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Suppose I set the voltage value of an isolated stimulator with a floating ground. I place one electrode above the spinal cord (positive) and the other placed subcutaneously far away from the spinal cord (negative). The circuitry within the stimulator will operate in such a way that the electrical potential difference between these two electrodes is the value I specify.
The stimulator can fix the electrical potential difference between the two electrodes in multiple ways:
- Hold the positive electrode's potential constant while changing the negative electrode's potential.
- Hold the negative electrode's potential constant while changing the positive electrode's potential.
- Adjusting both electrode potentials using a rule.
Although each of these ways can fix the voltage between the electrodes, they do not have equal effects on the biological environment that the electrodes are placed in. Going back to my example, if the stimulator obeys 1, I would expect a larger effect to be induced local to the spinal cord, in contrast to 2, which would would have a greater effect in the subcutaneous area. This is because there is an initial heterogenous charge distribution.
How should I assume my stimulator is fixing the potential difference between my electrodes?
Electrocardiography is the process of producing an electrocardiogram (ECG or EKG [a] ). It is a graph of voltage versus time of the electrical activity of the heart  using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances (such as atrial fibrillation and ventricular tachycardia), inadequate coronary artery blood flow (such as myocardial ischemia and myocardial infarction), and electrolyte disturbances (such as hypokalemia and hyperkalemia).
In a conventional 12-lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall magnitude of the heart's electrical potential is then measured from twelve different angles ("leads") and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the cardiac cycle. 
There are three main components to an ECG: the P wave, which represents the depolarization of the atria the QRS complex, which represents the depolarization of the ventricles and the T wave, which represents the repolarization of the ventricles. 
During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads throughout the atrium, and passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers, spreading down and to the left throughout the ventricles.  This orderly pattern of depolarization gives rise to the characteristic ECG tracing. To the trained clinician, an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system.  Among other things, an ECG can be used to measure the rate and rhythm of heartbeats, the size and position of the heart chambers, the presence of any damage to the heart's muscle cells or conduction system, the effects of heart drugs, and the function of implanted pacemakers. 
Bioelectric potentials in relation to movement in amoebae *
Evidence is presented to show that, in Amoeba proteus, the membrane potential (— 72 mV) and the streaming velocity (40 microns/sec) are both affected by changes in the ionic composition of the medium.
The membrane potential and streaming velocity are inverse functions of log external potassium chloride concentration over a wide range. Distilled water decreases the streaming velocity, although the membrane potential is raised.
Single penetrations in different parts of the cell and direct measurements using two internal micro-electrodes provide evidence for an electrical potential gradient in the cytoplasm along the axis of movement which approaches 1 volt/cm. The tip of a pseudopod appears to be positive with respect to the rear of the cell and negative with respect to the external medium.
Electrical potentials, positive or negative, applied to the rear of the cell, can be used to redirect the streaming in a chosen direction. Streaming is away from a negative electrode and towards a positive one.
The relationships between the membrane potential, the internal potential gradient and the motive force for streaming are discussed. A new theory is put forward for the bioelectric control of amoeboid movement.
Law defines the relationship between electrical current, voltage, and resistance. Current = Voltage/Resistance
- Current flow is directly proportional to voltage: INCREASE Voltage = INCREASE Current, DECREASE Voltage = DECREASE Current
- Current flow is inversely proportional to resistance: INCREASE Resistance = DECREASE Current, DECREASE Resistance = INCREASE Current
Biological tissues such as nerve and muscle membrane have the ability to simultaneously store and electric charge and oppose change in current flow. This characteristic is called capacitance. Skin and adipose act as resistors, or oppose current slow. Current always takes the "path of least resistance" when faced with multiple resistors.
Current will flow under 2 conditions:
- There is an energy source creating a difference in electrical potential
- There is a conducting pathway between the two potentials
Ionic Flow occurs in the body because like charges REPEL and opposite charges ATTRACT.
Anode = positive (+) electrode
Anion = negative (-) ion
Cathode = negative (-) electrode, often referred to as"active" electrode
Cation = positive (+) ion
Note the names are paired by attraction
At rest, a nerve holds a positive charge on the inside and negative on the outside.
Chemical reactions that occur under each electrode (Brethren Figures 8-2 and 8-3)
Cathode: positive Nan+ sodium ions migrate to the negative pole and combine with water to form Nao sodium hydroxide Base = increased alkalinity, promotes liquification of protein, and tissue softening
Anode: negative chlorine (Cl-) ions migrate to the positive pole and combine with water to form hydrochloric acid (HCL) = increased acidity, promotes coagulation of protein, and hardening of tissues.
Circulation improves as the body attempts to balance back to homeostasis and neutral pH level.
Noninvasive electrical stimulation of the eyes has been studied as a promising therapeutic tool to recover visual functions in patients suffering from various eye diseases 1 . There are two well-known methods that deliver the electric current to the eye noninvasively. One is transcorneal electrical stimulation that delivers the currents via a contact-lens-type electrode attached right above the cornea 2,3 . Previous studies reported that transcorneal electrical stimulation has beneficial effects on the improvement of visual functions in patients with optic neuropathy 2 and retinitis pigmentosa (RP) 4,5,6 . According to the studies that used animal models with eye diseases, improvement of visual functions resulting from transcorneal electrical stimulation was closely associated with the survival of the retinal ganglion cells (RGCs) and photoreceptors preserved from the degeneration, suggesting that the neuroprotective effect on retinal cells determines the outcome of the transcorneal electrical stimulation 7 . Additionally, it was found that the increase in the survival of RGCs after transcorneal electrical stimulation is related to an increase in the insulin-like growth factor 1 (IGF-1), brain-derived neurotrophic factor (BDNF), and ciliary neurotrophic factor (CNF), which are released from Müller cells in the retina 8,9 .
The other method is transorbital electrical stimulation (tES) that delivers weak electrical current to the eye via electrodes attached to the skin around the eye. The stimulation parameters such as electrode configurations, current waveforms, injection current intensities differed among studies. Generally, square pulses in bursts with the frequency range of 5–30 Hz were applied for tES 10 . Compared to transcorneal electrical stimulation, tES is less invasive with no side effects such as dry-eye and punctate keratitis and easier to apply 11 . Repetitive tES, applied to patients with optic nerve damage, has been reported to improve visual field size, visual acuity, and detection ability 12,13 . Repetitive tES has also reportedly strengthened the alpha-band functional connectivity in patients with chronic prechiasmatic visual system damage 14 . Another study has demonstrated that the tES-treated group showed a significant improvement in visual fields and reaction times during the visual-field-related task compared to the sham stimulation group 15 . Furthermore, tES has also been effective in improving visual function in patients with RP 16 . A previous study reported that the effectiveness of tES was related to the synchronization of cortical activities after retinal cells were stimulated 10 . Another study insisted that improvement of visual functions together with changes in the spectral EEG alpha band power and connectivity in the occipital lobe after tES might be caused by a retinofugal entrainment through firing of RGCs 15 . Indeed, a previous in vivo experimental study with rats also demonstrated that electrically evoked, tES-induced responses stemmed from the retina 17 . These series of findings suggest that a stronger electric field should be delivered to the cells in the retina to increase the therapeutic effect of tES.
Generally, the conventional electrode configuration used for tES comprises two active electrodes attached to the skin near the orbital cavity and a single reference electrode placed on the occipital pole or extra-cephalic regions like the wrist and neck 10,12,18 . According to a numerical simulation study with the conventional electrode montage, most electric fields were delivered to the anterior part of the eye 15 . Therefore, the conventional tES dominantly stimulated the anterior part of the retina despite a large number of retinal cells, including RGCs and Müller cells, being densely distributed in the posterior part of the retina, particularly around the fovea 19 . Therefore, considering the abovementioned action mechanisms of both tES, a stronger electric field should be delivered to the posterior part of the retina to increase the effectiveness of tES. In the conventional tES, however, electric field delivered to the peripheral side of the retina (anterior retina) reaches an individual phosphene threshold 20 , which represents the maximally allowable injection current in tES that does not evoke phosphenes in an individual, before a sufficient amount of stimulation current is delivered to the posterior retina. Therefore, it is necessary to reduce the electric field delivered to the anterior retina in relative to that delivered to the posterior retina to maximize the overall therapeutic effects of tES.
This study proposes a novel tES montage with eight active electrodes, with the diameter of 1 cm, attached around the eye (approximately 2 cm away from the center of the cornea) and a reference electrode on the occipital pole to reduce the difference in the electric field intensities delivered to the anterior and posterior retina. In other words, the study aims to maximize the electric field delivered to the posterior retina when that delivered to the anterior retina reaches the individual phosphene threshold. As aforementioned, short duration square pulses at a specific frequency are generally employed for tES. Although the electrical conductivity values of tissues are dependent on the frequency of injected current 21,22,23 , we employed tissue electrical conductivity values at DC frequency and solved a quasi-static Laplace equation because the frequency range used for tES (5 – 30 Hz) was low enough for the quasi–static approximation. Indeed, it was reported that there was no difference between the electric fields calculated assuming DC and AC with a relatively high frequency (
1 kHz) 24 . The optimal injection currents of the active electrodes were determined to maximize the electric field delivered to the posterior retina, near the fovea, by employing a constrained convex optimization approach. The efficacy of the new stimulation conditions was evaluated by comparing it with the conventional electrode montage.
If the sampling frequency is lower than half of the signal frequency, the recording is distorted. This phenomenon is called “aliasing”. EEG electrodes are able to pick up electrical noise at high frequency, e.g., from the muscles in the scalp, which results in the induction of artifacts, etc. To prevent distortion of the recordings due to these higher frequency components, signals are filtered using an analogue filter or anti-aliasing filter before analogue-to-digital conversion (figure 5). The effect of this filter is permanent.
After analogue-to-digital conversion, the digital signals can be further filtered, using selective digital filters. However, the effect of the digital filters is not permanent, since this is merely post-processing of the digital recordings. Many of the undesired signals are within frequency ranges that differ from those of EEG signals generated from the brain, and therefore can be removed or attenuated using digital filters when one is reviewing recordings. However, there are several important pitfalls when using digital filters. If one filters out a component within the range 0.1–70 Hz, then one can potentially lose relevant EEG data. Frequency components within the range of the filter values, and close to them, become distorted.
High-frequency or low-pass filters attenuate the components of frequency higher than the value of the filter. The filters attenuate the amplitude of signals at the cut-off value of the filter by 20–30%, and even higher frequencies are attenuated to a greater degree, even to complete elimination. Muscle artifacts, including surface-EMG signals from the muscles in the scalp, have higher frequency components than most EEG signals. However, filtering them out may also remove a high-frequency burst of EEG signals such as spikes. The other problem is that filtering out the muscle activity only attenuates these signals, and what is left may resemble a burst of spikes/sharp-waves and lead to erroneous interpretation.
Low-frequency or high-pass filters attenuate the slow components. The low-frequency range includes movement and sweating artifacts, which can be greatly attenuated by the use of these filters. The downside of too much low-frequency filtering is a loss or attenuation of actual focal or generalized pathological activity. In an extreme case, such as a recording of hypsarrhythmia, the EEG can almost be made to appear relatively normal. The influence of the low-frequency filter is determined by the time constant which is defined as the time required for the amplitude of a square wave to decrease to 37% of its original value (Sharbrough, 1997 ). The cut-off frequency can be calculated by dividing 0.16 (1/2π) by the numerical value of the time constant. For example, with a cut-off frequency of 0.16 Hz, the time constant is 1 second. Increasing the value of the low-frequency filter too much can remove some clinically important slow waves, or distort the form of the slow EEG pattern. An example would be the distortion of a blink artifact to such a degree that the correct interpretation becomes difficult.
Notch filters affect only a narrow frequency range and are primarily used to eliminate the electrical noise caused by power-line current 60 Hz in North America and 50 Hz in Europe. It is important to begin all recordings with the notch filter deactivated so that the technician can be alerted to “bad electrodes” with weak contacts between electrode, paste, and scalp. When one starts to review an EEG recording, the recommended filter settings are 0.5 Hz to 1 Hz for the high-pass and 70 Hz for the low-pass digital filter, with the selective 60-Hz or 50-Hz filter turned off (Sinha et al., 2016 ).
Types of Electrode: 4 Types (With Diagram)
This article throws light upon the four types of electrode used in electrochemical techniques.
The four types of electrode are: (1) The pH Electrode (2) Ion Selective and Gas Sensing Electrodes (3) The Clark Oxygen Electrode and (4) The Leaf Disc Electrode.
Type # 1. The pH Electrode:
Perhaps the most convenient and accurate way of determining pH is by using a glass electrode. The pH electrode depends on ion exchange in the hydrated layers formed on the glass electrode surface.
Glass consists of a silicate network amongst which are metal ions coordinated to oxygen atom, and it is the metal ions that exchange with H + . The glass electrode acts like a battery whose voltage depends on the H + activity of the solution in which it is immersed.
The size of the potential (E) due to H + is given by the equation:
where [H + ] and [H + ]o are the molar concentrations of H + inside and outside the glass electrode respectively. In practice, [H + ] is generally 10 -1 , because the electrode contains 0.1 M HCL. Since pH= – log [H + ], it follows that the developed potential is directly proportional to the pH of the solution outside the electrode. Glass electrodes are particularly useful because of lack of interference from the components of solution.
On the whole these molecules are not readily contaminated by molecules in solution, and if other ions are present they do not cause any significant interference. However, at high pH they do respond to sodium. Inaccuracies also occur under very acid conditions.
A glass electrode consists of a thin, soft glass membrane that is situated at the end of a hard glass tube, or sometimes an epoxy body. Also present in the glass electrode is an internal reference electrode of the silver/silver chloride (Ag/AgCL) surrounded by electrolyte of 0.1 M HCL. This internal reference electrode gives rise to a steady potential.
Thus the varying potential of the glass electrode can be compared with a- steady potential produced by an external reference electrode such as the standard calomel electrode by joining internal and external reference electrode.
The external reference electrode can either be a separate probe or built around glass electrode giving a combination electrode. If a combination electrode is used, the level of the test solution must be high enough to cover the porous plug (liquid junction) but not as high as the level of salt bridge solution (KCL) in the external electrode because it is essential for KCl to diffuse out slowly into the test solution.
Whatever reference electrode is used the measured voltage is the result of the difference between that of the reference and the glass electrodes. In practice, however, there are other potentials present in the system. These include so-called asymmetric potential, which is poorly understood but which is present across the glass membrane even when the H + concentration is the same on both sides.
Also included are the potentials due to Ag/AgCl and to the liquid junction to the reference electrode, which gives the potential because the K + and CI – do not diffuse at exactly the same rate and, therefore, generate a small potential at the boundary between the sample and the KCl in the reference electrode. The measured potential for glass electrode should thus also include constants to account for the additional potential within the device.
Therefore, the equation becomes:
where E* includes the standard electrode potential for glass electrode, and the constant junction potential present in the system.
At 25°C this equation becomes:
where E* now also includes a term to account for the internal H + concentration. As already known there is a 59 mV change for a 10-fold change in the activity of a monovalent ion this means that a change of one pH unit produces a 59 mV change.
A pH electrode is used in conjunction with a pH meter. This records the potential due to H + concentration but is designed to take a little current from the circuit. A large current flow will cause changes in the ion concentration and hence changes in pH this is prevented by having a high resistance present. The pH meter, glass electrode and reference calomel electrode are designed so that pH gives a zero potential.
Operation Of pH Electrode/Meter:
pH electrodes are available in variety of different shapes and sizes for many different applications. Intracellular pH can also be measured by using a miniature probes (micro electrodes). However, majority of them are based on same principle and operated in a similar fashion.
It is important that the outer layer of glass on glass electrode remains hydrated, and so it is normally immersed in a solution. Thus thin glass layer is fragile and thus care must be taken not to break it or scratch it, or to cause a buildup of static electric charge by rubbing it. Gelatinous and protein containing solutions should not be allowed to dry out on the glass surface as they would inhibit response.
As it is clear from above equations that potential produced is temperature dependent (each pH unity change represents 54.2 mV at 0°C and 61.5 mV at 37°C). This effect is predictable and can be compensated for. The pH meter will thus have a temperature compensation dial that must be correctly set before the meter is calibrated.
Calibration will necessitate the use of two solutions of widely differing pH. Usually calibration is first performed with buffer of pH 7, followed by a pH 4 buffer (if the sample is expected to be a acid) or a pH 9 buffer (if the sample is expected to be basic). Once the pH electrode is calibrated, it can simply be immersed in the solution to be measured and a rapid and accurate measurement estimate of pH can be made.
Type # 2. Ion Selective and Gas Sensing Electrodes:
The glass pH electrode is really a kind of ion-selective electrode (ISE) that is sensitive to H + . Similar potentiometric electrodes have been developed which are responsive to other ions, e.g., Na + , NH + 4, Cl – and NO – 3. The active material within these devices may be glass, an insoluble organic salt, or an ion exchange material.
Glass is the active material within the pH electrode, but modified aluminium silicate glasses can also be used to produce a variety of monovalent cation responsive electrodes. Insoluble inorganic salts like silver sulphite can be used to produce electrodes responsive to Cu 2+ , Pb 2+ and Cd 2+ , whereas lanthanum fluoride may be used to produce electrodes responsive to F – .
Ion selective electrode responds to the activity of particular ion. However, if the instrument is calibrated with a standard of known concentration then, provided the ionic strength of solution are similar, the concentration of test solution will be recorded. If some of the ions are not free and exist in complex form or an insoluble precipitate, these electrodes will give a much lower reading, then with a method that detects all of the ions present. Generally used ion selective electrodes are Ca 2+ , K + , and NO – 3.
An electrode may be ion selective but not ion specific. As with glass electrodes, these can be fouled by proteins forming a surface film. A reference electrode is also needed with these ISE so that the varying potential of these ISE can be compared with the steady potential produced by reference electrode.
Gas Sensing Electrodes:
They are used generally to estimate the concentration of gas by its interaction in a thin layer surrounding an ion sensitive electrode, commonly a pH electrode. Carbon dioxide, sulphur dioxide, ammonia can all be measured by their dissolution in a thin layer surrounding the pH electrode, and measuring the resultant pH of the layer.
Miniaturisation and Applications of Ion Sensitive Electrodes:
Miniaturisation of ion selective electrodes has been achieved by modification of field effect transistor to respond to specific ions. Such ion selective field effect transistors (ISFETs) are likely to have great clinical value. Multifunctional ISFETs are already available which are used to measure pH, Na + , K + , and Ca 2+ .
Type # 3. The Clark Oxygen Electrode:
It consists of a platinum cathode and silver anode, both immersed in same solution of saturated potassium chloride and separated from the test solution by a oxygen permeable membrane. When a potential difference of—0.6 V is applied across the electrodes such that platinum cathode is made negative with respect to silver anode, electrons are generated at anode and are then used to reduce oxygen at cathode.
The oxygen tension at cathode drops and so to make this deficit more oxygen moves towards cathode. Since the rate of diffusion of oxygen from the membrane is the limiting step in the reduction process, the current produced by the electrode is proportional to the oxygen tension in the sample.
These electrode reactions may be summarized as under:
At silver anode 4Ag + CI ‑ → 4AgCl + 4e –
At platinum cathode O2 + 4H + + 4e – → 2H2O
Operation of Rank Oxygen Electrode (Clark Electrode):
These allow the sample to be placed in upper reaction chamber by an oxygen permeable and ion impermeable membrane. Teflon is the usual choice, though cellophane, polythene, silicon rubber and cling film have been used with varying degree of success. Care must be taken that membrane must not become contaminated.
Thinner membranes give more response but are more fragile. The membrane covers the electrodes and allows oxygen to diffuse towards them whilst preventing other reaction elements to reach electrode and poison them. The electrodes are maintained in electric continuity with potassium chloride solution.
Oxygen electrode is mounted above a stirring motor, which is able to rotate a magnetic follower (flea) when inserted into the reaction vessel which is important as the platinum cathode reduces oxygen to produce the electric current. A correct set-up will show reduction in current when stirrer is switched off due to depletion of oxygen in the potassium chloride filled electrode chamber.
Resumption of stirring will result in a return of current (oxygen tension in potassium chloride) to its previous level prior to the stirrer being switched off. Since both solubility and rate of diffusion is affected by temperature, therefore some form of temperature control is necessary for better results which are done by circulating water bath.
Calibration of the instrument should be carried out at the same temperature as that of experiment. Many chemicals are adsorbed onto the surface of the membrane and reaction vessel hence it is important that the apparatus is thoroughly cleaned after each experiment.
Applications of Rank Oxygen Electrode:
Due to their ability to give continuous trace, oxygen electrodes have largely replaced manometric techniques in the study of reactions involving oxygen uptake and evolution.
i. Mitochondrial studies:
The study of respiratory control and effect of inhibitors on mitochondrial respiration and the measurement of phosphorylation: oxidation (P: O) ratios are best done by oxygen electrodes.
ii. The sites of action of electron transport inhibitors can also be determined using an oxygen electrode.
iii. Micro-organism that uses oxygen as the terminal electron acceptor of respiratory electron transport can be studied using oxygen electrode and the effect of electron transport inhibitors determined.
Enzymes are readily studied using Clark oxygen electrode, provided oxygen is involved in the reaction. Glucose oxidase, D-amino acid oxidase, and catalase are examples whose properties can be studied in this way.
Probe Type Clark Electrode:
These rely on same principle of operation as the rank electrode. However, the cathode and retaining membrane are arranged at the end of the probe to enable insertion into a liquid phase. It has disadvantage that it does not have stirring arrangements. It has a variety of uses.
Measurement of oxygen in bulk liquids:
Oxygen concentrations are routinely monitored in fermentation processes, sewage and industrial waste treatment and in inland, coastal and oceanic waters. This involves the variation of the Clark electrode called flush top sensor.
Early clinical use of oxygen electrode is to measure heart-lung machines during open heart surgery. They are also used for testing patients who were treated with oxygen. Small samples of blood are taken from patient and oxygen content is measured in a small Clark type pO2 electrode.
Type # 4. The Leaf Disc Electrode:
Whilst the rank oxygen electrode is ideally suited to many applications requiring a measurement of oxygen in aqueous samples, a leaf disc electrode such as the Hanasatech LD2 is of more use if gaseous oxygen measurement is required. Since the measurement of oxygen evolution is one of the easiest ways of following photosynthetic process in leaves, this instrument has found much biological application.
This device measures oxygen amperometrically using the same principle as the rank electrode. However, instead of being a liquid -filled reaction vessel, the reaction chamber is designed to allow a leaf to be held in place and provided with saturating carbon dioxide (or bicarbonate as a source of carbon dioxide). Illumination is usually provided by an array of light-emitting diodes (which produce little heat) and the oxygen emitted by leaf during photosynthesis can be measured.
Calibration of this electrode is bit complex as compared to rank electrode. A zero oxygen signal can be produced by passing nitrogen through the reaction chamber. Once this is stopped and air is passed through the chamber, signal corresponding to 21% of the oxygen can be determined. However, in closed chamber system, the amount of oxygen is related to the oxygen concentration and to volume of the chamber.
In practice, because the leaf disc itself may reduce the effective volume of the chamber, calibration involves injecting known volumes of air into the chamber and measuring the voltage response to obtain the effective volume of the chamber and hence a precise calibration of the electrode.
The leaf disc electrode has been used extensively for the study of the relationship between photosynthetic oxygen evolution under saturating carbon dioxide and the intensity of illumination, enabling calculation of quantum yield, the inclusion of probes to measure emitted fluorescence from the leaf disc at the same time as the oxygen evolutions measurement are made has resulted in a device that provides variety of information.
Applications of these devices are diverse, ranging from studies of micro-propagated plants to those plants suffering from atmospheric pollution. Though leaf disc electrode is clearly designed for whole leaf studies, photosynthetic rates of microalgae have also been studied using theses electrodes.
3 Physarum Neurons
We represent a Physarum neuron by a physically localized and almost everywhere isolated locus of Physarum, such as the blobs of agar colonized by Physarum in Figure 2a. There is no difference between axons and dendrites in the Physarum analogue models of a neural network, so we use a general term “connection” or “pathway.” A connection is a protoplasmic tube linking two Physarum neurons. An example is shown in Figure 2a, and the key elements are enhanced in the drawing in Figure 2b. The protoplasmic tube is conductive , propagating patterns of calcium waves, electrical potential, and peristaltic waves from one neuron to another.
An undisturbed Physarum exhibits periodic changes, or oscillations, of its surface electrical potential see the example in Figure 2b and further below. A typical normal oscillation of a surface potential has amplitude of 0.1 to 5 mV (sometimes less, depending on the location of the electrodes) and period 1–4 min [39, 41, 43]. The exact pattern of electric potential oscillations depends on the physiological state and age of the Physarum culture and the details of the experimental setup . In 1939 Heilbrunn and Daugherty discovered that the peristaltic activity of protoplasmic tubes is governed by oscillations of electrical potential propagating along the tubes . The exact nature of the correlation between electrical and contractile oscillation of plasmodium is still unclear there is a view that these two oscillations are governed by the same mechanism but may occur independently of each other .
The oscillations can be tuned by external electrical stimulation. In the example shown in Figure 3 we stimulated Physarum with triangular waveforms, frequency 0.009 Hz. Physarum oscillations were irregular before stimulation with average amplitude 0.42 mV. After ≈18 min of stimulation with the waveforms, Physarum's oscillatory activity regularized and its average amplitude almost doubled, increasing to 0.74 mV (Figure 3).
Stimulation of Physarum neuron with triangular waveforms. The stimulating waveforms on the graph look distorted due to the low frequency of sampling during recording.
Stimulation of Physarum neuron with triangular waveforms. The stimulating waveforms on the graph look distorted due to the low frequency of sampling during recording.
The oscillatory pattern of a single Physarum neuron is stable, apart from some possible drifts in the baseline potential due to mass transfer of the propagating Physarum. Physarum neurons linked electrically may exhibit high-amplitude spikes. An example of such very low-frequency irregular high-amplitude spikes is shown in Figure 4. Three petri dishes (a single dish is shown in Figure 2a) were connected with electrodes in series, and the potential difference was measured between the two most distant electrodes. This Physarum neural network shows a low amplitude of electrical potential oscillations, about 1 mV. High-amplitude spikes were observed at ≈1800 s (13.2 mV), ≈5500 s (16.9 mV), ≈11,500 s (16.7 mV), ≈1200 s (17.6 mV), ≈13,300 s (36.3 mV), ≈14,100 s (44.6 mV), and ≈17,200 s (27.2 mV).
Large-amplitude spiking activity in three pairs of Physarum blobs (Figure 2a) connected in series. Zoomed are domains of normal oscillatory activity.
Large-amplitude spiking activity in three pairs of Physarum blobs (Figure 2a) connected in series. Zoomed are domains of normal oscillatory activity.
Physarum neural networks do not have synapses represented as discrete structural elements. Synaptic-like morphological contacts could not be formed: When two pieces of Physarum are inoculated at a distance from each other, they start exploring the space around them and form branching networks of protoplasmic tubes. When two networks grown from different sites of inoculation come into contact, they usually fuse, forming a single united network. However, there is a functional analogue of synapses that is an intrinsic feature of Physarum protoplasmic tubes and makes any locus of a Physarum network a synapse. This is the memristive property.
A memristor is a resistor with a memory, whose resistance depends on how much current has flowed through the device. Postulated theoretically by Chua in 1971  and implemented practically by Strukov et al. , memristors have influenced the recent development of computing circuits [64, 70, 30] and neuromorphic architectures [60, 40, 52, 37, 27, 28].
In laboratory experiments  we demonstrated that protoplasmic tubes of acellular slime mold P. polycephalum show current-versus-voltage profiles consistent with memristive systems. Experimental laboratory studies show pronounced hysteresis and memristive effects exhibited by the slime mold. The memristor is an analogue of a synaptic connection [52, 23], and in fact is capable of direct emulation of the temporal dynamics of real-life synapses . As a living memristor, each protoplasmic tube of Physarum is a synaptic element with memory, whose state is modified depending on its presynaptic and postsynaptic activities. As with memristors, several protoplasmic tubes in a Physarum network can form an associative memory network . The synapses shown in Figure 1 correspond to protoplasmic tubes with memristive properties.
Attention-Deficit Hyperactivity Disorder
ADHD is also a developmental disorder characterized by two types of symptoms: (i) inattentiveness and (ii) hyperactivity/impulsiveness. Most cases are diagnosed at the ages of 6 years. Symptoms become particularly noticeable when circumstances change. Moreover, ADHD is commonly comorbid with other psychiatric disorders (e.g., depression and anxiety disorder), causing a substantial burden for patients and their families. Thus far, the medication-based intervention can achieve short-term effects, and the long-term effects of treatment for ADHD remain uncertain (Posner et al., 2020). It is essential to develop novel alternative strategies for treating ADHD.
There are four ADHD-related articles listed in Table 4, and all the studies used EEG for neuroimaging. Event-related potentials (P200 and P300) were employed in two of the papers and were used to evaluate the effects of tDCS (Breitling et al., 2020) and tACS (Dallmer-Zerbe et al., 2020), respectively. The remaining studies extracted functional brain connectivity (Cosmo et al., 2015b), power spectra (Dallmer-Zerbe et al., 2020), and statistical analysis (Cosmo et al., 2015a) as features for examining the neurological changes following tDCS. In all investigations, 20 min of tES therapy at low current intensity (1 mA) was applied. In the tACS study (Dallmer-Zerbe et al., 2020), the authors applied a stimulation with a mean frequency of 3 Hz, delivered from multiple electrodes (anodes: C3, C4, CP3, CP4, P3, and P4) and returned by cathodes at T7, T8, TP7, TP8, P7, and P8 (the distribution of electrodes following the international 10 EEG system). The neurological results following tACS demonstrated that the P300 amplitude significantly increased, accompanied by a decrease in omission errors compared to pre-tACS. Both tDCS studies share the same experimental paradigm (current intensity, duration, and stimulation location), but the results were reversed. The first study (Cosmo et al., 2015b) indicated that resting-state brain connectivity increased in individuals after DLPFC stimulation. The authors of the second study (Cosmo et al., 2015a) found no evidence supporting the capability of tDCS to improve inhibitory control by stimulating the left DLPFC in patients with ADHD performing the go/no–go task. A recent investigation (Breitling et al., 2020) compared the effectiveness of conventional (with one anodal electrode) and high-definition tDCS (with four anodal electrodes) for improving working memory performance, with the anode located near the right inferior frontal gyrus and the cathode placed over the contralateral supraorbital region. The results for working memory behavior were not generally influenced by conventional and high-definition tDCS (HD-tDCS). However, elevated P300 and N200 were observed after conventional and HD-tDCS since the current intensity differed between conventional tDCS (1 mA) and HD-tDCS (0.5 mA). The conclusion, which may be difficult to accept, is that HD-tDCS is equally suitable as conventional tDCS for improving the working memory performance of patients with ADHD. Therefore, comprehensive investigations are required to assess the effectiveness of tES for treating ADHD in the future.
Table 4. Studies and experimental characteristics of tES literature for ADHD.
Action Potential Generation
All living cells generate an electrical potential across their membranes, with the intracellular region relatively negative compared with the extracellular region. 28 This potential difference across the cell membrane is referred to as the resting membrane potential. The development and maintenance of the resting membrane potential can be explained by a simple model.
We can partition a beaker into right and left halves with an impermeable membrane containing multiple closed potassium channels. Assume this membrane separates two different concentrations of a potassium chloride (KCl) solution with a 10-mM concentration on one side and 100-mmol/L concentration on the other (Figure 9-1). 17 Potassium chloride in solution exists as positive potassium (K + ) ions (cations) and negative chloride (Cl – ) ions (anions). A voltmeter (a device that detects potential differences) measuring the two solutions fails to detect a potential difference because there is a lack of physical continuity between the left and right halves of the beaker when the partition’s potassium channels are closed. In other words, the two solutions are electrically independent.
(Redrawn from Dumitru D, Amato AA, Zwarts MJ: Electrodiagnostic medicine, ed 2, Philadelphia, 2002, Hanley & Belfus, with permission.)
If we now open the membrane’s potassium channels, K + cations will flow “down” their concentration gradient from the high (100 mm/L) to low (10 mm/L) ion concentration side of the beaker (see Figure 9-1). The potassium ions will continue this directional flow until there is a balance between (1) the forces of the physical concentration gradient difference driving potassium to the lower-concentration region, and (2) the electrical gradient opposing this directional ion flow. Because there are only potassium channels, the negative chloride ions cannot pass through the membrane and remain on their respective sides of the beaker. As more and more positive potassium ions leave one side of the beaker, there begins to develop an unbalanced or “excess” amount of negative charges (Cl – ) on the high-concentration side of the beaker, with an equal buildup of “excess” positive charges (K + ) on the other side of the beaker. The increasing net negative charge of the beaker half with the increasing number of unbalanced chloride ions begins to make it increasingly difficult for the positive potassium charges to leave the high-concentration side of the beaker. This is because the progressively building net negative charge increasingly attracts those remaining positive potassium ions. Similarly, a growing amount of positive potassium ions on the formerly low potassium concentration side of the beaker begin to increasingly repel additional potassium ions attempting to enter this side of the beaker.