postsynaptic potentials.

Neurotransmission can also occur through electrical synapses. [22] Due to the direct connection between excitable cells in the form of gap junctions, an action potential can be transmitted directly from one cell to the next in either direction. The free flow of ions between cells enables rapid non-chemical-mediated transmission. Rectifying channels ensure that action potentials move only in one direction through an electrical synapse. [ citation needed ] Electrical synapses are found in all nervous systems, including the human brain, although they are a distinct minority. [23]

"All-or-none" principle Edit

The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all. [d] [e] [f] This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus. [24] In both cases, the frequency of action potentials is correlated with the intensity of a stimulus.

Sensory neurons Edit

In sensory neurons, an external signal such as pressure, temperature, light, or sound is coupled with the opening and closing of ion channels, which in turn alter the ionic permeabilities of the membrane and its voltage. [25] These voltage changes can again be excitatory (depolarizing) or inhibitory (hyperpolarizing) and, in some sensory neurons, their combined effects can depolarize the axon hillock enough to provoke action potentials. Some examples in humans include the olfactory receptor neuron and Meissner's corpuscle, which are critical for the sense of smell and touch, respectively. However, not all sensory neurons convert their external signals into action potentials some do not even have an axon. [26] Instead, they may convert the signal into the release of a neurotransmitter, or into continuous graded potentials, either of which may stimulate subsequent neuron(s) into firing an action potential. For illustration, in the human ear, hair cells convert the incoming sound into the opening and closing of mechanically gated ion channels, which may cause neurotransmitter molecules to be released. In similar manner, in the human retina, the initial photoreceptor cells and the next layer of cells (comprising bipolar cells and horizontal cells) do not produce action potentials only some amacrine cells and the third layer, the ganglion cells, produce action potentials, which then travel up the optic nerve.

Pacemaker potentials Edit

In sensory neurons, action potentials result from an external stimulus. However, some excitable cells require no such stimulus to fire: They spontaneously depolarize their axon hillock and fire action potentials at a regular rate, like an internal clock. [27] The voltage traces of such cells are known as pacemaker potentials. [28] The cardiac pacemaker cells of the sinoatrial node in the heart provide a good example. [g] Although such pacemaker potentials have a natural rhythm, it can be adjusted by external stimuli for instance, heart rate can be altered by pharmaceuticals as well as signals from the sympathetic and parasympathetic nerves. [29] The external stimuli do not cause the cell's repetitive firing, but merely alter its timing. [28] In some cases, the regulation of frequency can be more complex, leading to patterns of action potentials, such as bursting.

The course of the action potential can be divided into five parts: the rising phase, the peak phase, the falling phase, the undershoot phase, and the refractory period. During the rising phase the membrane potential depolarizes (becomes more positive). The point at which depolarization stops is called the peak phase. At this stage, the membrane potential reaches a maximum. Subsequent to this, there is a falling phase. During this stage the membrane potential becomes more negative, returning towards resting potential. The undershoot, or afterhyperpolarization, phase is the period during which the membrane potential temporarily becomes more negatively charged than when at rest (hyperpolarized). Finally, the time during which a subsequent action potential is impossible or difficult to fire is called the refractory period, which may overlap with the other phases. [30]

The course of the action potential is determined by two coupled effects. [31] First, voltage-sensitive ion channels open and close in response to changes in the membrane voltage Vm. This changes the membrane's permeability to those ions. [32] Second, according to the Goldman equation, this change in permeability changes the equilibrium potential Em, and, thus, the membrane voltage Vm. [h] Thus, the membrane potential affects the permeability, which then further affects the membrane potential. This sets up the possibility for positive feedback, which is a key part of the rising phase of the action potential. [6] [9] A complicating factor is that a single ion channel may have multiple internal "gates" that respond to changes in Vm in opposite ways, or at different rates. [33] [i] For example, although raising Vm opens most gates in the voltage-sensitive sodium channel, it also closes the channel's "inactivation gate", albeit more slowly. [34] Hence, when Vm is raised suddenly, the sodium channels open initially, but then close due to the slower inactivation.

The voltages and currents of the action potential in all of its phases were modeled accurately by Alan Lloyd Hodgkin and Andrew Huxley in 1952, [i] for which they were awarded the Nobel Prize in Physiology or Medicine in 1963. [lower-Greek 2] However, their model considers only two types of voltage-sensitive ion channels, and makes several assumptions about them, e.g., that their internal gates open and close independently of one another. In reality, there are many types of ion channels, [35] and they do not always open and close independently. [j]

Stimulation and rising phase Edit

A typical action potential begins at the axon hillock [36] with a sufficiently strong depolarization, e.g., a stimulus that increases Vm. This depolarization is often caused by the injection of extra sodium cations into the cell these cations can come from a wide variety of sources, such as chemical synapses, sensory neurons or pacemaker potentials.

For a neuron at rest, there is a high concentration of sodium and chloride ions in the extracellular fluid compared to the intracellular fluid, while there is a high concentration of potassium ions in the intracellular fluid compared to the extracellular fluid. The difference in concentrations, which causes ions to move from a high to a low concentration, and electrostatic effects (attraction of opposite charges) are responsible for the movement of ions in and out of the neuron. The inside of a neuron has a negative charge, relative to the cell exterior, from the movement of K + out of the cell. The neuron membrane is more permeable to K + than to other ions, allowing this ion to selectively move out of the cell, down its concentration gradient. This concentration gradient along with potassium leak channels present on the membrane of the neuron causes an efflux of potassium ions making the resting potential close to EK ≈ –75 mV. [37] Since Na + ions are in higher concentrations outside of the cell, the concentration and voltage differences both drive them into the cell when Na + channels open. Depolarization opens both the sodium and potassium channels in the membrane, allowing the ions to flow into and out of the axon, respectively. If the depolarization is small (say, increasing Vm from −70 mV to −60 mV), the outward potassium current overwhelms the inward sodium current and the membrane repolarizes back to its normal resting potential around −70 mV. [6] [7] [8] However, if the depolarization is large enough, the inward sodium current increases more than the outward potassium current and a runaway condition (positive feedback) results: the more inward current there is, the more Vm increases, which in turn further increases the inward current. [6] [9] A sufficiently strong depolarization (increase in Vm) causes the voltage-sensitive sodium channels to open the increasing permeability to sodium drives Vm closer to the sodium equilibrium voltage ENa≈ +55 mV. The increasing voltage in turn causes even more sodium channels to open, which pushes Vm still further towards ENa. This positive feedback continues until the sodium channels are fully open and Vm is close to ENa. [6] [7] [38] [39] The sharp rise in Vm and sodium permeability correspond to the rising phase of the action potential. [6] [7] [38] [39]

The critical threshold voltage for this runaway condition is usually around −45 mV, but it depends on the recent activity of the axon. A cell that has just fired an action potential cannot fire another one immediately, since the Na + channels have not recovered from the inactivated state. The period during which no new action potential can be fired is called the absolute refractory period. [40] [41] [42] At longer times, after some but not all of the ion channels have recovered, the axon can be stimulated to produce another action potential, but with a higher threshold, requiring a much stronger depolarization, e.g., to −30 mV. The period during which action potentials are unusually difficult to evoke is called the relative refractory period. [40] [41] [42]

Peak phase Edit

The positive feedback of the rising phase slows and comes to a halt as the sodium ion channels become maximally open. At the peak of the action potential, the sodium permeability is maximized and the membrane voltage Vm is nearly equal to the sodium equilibrium voltage ENa. However, the same raised voltage that opened the sodium channels initially also slowly shuts them off, by closing their pores the sodium channels become inactivated. [34] This lowers the membrane's permeability to sodium relative to potassium, driving the membrane voltage back towards the resting value. At the same time, the raised voltage opens voltage-sensitive potassium channels the increase in the membrane's potassium permeability drives Vm towards EK. [34] Combined, these changes in sodium and potassium permeability cause Vm to drop quickly, repolarizing the membrane and producing the "falling phase" of the action potential. [40] [43] [39] [44]

Afterhyperpolarization Edit

The depolarized voltage opens additional voltage-dependent potassium channels, and some of these do not close right away when the membrane returns to its normal resting voltage. In addition, further potassium channels open in response to the influx of calcium ions during the action potential. The intracellular concentration of potassium ions is transiently unusually low, making the membrane voltage Vm even closer to the potassium equilibrium voltage EK. The membrane potential goes below the resting membrane potential. Hence, there is an undershoot or hyperpolarization, termed an afterhyperpolarization, that persists until the membrane potassium permeability returns to its usual value, restoring the membrane potential to the resting state. [45] [43]

Refractory period Edit

Each action potential is followed by a refractory period, which can be divided into an absolute refractory period, during which it is impossible to evoke another action potential, and then a relative refractory period, during which a stronger-than-usual stimulus is required. [40] [41] [42] These two refractory periods are caused by changes in the state of sodium and potassium channel molecules. When closing after an action potential, sodium channels enter an "inactivated" state, in which they cannot be made to open regardless of the membrane potential—this gives rise to the absolute refractory period. Even after a sufficient number of sodium channels have transitioned back to their resting state, it frequently happens that a fraction of potassium channels remains open, making it difficult for the membrane potential to depolarize, and thereby giving rise to the relative refractory period. Because the density and subtypes of potassium channels may differ greatly between different types of neurons, the duration of the relative refractory period is highly variable.

The absolute refractory period is largely responsible for the unidirectional propagation of action potentials along axons. [46] At any given moment, the patch of axon behind the actively spiking part is refractory, but the patch in front, not having been activated recently, is capable of being stimulated by the depolarization from the action potential.

The action potential generated at the axon hillock propagates as a wave along the axon. [47] The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches. This basic mechanism was demonstrated by Alan Lloyd Hodgkin in 1937. After crushing or cooling nerve segments and thus blocking the action potentials, he showed that an action potential arriving on one side of the block could provoke another action potential on the other, provided that the blocked segment was sufficiently short. [k]

Once an action potential has occurred at a patch of membrane, the membrane patch needs time to recover before it can fire again. At the molecular level, this absolute refractory period corresponds to the time required for the voltage-activated sodium channels to recover from inactivation, i.e., to return to their closed state. [41] There are many types of voltage-activated potassium channels in neurons. Some of them inactivate fast (A-type currents) and some of them inactivate slowly or not inactivate at all this variability guarantees that there will be always an available source of current for repolarization, even if some of the potassium channels are inactivated because of preceding depolarization. On the other hand, all neuronal voltage-activated sodium channels inactivate within several milliseconds during strong depolarization, thus making following depolarization impossible until a substantial fraction of sodium channels have returned to their closed state. Although it limits the frequency of firing, [48] the absolute refractory period ensures that the action potential moves in only one direction along an axon. [46] The currents flowing in due to an action potential spread out in both directions along the axon. [49] However, only the unfired part of the axon can respond with an action potential the part that has just fired is unresponsive until the action potential is safely out of range and cannot restimulate that part. In the usual orthodromic conduction, the action potential propagates from the axon hillock towards the synaptic knobs (the axonal termini) propagation in the opposite direction—known as antidromic conduction—is very rare. [50] However, if a laboratory axon is stimulated in its middle, both halves of the axon are "fresh", i.e., unfired then two action potentials will be generated, one traveling towards the axon hillock and the other traveling towards the synaptic knobs.

Myelin and saltatory conduction Edit

In order to enable fast and efficient transduction of electrical signals in the nervous system, certain neuronal axons are covered with myelin sheaths. Myelin is a multilamellar membrane that enwraps the axon in segments separated by intervals known as nodes of Ranvier. It is produced by specialized cells: Schwann cells exclusively in the peripheral nervous system, and oligodendrocytes exclusively in the central nervous system. Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node. [l] [m] [n] Myelination is found mainly in vertebrates, but an analogous system has been discovered in a few invertebrates, such as some species of shrimp. [o] Not all neurons in vertebrates are myelinated for example, axons of the neurons comprising the autonomous nervous system are not, in general, myelinated.

Myelin prevents ions from entering or leaving the axon along myelinated segments. As a general rule, myelination increases the conduction velocity of action potentials and makes them more energy-efficient. Whether saltatory or not, the mean conduction velocity of an action potential ranges from 1 meter per second (m/s) to over 100 m/s, and, in general, increases with axonal diameter. [p]

Action potentials cannot propagate through the membrane in myelinated segments of the axon. However, the current is carried by the cytoplasm, which is sufficient to depolarize the first or second subsequent node of Ranvier. Instead, the ionic current from an action potential at one node of Ranvier provokes another action potential at the next node this apparent "hopping" of the action potential from node to node is known as saltatory conduction. Although the mechanism of saltatory conduction was suggested in 1925 by Ralph Lillie, [q] the first experimental evidence for saltatory conduction came from Ichiji Tasaki [r] and Taiji Takeuchi [s] [51] and from Andrew Huxley and Robert Stämpfli. [t] By contrast, in unmyelinated axons, the action potential provokes another in the membrane immediately adjacent, and moves continuously down the axon like a wave.

Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter (roughly 1 micrometre), myelination increases the conduction velocity of an action potential, typically tenfold. [v] Conversely, for a given conduction velocity, myelinated fibers are smaller than their unmyelinated counterparts. For example, action potentials move at roughly the same speed (25 m/s) in a myelinated frog axon and an unmyelinated squid giant axon, but the frog axon has a roughly 30-fold smaller diameter and 1000-fold smaller cross-sectional area. Also, since the ionic currents are confined to the nodes of Ranvier, far fewer ions "leak" across the membrane, saving metabolic energy. This saving is a significant selective advantage, since the human nervous system uses approximately 20% of the body's metabolic energy. [v]

The length of axons' myelinated segments is important to the success of saltatory conduction. They should be as long as possible to maximize the speed of conduction, but not so long that the arriving signal is too weak to provoke an action potential at the next node of Ranvier. In nature, myelinated segments are generally long enough for the passively propagated signal to travel for at least two nodes while retaining enough amplitude to fire an action potential at the second or third node. Thus, the safety factor of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. However, action potentials may end prematurely in certain places where the safety factor is low, even in unmyelinated neurons a common example is the branch point of an axon, where it divides into two axons. [53]

Some diseases degrade myelin and impair saltatory conduction, reducing the conduction velocity of action potentials. [w] The most well-known of these is multiple sclerosis, in which the breakdown of myelin impairs coordinated movement. [54]

Cable theory Edit

The flow of currents within an axon can be described quantitatively by cable theory [55] and its elaborations, such as the compartmental model. [56] Cable theory was developed in 1855 by Lord Kelvin to model the transatlantic telegraph cable [x] and was shown to be relevant to neurons by Hodgkin and Rushton in 1946. [y] In simple cable theory, the neuron is treated as an electrically passive, perfectly cylindrical transmission cable, which can be described by a partial differential equation [55]

where V(x, t) is the voltage across the membrane at a time t and a position x along the length of the neuron, and where λ and τ are the characteristic length and time scales on which those voltages decay in response to a stimulus. Referring to the circuit diagram on the right, these scales can be determined from the resistances and capacitances per unit length. [57]

These time and length-scales can be used to understand the dependence of the conduction velocity on the diameter of the neuron in unmyelinated fibers. For example, the time-scale τ increases with both the membrane resistance rm and capacitance cm. As the capacitance increases, more charge must be transferred to produce a given transmembrane voltage (by the equation Q = CV) as the resistance increases, less charge is transferred per unit time, making the equilibration slower. In a similar manner, if the internal resistance per unit length ri is lower in one axon than in another (e.g., because the radius of the former is larger), the spatial decay length λ becomes longer and the conduction velocity of an action potential should increase. If the transmembrane resistance rm is increased, that lowers the average "leakage" current across the membrane, likewise causing λ to become longer, increasing the conduction velocity.

Chemical synapses Edit

In general, action potentials that reach the synaptic knobs cause a neurotransmitter to be released into the synaptic cleft. [z] Neurotransmitters are small molecules that may open ion channels in the postsynaptic cell most axons have the same neurotransmitter at all of their termini. The arrival of the action potential opens voltage-sensitive calcium channels in the presynaptic membrane the influx of calcium causes vesicles filled with neurotransmitter to migrate to the cell's surface and release their contents into the synaptic cleft. [aa] This complex process is inhibited by the neurotoxins tetanospasmin and botulinum toxin, which are responsible for tetanus and botulism, respectively. [ab]

Electrical synapses Edit

Some synapses dispense with the "middleman" of the neurotransmitter, and connect the presynaptic and postsynaptic cells together. [ac] When an action potential reaches such a synapse, the ionic currents flowing into the presynaptic cell can cross the barrier of the two cell membranes and enter the postsynaptic cell through pores known as connexons. [ad] Thus, the ionic currents of the presynaptic action potential can directly stimulate the postsynaptic cell. Electrical synapses allow for faster transmission because they do not require the slow diffusion of neurotransmitters across the synaptic cleft. Hence, electrical synapses are used whenever fast response and coordination of timing are crucial, as in escape reflexes, the retina of vertebrates, and the heart.

Neuromuscular junctions Edit

A special case of a chemical synapse is the neuromuscular junction, in which the axon of a motor neuron terminates on a muscle fiber. [ae] In such cases, the released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor, an integral membrane protein in the membrane (the sarcolemma) of the muscle fiber. [af] However, the acetylcholine does not remain bound rather, it dissociates and is hydrolyzed by the enzyme, acetylcholinesterase, located in the synapse. This enzyme quickly reduces the stimulus to the muscle, which allows the degree and timing of muscular contraction to be regulated delicately. Some poisons inactivate acetylcholinesterase to prevent this control, such as the nerve agents sarin and tabun, [ag] and the insecticides diazinon and malathion. [ah]

Cardiac action potentials Edit

The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual. [ai] This plateau is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.

The cardiac action potential plays an important role in coordinating the contraction of the heart. [ai] The cardiac cells of the sinoatrial node provide the pacemaker potential that synchronizes the heart. The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles. Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers. [note 2] Conversely, anomalies in the cardiac action potential—whether due to a congenital mutation or injury—can lead to human pathologies, especially arrhythmias. [ai] Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, and verapamil. [aj]

Muscular action potentials Edit

The action potential in a normal skeletal muscle cell is similar to the action potential in neurons. [58] Action potentials result from the depolarization of the cell membrane (the sarcolemma), which opens voltage-sensitive sodium channels these become inactivated and the membrane is repolarized through the outward current of potassium ions. The resting potential prior to the action potential is typically −90mV, somewhat more negative than typical neurons. The muscle action potential lasts roughly 2–4 ms, the absolute refractory period is roughly 1–3 ms, and the conduction velocity along the muscle is roughly 5 m/s. The action potential releases calcium ions that free up the tropomyosin and allow the muscle to contract. Muscle action potentials are provoked by the arrival of a pre-synaptic neuronal action potential at the neuromuscular junction, which is a common target for neurotoxins. [ag]

Plant action potentials Edit

Plant and fungal cells [ak] are also electrically excitable. The fundamental difference from animal action potentials is that the depolarization in plant cells is not accomplished by an uptake of positive sodium ions, but by release of negative chloride ions. [al] [am] [an] In 1906, J. C. Bose published the first measurements of action potentials in plants, which had previously been discovered by Burdon-Sanderson and Darwin. [59] An increase in cytoplasmic calcium ions may be the cause of anion release into the cell. This makes calcium a precursor to ion movements, such as the influx of negative chloride ions and efflux of positive potassium ions, as seen in barley leaves. [60]

The initial influx of calcium ions also poses a small cellular depolarization, causing the voltage-gated ion channels to open and allowing full depolarization to be propagated by chloride ions.

Some plants (e.g. Dionaea muscipula) use sodium-gated channels to operate movements and essentially "count". Dionaea muscipula, also known as the Venus flytrap, is found in subtropical wetlands in North and South Carolina. [61] When there are poor soil nutrients, the flytrap relies on a diet of insects and animals. [62] Despite research on the plant, there lacks an understanding behind the molecular basis to the Venus flytraps, and carnivore plants in general. [63]

However, plenty of research has been done on action potentials and how they affect movement and clockwork within the Venus flytrap. To start, the resting membrane potential of the Venus flytrap (-120mV) is lower than animal cells (usually -90mV to -40mV). [63] [64] The lower resting potential makes it easier to activate an action potential. Thus, when an insect lands on the trap of the plant, it triggers a hair-like mechanoreceptor. [63] This receptor then activates an action potential which lasts around 1.5 ms. [65] Ultimately, this causes an increase of positive Calcium ions into the cell, slightly depolarizing it.

However, the flytrap doesn't close after one trigger. Instead, it requires the activation of 2 or more hairs. [62] [63] If only one hair is triggered, it throws the activation as a false positive. Further, the second hair must be activated within a certain time interval (0.75 s - 40 s) for it to register with the first activation. [63] Thus, a buildup of calcium starts and slowly falls from the first trigger. When the second action potential is fired within the time interval, it reaches the Calcium threshold to depolarize the cell, closing the trap on the prey within a fraction of a second. [63]

Together with the subsequent release of positive potassium ions the action potential in plants involves an osmotic loss of salt (KCl). Whereas, the animal action potential is osmotically neutral because equal amounts of entering sodium and leaving potassium cancel each other osmotically. The interaction of electrical and osmotic relations in plant cells [ao] appears to have arisen from an osmotic function of electrical excitability in a common unicellular ancestors of plants and animals under changing salinity conditions. Further, the present function of rapid signal transmission is seen as a newer accomplishment of metazoan cells in a more stable osmotic environment. [66] It is likely that the familiar signaling function of action potentials in some vascular plants (e.g. Mimosa pudica) arose independently from that in metazoan excitable cells.

Unlike the rising phase and peak, the falling phase and after-hyperpolarization seem to depend primarily on cations that are not calcium. To initiate repolarization, the cell requires movement of potassium out of the cell through passive transportation on the membrane. This differs from neurons because the movement of potassium does not dominate the decrease in membrane potential In fact, to fully repolarize, a plant cell requires energy in the form of ATP to assist in the release of hydrogen from the cell – utilizing a transporter commonly known as H+-ATPase. [67] [63]

Action potentials are found throughout multicellular organisms, including plants, invertebrates such as insects, and vertebrates such as reptiles and mammals. [ap] Sponges seem to be the main phylum of multicellular eukaryotes that does not transmit action potentials, although some studies have suggested that these organisms have a form of electrical signaling, too. [aq] The resting potential, as well as the size and duration of the action potential, have not varied much with evolution, although the conduction velocity does vary dramatically with axonal diameter and myelination.

Comparison of action potentials (APs) from a representative cross-section of animals [68]
Animal Cell type Resting potential (mV) AP increase (mV) AP duration (ms) Conduction speed (m/s)
Squid (Loligo) Giant axon −60 120 0.75 35
Earthworm (Lumbricus) Median giant fiber −70 100 1.0 30
Cockroach (Periplaneta) Giant fiber −70 80–104 0.4 10
Frog (Rana) Sciatic nerve axon −60 to −80 110–130 1.0 7–30
Cat (Felis) Spinal motor neuron −55 to −80 80–110 1–1.5 30–120

Given its conservation throughout evolution, the action potential seems to confer evolutionary advantages. One function of action potentials is rapid, long-range signaling within the organism the conduction velocity can exceed 110 m/s, which is one-third the speed of sound. For comparison, a hormone molecule carried in the bloodstream moves at roughly 8 m/s in large arteries. Part of this function is the tight coordination of mechanical events, such as the contraction of the heart. A second function is the computation associated with its generation. Being an all-or-none signal that does not decay with transmission distance, the action potential has similar advantages to digital electronics. The integration of various dendritic signals at the axon hillock and its thresholding to form a complex train of action potentials is another form of computation, one that has been exploited biologically to form central pattern generators and mimicked in artificial neural networks.

The common prokaryotic/eukaryotic ancestor, which lived perhaps four billion years ago, is believed to have had voltage-gated channels. This functionality was likely, at some later point, cross-purposed to provide a communication mechanism. Even modern single-celled bacteria can utilize action potentials to communicate with other bacteria in the same biofilm. [69]

The study of action potentials has required the development of new experimental methods. The initial work, prior to 1955, was carried out primarily by Alan Lloyd Hodgkin and Andrew Fielding Huxley, who were, along John Carew Eccles, awarded the 1963 Nobel Prize in Physiology or Medicine for their contribution to the description of the ionic basis of nerve conduction. It focused on three goals: isolating signals from single neurons or axons, developing fast, sensitive electronics, and shrinking electrodes enough that the voltage inside a single cell could be recorded.

The first problem was solved by studying the giant axons found in the neurons of the squid (Loligo forbesii and Doryteuthis pealeii, at the time classified as Loligo pealeii). [ar] These axons are so large in diameter (roughly 1 mm, or 100-fold larger than a typical neuron) that they can be seen with the naked eye, making them easy to extract and manipulate. [i] [as] However, they are not representative of all excitable cells, and numerous other systems with action potentials have been studied.

The second problem was addressed with the crucial development of the voltage clamp, [at] which permitted experimenters to study the ionic currents underlying an action potential in isolation, and eliminated a key source of electronic noise, the current IC associated with the capacitance C of the membrane. [71] Since the current equals C times the rate of change of the transmembrane voltage Vm, the solution was to design a circuit that kept Vm fixed (zero rate of change) regardless of the currents flowing across the membrane. Thus, the current required to keep Vm at a fixed value is a direct reflection of the current flowing through the membrane. Other electronic advances included the use of Faraday cages and electronics with high input impedance, so that the measurement itself did not affect the voltage being measured. [72]

The third problem, that of obtaining electrodes small enough to record voltages within a single axon without perturbing it, was solved in 1949 with the invention of the glass micropipette electrode, [au] which was quickly adopted by other researchers. [av] [aw] Refinements of this method are able to produce electrode tips that are as fine as 100 Å (10 nm), which also confers high input impedance. [73] Action potentials may also be recorded with small metal electrodes placed just next to a neuron, with neurochips containing EOSFETs, or optically with dyes that are sensitive to Ca 2+ or to voltage. [ax]

While glass micropipette electrodes measure the sum of the currents passing through many ion channels, studying the electrical properties of a single ion channel became possible in the 1970s with the development of the patch clamp by Erwin Neher and Bert Sakmann. For this discovery, they were awarded the Nobel Prize in Physiology or Medicine in 1991. [lower-Greek 3] Patch-clamping verified that ionic channels have discrete states of conductance, such as open, closed and inactivated.

Optical imaging technologies have been developed in recent years to measure action potentials, either via simultaneous multisite recordings or with ultra-spatial resolution. Using voltage-sensitive dyes, action potentials have been optically recorded from a tiny patch of cardiomyocyte membrane. [ay]

Several neurotoxins, both natural and synthetic, are designed to block the action potential. Tetrodotoxin from the pufferfish and saxitoxin from the Gonyaulax (the dinoflagellate genus responsible for "red tides") block action potentials by inhibiting the voltage-sensitive sodium channel [az] similarly, dendrotoxin from the black mamba snake inhibits the voltage-sensitive potassium channel. Such inhibitors of ion channels serve an important research purpose, by allowing scientists to "turn off" specific channels at will, thus isolating the other channels' contributions they can also be useful in purifying ion channels by affinity chromatography or in assaying their concentration. However, such inhibitors also make effective neurotoxins, and have been considered for use as chemical weapons. Neurotoxins aimed at the ion channels of insects have been effective insecticides one example is the synthetic permethrin, which prolongs the activation of the sodium channels involved in action potentials. The ion channels of insects are sufficiently different from their human counterparts that there are few side effects in humans.

The role of electricity in the nervous systems of animals was first observed in dissected frogs by Luigi Galvani, who studied it from 1791 to 1797. [ba] Galvani's results stimulated Alessandro Volta to develop the Voltaic pile—the earliest-known electric battery—with which he studied animal electricity (such as electric eels) and the physiological responses to applied direct-current voltages. [bb]

Scientists of the 19th century studied the propagation of electrical signals in whole nerves (i.e., bundles of neurons) and demonstrated that nervous tissue was made up of cells, instead of an interconnected network of tubes (a reticulum). [74] Carlo Matteucci followed up Galvani's studies and demonstrated that cell membranes had a voltage across them and could produce direct current. Matteucci's work inspired the German physiologist, Emil du Bois-Reymond, who discovered the action potential in 1843. [75] The conduction velocity of action potentials was first measured in 1850 by du Bois-Reymond's friend, Hermann von Helmholtz. [76] To establish that nervous tissue is made up of discrete cells, the Spanish physician Santiago Ramón y Cajal and his students used a stain developed by Camillo Golgi to reveal the myriad shapes of neurons, which they rendered painstakingly. For their discoveries, Golgi and Ramón y Cajal were awarded the 1906 Nobel Prize in Physiology. [lower-Greek 4] Their work resolved a long-standing controversy in the neuroanatomy of the 19th century Golgi himself had argued for the network model of the nervous system.

The 20th century was a significant era for electrophysiology. In 1902 and again in 1912, Julius Bernstein advanced the hypothesis that the action potential resulted from a change in the permeability of the axonal membrane to ions. [bc] [77] Bernstein's hypothesis was confirmed by Ken Cole and Howard Curtis, who showed that membrane conductance increases during an action potential. [bd] In 1907, Louis Lapicque suggested that the action potential was generated as a threshold was crossed, [be] what would be later shown as a product of the dynamical systems of ionic conductances. In 1949, Alan Hodgkin and Bernard Katz refined Bernstein's hypothesis by considering that the axonal membrane might have different permeabilities to different ions in particular, they demonstrated the crucial role of the sodium permeability for the action potential. [bf] They made the first actual recording of the electrical changes across the neuronal membrane that mediate the action potential. [lower-Greek 5] This line of research culminated in the five 1952 papers of Hodgkin, Katz and Andrew Huxley, in which they applied the voltage clamp technique to determine the dependence of the axonal membrane's permeabilities to sodium and potassium ions on voltage and time, from which they were able to reconstruct the action potential quantitatively. [i] Hodgkin and Huxley correlated the properties of their mathematical model with discrete ion channels that could exist in several different states, including "open", "closed", and "inactivated". Their hypotheses were confirmed in the mid-1970s and 1980s by Erwin Neher and Bert Sakmann, who developed the technique of patch clamping to examine the conductance states of individual ion channels. [bg] In the 21st century, researchers are beginning to understand the structural basis for these conductance states and for the selectivity of channels for their species of ion, [bh] through the atomic-resolution crystal structures, [bi] fluorescence distance measurements [bj] and cryo-electron microscopy studies. [bk]

Julius Bernstein was also the first to introduce the Nernst equation for resting potential across the membrane this was generalized by David E. Goldman to the eponymous Goldman equation in 1943. [h] The sodium–potassium pump was identified in 1957 [bl] [lower-Greek 6] and its properties gradually elucidated, [bm] [bn] [bo] culminating in the determination of its atomic-resolution structure by X-ray crystallography. [bp] The crystal structures of related ionic pumps have also been solved, giving a broader view of how these molecular machines work. [bq]

Mathematical and computational models are essential for understanding the action potential, and offer predictions that may be tested against experimental data, providing a stringent test of a theory. The most important and accurate of the early neural models is the Hodgkin–Huxley model, which describes the action potential by a coupled set of four ordinary differential equations (ODEs). [i] Although the Hodgkin–Huxley model may be a simplification with few limitations [78] compared to the realistic nervous membrane as it exists in nature, its complexity has inspired several even-more-simplified models, [79] [br] such as the Morris–Lecar model [bs] and the FitzHugh–Nagumo model, [bt] both of which have only two coupled ODEs. The properties of the Hodgkin–Huxley and FitzHugh–Nagumo models and their relatives, such as the Bonhoeffer–Van der Pol model, [bu] have been well-studied within mathematics, [80] [bv] computation [81] and electronics. [bw] However the simple models of generator potential and action potential fail to accurately reproduce the near threshold neural spike rate and spike shape, specifically for the mechanoreceptors like the Pacinian corpuscle. [82] More modern research has focused on larger and more integrated systems by joining action-potential models with models of other parts of the nervous system (such as dendrites and synapses), researchers can study neural computation [83] and simple reflexes, such as escape reflexes and others controlled by central pattern generators. [84] [bx]

The impact of negative thoughts on our brains

Karen Lawson, M.D., says that when we express our emotions without any attachment or judgment, we give them the freedom to flow out of our bodies and release the weight of this heavier energy. However, trapping them inside ourselves and holding onto these toxic thoughts can cause a variety of problems. Such conditions include high blood pressure and digestive troubles.

Chronic stress can actually decrease your life span by shortening your telomeres. Those are the “end caps” of DNA strands, which have a massive impact on aging.

So, how does the brain actually react when you think negative thoughts? Well, each time you have a thought, your brain creates more synapses and pathways in alignment with your thought process at the time. So thinking predominantly negative thoughts will only breed more of them. However, the reverse is true as well. According to many scientists, negative thinking and emotions inhibit signals from being transmitted between the central nervous system and the brain. In turn, this creates a “brain fog.” And that condition can compromise your immune system, memory, sleep patterns, and much more.

Negative feelings cause stress

When you experience negative emotions, the stress hormone cortisol is released into the body to help it cope with the immediate stressors. Reacting to a dangerous situation is a normal part of being human, but allowing the stress to continue after the event has occurred can lead to many health problems, including migraines, muscle and chest pains, and sleep disruptions.

Feeling afraid, which so often occurs when we focus too much on the future, can actually decrease activity in your cerebellum, which impedes the brain’s capability of processing new information and hinders creative problem-solving skills. Also, fear can affect the left temporal lobe, which controls everything from mood to memory to impulse control.

Your frontal lobe, particularly your prefrontal cortex, plays a big role in orchestrating thoughts and actions based on your internal goals and beliefs. Therefore, when you replay negative thoughts and feelings in your brain, you initiate the expansion of this thought process as more synapses and neurons are created to replicate your predominant thoughts.

So, now that you know just how your thoughts and emotions can affect your health, how can you ensure that your thoughts stay positive?

First of all, make sure to release your negative emotions in some manner. This can be a conversation with a friend, writing, drawing, or some of the other artistic expressions. The longer you ruminate on negative thoughts, the more you will see them manifest in your brain. Ultimately, you’ll change your reality.

Secondly, meditation helps immensely to dispel negative thinking and rewire your brain for positive thoughts. If you haven’t yet established regular meditation practice, make sure to read our article about meditation tips for beginners to get you started.

Also, make sure you surround yourself with positive influences, because energy is contagious. Allowing negative people into your life will only bring down your own energy levels. Therefore, you’ll be exposed to toxic energy on a regular basis. And that circumstance can do great damage to the spirit. Keep the company of those who inspire, uplift, and encourage you to live the best life possible!

Finally, make sure you take enough time for self-care and commit to living the life you love. Don’t compromise your own happiness for someone else’s. So the only way to evolve into your highest self is to follow your own heart. Trust it to take you where you need to go. Negative emotions and thoughts can’t survive in a heart and mind full of life, love, and true happiness.

Q: What happens in the depolarization of a nerve?

(A). Sodium ions exit the nerve membrane
(B). Sodium ions enter the nerve membrane
(C). Both sodium ions and potassium ions enter the nerve membrane
(D). Both sodium ions and potassium ions exit the nerve membrane

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Keyword: depolarization

Nerve conduction (and recovery) follows the steps of depolarization, repolarization, hyperpolarization, and refractory period.

  • Depolarization occurs when a stimulus reaches a resting neuron. During the depolarization phase, the gated sodium ion channels on the neuron’s membrane suddenly open and allow sodium ions (Na+) present outside the membrane to rush into the cell.
  • As the sodium ions quickly enter the cell, the internal charge of the nerve changes from -70 mV to -55 mV.
  • When this firing threshold of -55 mV is reached, membrane permeability to sodium increases dramatically and sodium ions enter the axoplasm (the inner portion of the nerve cell) even more rapidly.
  • As a result, the inner portion of the nerve cell reaches +40 mV.
  • With repolarization, the potassium channels open to allow the potassium ions (K+) to move out of the membrane (efflux). As this happens, the electrical potential gradually becomes more negative inside the nerve cell until the original resting potential of -70 mV is attained again.

To summarize, sodium ions (Na+) enter the nerve membrane during depolarization and potassium ions (K+) leave the nerve membrane during repolarization.

Answer: (B). Sodium ions enter the nerve membrane

Now, what does local anesthesia do in this process? Why do we use local anesthesia in dental hygiene to numb the patient? Local anesthetic molecules work by blocking the sodium channel and interfering with depolarization. As a result, the nerve is NOT “fired” (triggered) and therefore, the patient does NOT feel the pain. This process is explained on the right side of the image above.

If you have any questions regarding local anesthesia, just let me know. By the way, StudentRDH has a fantastic course that gives you everything you need without the pain of reading the entire textbook. The quizzes also give you immediate feedback so you can learn faster. Taking the WREB or CDCA boards? No problem! There is a mock exam that simulates that exact format for the local anesthesia boards. You know where to find me! At [email protected]

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Claire Jeong, RDH, MS

Claire is an entrepreneur, author, educator, researcher, and international speaker. She is the founder of StudentRDH and SmarterDA, dental hygiene and assisting exam prep solutions. Through her live and online courses, Claire helped tens of thousands of people gain valuable dental knowledge and clinical skills. She combines the WakeUp Memory Technique™ in her courses and teaches educators to apply this method in their own classrooms. To her audience’s testimony: "Learning is now addictive."


The nervous system is divided into two camps. There is the central nervous system (CNS) which is your brain and your spinal cord. Then there is the peripheral nervous system (PNS), which is everything else.

In the central nervous system (the brain and spinal cord), if you injure that - as far as we know - it's permanent. The nerve cells may die or they may not die but they certainly don't reconnect with where they should connect. That stops signals getting through which is why you get problems of paralysis or loss of sensation, depending on where the damage is. That's why a stroke is so disabling.

In the skin the nerve cells there seem to be able to survive injury. They also seem to be able to re-grow to their targets so they go back to where they connected to in the first place. So if it was a muscle they were supposed to be supplying, they'll reconnect with the muscle. If it was a patch of skin they can branch out and re-supply the skin so you do get sensation back.

But nerves grow quite slowly, probably a couple of millimetres a day. So if you've got a big injury the length of your arm it can take a few weeks before the nerves can get back to your arm. The sensation may not be absolutely perfect because some nerve cells might die but you should get coverage of the skin back afterwards.

What happens when you break or interrupt a nerve is that the actual cell inside is just one massive long cell. The distal bit (the bit downstream of the cut site) will degenerate. It retracts and forms this little lump bulb. This then grows back along the original path of the nerve so it uses the original pathway of the nerve as a guide, rather like a motorway cone. It uses the cones and lays down a new road surface, which is the nerve, and it gets back to where it was supposed to attach. The distal site it was supposed to attach to switches on various markers so it can recognise it and off it goes!

General Biology Exam 3

1b.) Describes a solution/cell that has low solute-high solvent concentration.
2b.) Because of this, water moves into the cell.
3b.) Plant cells become "turgid" (inflated but not exploded due to plant Cell Wall)
4b.) Animal cells become "lysed" (exploded)
5b.) HEMOLYSIS describes the fatal state of an animal cell in a hypotonic environment.

2.) Active Transport
(moving from an area of low to high concentration--energy "required")

3.) Endocytosis (Taking in substances to be digested in the lysosome)

b.) Facilitated Diffusion and Osmosis.

b.) Polar (i.e. glucose) , large (i.e. non-electrolytes) , & charged (i.e. cations/anions) molecules.

2.) Sodium-Potassium (Na-K) pumps are involved.

c.) Proton pump (pumps H+ ions out of the cell, so the cell becomes less positively charged [negative] and the extracellular fluid is more positively charged [positive])

b.) Na-K pumps require energy in the form of ATP in order to transport substances from an are of low to high area of concentration.

2.) A distant cell responds to the SECRETED hormonal signals

--> NOTE: Hormonal signals are released in small amounts and travel exclusively through the bloodstream.

2.) The target cell detects the signaling molecule when the signaling molecule binds to a receptor protein embedded in the surface of the target cell.

3.) Once the signaling molecule binds to the receptor site, the receptor protein changes shape.

2.) Open and close ion channels

b.) Polar proteins cannot pass through the plasma membrane because of the negatively charged phosphate heads of the plasma membrane they are also too large.

b.) At the "reception" stage, Ligands bind to their specific receptor and causes the receptor protein to change its shape. At the "response" stage, the receptor "activates" a particular cellular response.

c.) G proteins are the most common type of transmembrane protein receptors embedded in the plasma membrane.
d.) A polar Ligand binds to the G protein and changes its shape. Once this is done, the GDP protein can bind to the G protein. The GDP is now GTP. The GTP then activates a nearby enzyme. When the ligand leaves the G protein, the G protein hydrolyzes GTP into GDP and pi, restarting the process.

e.) Receptor tyrosine kinases.
f.) When 2 signaling molecules bind to receptor tyrosine kinases, they link together (dimerization).
These "dimers" are activated by phosphorylation. The enzymatic receptor activates and phosphate groups (taken from ATP) attach to the tyrosine parts of the "dimer". Relay molecules (relay proteins) bind to the attached phosphates and are then activated. These relay proteins then leave to fulfill their roles as relay molecules in the signal-transduction pathway.

-->***NOTE for '1': This is a non-polar Ligand

2.) Aldosterone binds to a receptor protein located in the cytoplasm ("reception"), activating it ("response").

-->NOTE for '2':When this steroid hormone binds to the receptor protein, is changes that receptor into a "hormone-receptor complex" that triggers a particular response.

3.) The hormone-receptor complex
(as mentioned in 'NOTE') enters the nucleus and binds to DNA (transcription occurs soon).

4.) The bound protein acts as a "transcription factor", causing DNA to be transcribed into mRNA.

Studies on tissue from patients and mini-brains made from stem cells shed light on the disease.

In addition to plaques that accumulate outside of nerve cells in the brain, Alzheimer’s disease is also characterised by changes inside these cells. Researchers from the Cell Signalling research group at the Chair of Molecular Biochemistry at RUB, headed by Dr. Thorsten Müller, have been studying what exactly happens in these cells. They determined that various proteins and protein components accumulate in the cells, which also affect their functions. Moreover, they identified a correlation between the progression of the disease and certain corpuscles in the cell nuclei. They published their report in the journal Acta Neuropathological Communications on 13 April 2021.

Aggregates seem to have a function

Affecting over 50 million people, Alzheimer’s disease is the most common form of dementia and primarily occurs in people over the age of 65. The pathology of the disease in the brain is mainly characterised by two factors: beta-amyloid plaques outside the nerve cells and tau proteins. The tau protein stabilises tube-like structures (microtubules) inside cells, which are relevant for the transport of nutrients in nerve cells. Beta-amyloid is a protein present in the body that is formed by the cleavage of the amyloid precursor protein (APP).

APP is embedded in the cell membrane of the nerve cells and protrudes both on the inside and outside. Typically, it is cleaved once near the cell membrane The part inside the nerve cells is unstable and disintegrates. In Alzheimer’s patients, two cleavages take place, resulting in the formation of three parts. The new findings now indicate that the part inside the nerve cells is more stable in Alzheimer’s patients. It consists of only approximately 50 amino acids and can, under certain conditions, migrate into the cell nucleus together with other proteins such as FE65 and TIP60. This protein complex, also known as nuclear aggregates, has the capacity to manipulate the cell’s gene expression. “This suggests that the aggregates have a function in this region,” points out lead author David Marks from the Cell Signaling group at RUB. Closer analysis confirmed this evidence, as a protein was found within the nuclear aggregates that is involved in the modification of DNA.

Two more protein candidates in cells

“To understand this mechanism even better, we looked for other proteins that might be part of these aggregates and identified two more candidates involved in the nuclear aggregates, the so-called tumour suppressor proteins P53 and PML,” explains David Marks. The study team showed in experiments on living cells that nuclear aggregates formed from the proteins APP-CT50, FE65, TIP60 and PML fuse with each other over time and produce even larger nuclear aggregates.

In addition, they examined brain samples from older deceased persons as well as self-generated neuronal tissue from induced pluripotent stem cells. “These mini-brains, so-called cerebral organoids, reflect the embryonic and developmental phase of a brain fairly accurately,” explains Thorsten Müller. The research team found nearly no nuclear aggregates in the comparatively young cerebral organoids, whereas they were present in brain samples from older patients. “This leads us to conclude that the process depends on age,” says Müller.

Striking correlation

Furthermore, the researchers showed on these brain samples that APP-CT50 and FE65 can make up part of the so-called PML bodies, which occur naturally in large numbers of cell nuclei. Upon further analysis of hippocampal brain slices from Alzheimer’s patients, the authors identified a significant correlation between a reduced number of PML bodies in the nucleus in regions with a high beta-amyloid plaque load. In particular, such regions mean that there is higher expression and processing of APP. This may suggest that the transfer of APP-CT50 and FE65 into the nucleus is part of AD pathology and affects the fusion of PML bodies there.

The study was funded by the German Research Foundation under the funding codes MU3525/3-2 and INST 213/886-1 FUGG and by the Mercator Research Center under the funding code Pr-2016-0010.

David Marks et al.: Amyloid precursor protein elevates fusion of promyelocytic leukemia nuclear bodies in human hippocampal areas with high plaque load, in: Acta neuropathologica communications, 2021, DOI: 10.1186/s40478-021-01174-x

How proteins control information processing in the brain

A complicated interaction between different proteins is needed for information to pass from one nerve cell to the next. Researchers at the Martin Luther University Halle-Wittenberg (MLU) have now managed to study this process in the synaptic vesicles, which play an important role in this process. The study appeared in the journal Nature Communications.

Several billion nerve cells communicate with each other in the body so that humans and other living beings can perceive and react to their environment. A host of complex chemical and electrical processes occur within a few milliseconds. "Special messenger substances -- known as neurotransmitters -- are released at the synapses of the nerve cells. They transmit information between the individual nerve cells," explains Dr Carla Schmidt, an assistant professor at the Centre for Innovation Competence HALOmem at MLU. The messenger substances are packed into small vesicles called synaptic vesicles, which fuse with the cell membrane in response to an electrical impulse and release the messenger substances. The messenger substances are then recognised by special receptor proteins in the next nerve cell. For this to succeed, numerous proteins have to work together, meshing like cogs in a clockwork mechanism. However, too little is currently known about how this process precisely works, says Schmidt.

The researchers have used a special form of mass spectrometry to investigate the process. Cross-linking mass spectrometry helps identify the interaction sites of the proteins. These are mixed with a substance that binds together nearby proteins. This substance reacts at different places depending on how the proteins interact with one another. The mass spectrometer analyses the binding patterns, which can be used to draw conclusions about the arrangement of the proteins. This enables researchers to examine different stages of the vesicles and to detect which protein networks have formed.

The study from Halle enables a more thorough understanding of the process of signal transmission in nerve cells. Knowledge about normal processes helps scientists recognize and understand malfunctions that could trigger diseases such as Alzheimer's.

The study was supported bei the Federal Ministiry of Research and Education in Germany, the European Regional Development Fund (ERDF) and the Alexander von Humboldt-Foundation.

Top 38 Fun Facts About The Nervous System

The nervous system is a wonder in itself, a complicated system without which other systems would not function fully. Here are 38 fun facts about the nervous system:

There are 2 main parts in the nervous system.

  • There are two parts to the nervous system namely the central nervous system (CNS) and the peripheral nervous system (PNS).
  • The peripheral nervous system is further comprised of two subdivisions namely the somatic and the autonomic nervous system.

Neurons, Axons & Dendrites are the basic building blocks in the nervous system.

  • The basic functional unit of the nervous system is the neuron.
  • There are billions of neurons in the nervous system, most of them in the brain.
  • The neuron consists of long cables like extensions that protrude out of its body termed as axons and short, thick extensions termed as dendrites.
  • Together, axons and dendrites act as cables to carry messages to and from the brain and spinal cord.
  • Billions of these neurons work in conjunction with the brain to provide information about the surroundings to the human body.

The brain requires and utilizes more energy than any other organ in the body.

  • The brain uses more than 20% of the body’s total energy production.
  • Most of this energy is channeled through the brain towards the transmission of electrical impulses whether we are awake or asleep.
  • It is a well-known fact that the brain works tirelessly even during sleep.
Reference: “Why Does the Brain Need So Much Power? – Scientific American”. Accessed April 14, 2019. Link.

Watch the video: How thoughts are generated in our brain (July 2022).


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