Changes in the potential and polarity of organs, tissues, and cells of living organisms during life activities. It is a kind of physical, physico-chemical changes in the process of life activities, which is the manifestation of normal physiological activities and a basic feature of biological living tissues.

History

More than 2,000 years ago, humans discovered the fact that animals are electrically charged, and used the bioelectricity of electric rays to treat mental illness. At the end of the 18th century, L. Galvani discovered the phenomenon of contraction of frog muscles when they come into contact with loops made of different metals, and proposed the idea of "animal electricity". However, the overturning of the voltaic proved that the contraction of the frog muscle was only due to the conductive liquid contained in the frog muscle, which connected the different metals tied to the ends of the frog muscle into a closed loop, which was the key to generating electricity. Later works by C. Matiuschi, E. H. Dubois-Raymond and L. Hermann all proved the existence of bioelectricity. At the beginning of the 20th century, W. Einthofen used a sensitive string galvanometer to directly measure weak bioelectric currents. In 1922, H.S. Garser and J. Evlanger were the first to study neural action potentials with a cathode ray oscilloscope, laying the technical foundation of modern electrophysiology. In 1939, A.L. Hodgkin and A.F. Huxley inserted microelectrodes into the large nerve of the squid and directly measured the potential difference between the inside and outside of the nerve fiber membrane. This technological innovation has promoted the development of electrophysiology theory. In 1960, electronic computers began to be used in electrophysiological research, so that evoked potentials can be clearly distinguished from spontaneous brain waves, and the parameters emitted by cells can be accurately analyzed and calculated.

Resting potential

The potential difference between different parts of a biological tissue or cell in the absence of stress arousal. For example, there is a positive potential difference of 5~6 millivolts between the cornea of the eyeball and the back of the eyeball, and there is a potential difference of tens of millivolts between the inside and outside of the nerve cell membrane. The potential difference between the inside and outside of the cell membrane in the resting state is called the resting membrane potential, or membrane potential for short. Its size and polarity are mainly determined by the type of ions inside and outside the cell, the difference in ion concentration, and the permeability of the cell membrane to these ions. For example, nerve or muscle cells are more than a few tens of millivolts outside the membrane than inside the membrane. There is a potential difference of more than 100 millivolts between the inside and outside of the cell membrane of plant cells (e.g., Algae axlea). Changing the concentration of potassium ions in the extracellular fluid (or intracellular fluid) can alter the polarization state of the cell membrane. This indicates that the polarization state of the cell membrane is mainly determined by the difference in potassium concentration between inside and outside the cell. In the case of damage to the cell membrane (rupture of the cell membrane), the cell fluid at the site of the injury flows inside and out, and the membrane potential at the site of the injury disappears. As a result, there is a potential difference between the normal site and the site of injury, which is called the damage potential (or demarcation potential). In some biological cells, there is a potential difference not only between the inside and outside of the cell membrane, but also between different parts of the cell. These cells are called polar cells. In a tissue composed of polar cells, if the polar cells are arranged in different directions, the electric fields generated by them cancel each other out, and the tissue does not exhibit potential differences. If the polar cells are arranged in the same direction, there will be a certain polarity and potential difference between different parts of the tissue. Its polarity and potential magnitude depend on the sum of the vectors formed by the cell dipole vectors in parallel, series, or both. For example, the skin of a frog, where the epidermis is close to the dermis, has polar cells. These cells have the property of parallel dipoles, with the inner surface being tens of millivolts more positive than the outer surface. In other biological tissues, polar cells are arranged in series, such as the electrical organs of electric fish, which are strung together by "electromyography" formed by specialized muscles. The electrical organ of the electric eel is formed by 5000~6000 EMG plate units in series, and the discharge voltage of this electric organ can be as high as 600~866 volts because each EMG plate can generate a voltage of about 0.15 volts. The roots of some plants are also made up of polar cells in series. As a result, there may be a potential difference between the points from the tip of the root to the base of the root. A stressful living organism is stressful, that is, when it is stimulated by a certain intensity (threshold), it causes changes in the metabolism or function of the cell. The stimulus that causes change (abruptness) must have a certain rate of change, and slowly increasing the intensity of the stimulus does not cause a stress response. If direct current is used as stimulation, the stress response when the power is turned on occurs at the cathode, and the stress response when the power is off occurs at the anode. The stress response is followed by a period of recovery (refractory period) before it can respond to the stimulus again. During the stress response, it is often accompanied by changes in cell membrane potential or tissue polarity. Local electrical response of plants: Plants are slow to respond to irritability and tend to be confined to the stimulated area. The intensity of its response, which is determined by the intensity of the stimulus, produces a negative potential change at the point of action of the stimulus. For example, plant tissues are subjected to bending and bending (mechanical stimulation), which can cause a negative potential response of tens of millivolts. The electrical changes that occur in plant photosynthesis are an electrical reaction caused by metabolic changes. The intensity with which plants perform photosynthesis depends on the amount of chlorophyll. Therefore, if the light intensity or chlorophyll content of different parts is different, the metabolic intensity of different parts will be different. In this case, there is not only a difference in oxygen production and carbon dioxide consumption, but also a potential difference between different parts. For example, on the leaves of the sun grass, one part is given light, and the other part is not given light, then within a few minutes, a potential difference of 50~100 millivolts can be generated between the two parts. Within a certain range, the magnitude of the potential difference is proportional to the intensity of the light. Plant MovementSome plants produce motor responses when stimulated. At this time, there is often a change in conductive potential. For example, when a mimosa is stimulated, the closed-motion response of the leaf can be transmitted over a considerable distance. In this process, the negative potential change caused by the stimulus point can be spread outward at a speed of 2~10 mm per second. The potential change reaches the maximum value in 1~2 seconds, and its amplitude can reach 50~100 millivolts. However, the recovery time is long, and it takes dozens of minutes to return to the original polar state, and this period of negative potential change is its refractory period. The electrical changes that occur in local animal cells or tissues, especially nerves and muscles, are more pronounced when stimulated than in plants. If the nerve fiber is locally subjected to weak electrical stimulation, excitability at the cathode increases and the membrane potential decreases (depolarization), and excitability decreases at the anode and the membrane potential increases (hyperpolarization). In the case of a strong stimulus close to the threshold of arousing excitatory impulses, the potential change of the cathode is greater than that of the anode, which is a stress response. However, this potential change is only confined to the stimulation area and its adjacent parts, and does not spread outward, so it is called local reaction, and the potential that occurs is called local potential. Excitatory postsynaptic potentials, or inhibitory postsynaptic potentials, are generated on the postsynaptic membrane by one neuron in response to the excitatory impulses of another neuron. The former is the process of depolarization of the postsynaptic membrane, and the latter is the hyperpolarization process of the postsynaptic membrane. These potential changes, which are confined to the postsynaptic membrane and do not conduct outward, are also local potentials. If sensory cells or special nerve endings in the receptor are appropriately stimulated, such as photoreceptor cells in the eyeball stimulated by light, and nerve endings in the mechanoreceptor Burgton's body are stimulated by pressure, a local potential response will also be generated, which is called the receptor potential or initiation potential. Similarly, when muscle cells receive nerve impulses, there is a local, nonconductive negative potential change at the junction of the nerve and muscle (endplate), called the endplate potential. All of these local potentials are spread to a certain area in the vicinity, but they are not conducted. The closer you are to where the local potential occurs, the greater the potential and decays as a function of distance. The magnitude of the local potential increases with the increase of stimulation intensity, and the maximum can reach tens of millivolts. Disseminated electrical responses are more prevalent in disseminated animals. For example, when nerve cells are strongly stimulated, the local electrical response generated at the cathode increases with the enhancement of the stimulation, and when the threshold is exceeded, a nerve impulse that can be conducted along the nerve fiber will be triggered. The area where the nerve impulse arrives is accompanied by a change in the membrane potential, called the action membrane potential (action potential for short). This is a reverse polarization process of the membrane potential, that is, from the original positive outside the membrane to the negative outside the membrane. As a result, there is a potential difference between the excitation site and the resting site, and the excitation site is negative compared with the normal part, and the potential can reach more than 100 millivolts. This negative potential region can be conducted forward at a very fast speed, such as the conduction velocity of large nerve fibers in shrimp can reach 80~200 m/s. Excitatory postsynaptic potentials or receptor potentials, although not conductive excitatory waves, can affect the excitability of adjacent nervous tissues when they increase to a certain extent, and even nerve impulses with negative potential changes occur. The tissues or organs of the animal, in the event of a stress response, can also undergo electrical changes. Its size and polarity are determined by the vector sum of the electric fields generated when the cells that make up the tissue are excited. If the eye is stimulated by light, the change in the potential difference between the front and back of the eye can be recorded, which is called electroretinagram. Its waveform is very complex, and it is stimulated by light to produce receptor potentials in receptor cells, which in turn cause other cells in the retina to produce excitation and potential changes. Since the direction of the electric field of these electrical changes is inconsistent, the electroretinal mark is the vector sum of the electric fields produced by these cells. Different animals produce different electroretinal images due to different retinal structures, and factors such as light intensity and time will also affect the waveform of electroretina. A biological organism is a conductive volumetric conductor. When electrical changes occur in some cells or tissues, an electric field is generated within this volumetric conductor. Therefore, potential changes in the electric field can be guided in different parts of the electric field, and their magnitude and waveform are different. An electrocardiogram, for example, is the vector sum of complex potential changes that occur when heart cells are active. Depending on the location of the guide electrode, the recorded waveform is different, and the physiological meaning reflected is also different. In addition, the electric field generated in the central nervous system of higher animals has "spontaneous" rhythmic potential fluctuations on the scalp of humans and animals, whether at rest or active, called brain waves. It is the sum of the electric fields generated by the activity of a large number of nerve cells in the brain. In the resting state, the amplitude of potential change is higher, while the frequency of fluctuations is lower. When excited, the total potential is lower and the frequency of fluctuations is higher because the pace of activity of neurons in the brain is inconsistent (tends to be asynchronous). When receiving a specific stimulus from the outside world, the sum electric field is relatively strong, so a significant potential change can be recorded. Because this potential change is induced by an external stimulus, it is called an evoked potential. It is impossible for a theory to attempt a single theory to explain the different electrical phenomena that occur in various organisms. However, the various electrical phenomena that occur in animals, especially in the nervous system or muscular system, can basically be explained at the cellular level by the ion theory proposed by A. L. Hodgkin and A. F. Huxley. The ion theory was developed on the basis of the membrane theory proposed by J. Bernstein (1902). The ion theory holds that the cell membranes of nerves or muscles have varying degrees of permeability to different ions. And because the concentration of various ions in the cell, especially potassium, sodium and chloride, is different from the concentration in the extracellular fluid, the potential difference between the inner and outer sides of the cell membrane (according to the principle of F.G. Donnan's equilibrium) is the membrane potential. This is the basis of resting potential. Under different physiological conditions, the permeability of the cell membrane to various ions will change, so the membrane potential will also change, that is, various forms of action potentials will be formed. For example, in the resting state, the cell membrane of nerve or muscle cells has greater permeability to potassium ions, and the concentration of potassium ions in the cell is dozens of times higher than that outside the cell, thus forming a resting membrane potential of tens of millivolts outside the membrane compared with the positive in the membrane. When the extracellular (or intracellular) potassium concentration is changed, the resting membrane potential will change accordingly according to the Nernst formula. This proves the idea that the resting membrane potential is determined by the concentration of potassium ions inside and outside the cell. The resting membrane potential of some plant cells is also determined by the concentration of potassium ions inside and outside the cell. When nerve or muscle cells are excited, the permeability of the cell membrane to various ions changes, that is, the permeability to sodium ions suddenly increases, and it dominates the permeability of various ions. Therefore, the magnitude and polarity of the membrane potential are mainly determined by the concentration of sodium ions inside and outside the cell membrane. Because the concentration of sodium ions outside the cell is higher than that in the cell, the membrane potential suddenly changes from positive outside the membrane to positive in the membrane and outside the membrane in a short period of time, that is, the reverse polarization phenomenon occurs. At this time, the amplitude of the potential change (depolarization followed by reverse polarization) can reach more than 100 millivolts, which is the action potential. However, there is still a membrane potential that is different from that in the resting state, which is called the action membrane potential. The area where the action potential is located, i.e., the area where the excitatory impulse is located, is rapidly transmitted forward. The time for excitatory impulses to appear in a certain area is extremely short, only a few milliseconds. When the excitatory impulse passes, the membrane potential in this area gradually returns to its original resting state, that is, the resting membrane potential is restored. The changes in permeability that occur on different cells, or even on the cell membranes in different areas of the same cell, are not exactly the same. For example, optic cells in the vertebrate retina respond to an increase in membrane potential (hyperpolarization) when stimulated by light. However, optic cells in the invertebrate retina respond to a decrease in membrane potential (depolarization) when stimulated by light. For another example, on the membrane of the axon of the same spinal motor neuron, the depolarization or even depolarization response is manifested when excited. However, when the excitatory postsynaptic membrane of the same motor neuron receives an excitatory transmitter released from the nerve endings of another neuron, there is also a depolarization response, but the changes in ion permeability that occur at this time are different from those that occur on axons. When the excitatory subsynaptic membrane is excited, the permeability to sodium ions does not increase suddenly alone, but the permeability to various ions increases generally, so it does not appear to be reverse polarized (positive in the membrane than outside the membrane). The same is true on the inhibitory postsynaptic membrane of the same motor neuron when acting on inhibitory transmitters released from the nerve endings of another neuron. When the inhibitory subsynaptic membrane is excited, the permeability to potassium and chloride ions is increased, which makes the membrane potential hyperpolarized, and the extramembrane is corrected in the membrane. It can be seen that the membrane potential changes of different cells, and even different areas of the same cell, are different when excited. In general, changes in both resting membrane potential and various action membrane potentials can be explained by the difference in the permeability of cell membranes to various ions. Due to the different changes in permeability and the differences in the concentration of various ions inside and outside the membrane, various bioelectric phenomena with different polarity, amplitude, frequency and phase are exhibited. Most of the bioelectric phenomena that occur in tissues or organs are the vector sum of the bioelectricity produced by individual cells, so the mechanism of its occurrence can also be explained by the ion theory. However, the time course of some bioelectrical changes is very slow, such as the electrical changes produced during photosynthesis are closely related to the metabolic activities of cells, that is, a kind of bioelectrochemical potential. Some very slow potential fluctuations can also be detected in the cerebral cortex, some fluctuate several times in 1 minute, and some have obvious changes in minutes or even tens of minutes. This potential is distinct from rapid nerve cell excitatory activity and may also be a bioelectrochemical phenomenon caused by metabolic activity or related to glial cell activity.