Electric Signals In Cells

Brief table of contents 

• Neurons transmission of electrical signals
• Types of electrical signals in nerve cells. Propagation ofnerve impulses along with the fiber.
• Classes of electrical signals
• Signal transmission from the retina can be represented by the following simplified diagram:
• Technique for recording signals from neurons using electrodes
• Non-invasive methods for recording neural activity
• Distribution of local gradient potentials and passive electrical properties of neurons.
• Propagation of potential changes in bipolar cells and photoreceptors
• Properties of action potentials
• PD as a neural code
• Synapses: areas of intercellular communication
• Chemically mediated synaptic transmission
• Excitation and inhibition
• Electrical transmission
• Integrative Mechanisms
• The complexity of the information conveyed by action potentials
• Information processing can be presented in the following form:
• Electrical properties of neurons 

• Scientists reveal how electrical signals control muscles 

Neurons transmission of electrical signals

The amino alcohol choline and its ester acetylcholine 6.3 are widely distributed in nature. The latter substance is the most important animal metabolite that ensures the activity of the nervous system. It is produced at the endings of nerve cells called cholinergic neurons and functions as a neurotransmitter. So-called substances, the release of which in the nerve endings causes the transmission of an electrical signal to a neighboring neuron or to a muscle and other innervated organ. Basically, acetylcholine is involved in signal transmission from the spinal cord or brain to skeletal muscles, thereby providing voluntary body movements. In addition, it is involved in some involuntary muscle reactions, lowers blood pressure, slows down the heart rate, and intensifies the work of the gastrointestinal tract. 

    These cells have another interesting feature - they are electrically connected to each other. Using intracellular microelectrodes, it was shown that the conjugation coefficient between neighboring cells can reach 0.8 (this means that when an electrical signal is transmitted from one cell to another, its amplitude decreases only by 20%). Due to this feature, action potentials arising in any part of the salivary gland quickly spread to other areas, which leads to the rapid involvement of all glandular cells in the activity and to the synchronization of their work. Simultaneous activation of the right and left halves of the gland is achieved due to electrical coupling between paired neurons 4. 

    In a sense, almost every neuron behaves like a chemoreceptor in response to chemical stimulation in synapses, it generates an electrical signal. As we have already seen, chemical factors act on membrane ion channels, changing their permeability and thereby causing a shift in the membrane potential ( postsynaptic potential ). This electrical effect can smoothly change according to the intensity of the stimulus. For signal transmission over long distances the magnitude of the electrical signal at the input of the neuron is encoded by the frequency of the pulsed discharge at the output. Normally , a cell is able to adapt by attenuating its response to a constant stimulus , while at the same time generating a strong output when the strength of the stimulus changes. 

    To describe the phenomena associated with induced magnetic fields , the use of such a concept as a current dipole has turned out to be very fruitful. This is due to the very nature of the process of signal transmission to the brain.. When impulses are received from external receptors in different parts of the brain, concentrated groups of neurons fire, generating electrical and magnetic signals. Usually , several neural stations are sequentially excited, causing, along with spike impulses, relatively slowly changing (gradual) potentials and currents in the brain volume. As a result , the evoked signal can be registered on the surface, which is an oscillation of the potential (in the case of HEP) or magnetic field (MFF) in the form of a damped wave with a number of maxima and minima (Fig. 43). Each of the EEP and EMP peaks corresponds to the firing of a separate group of neurons and, generally speaking, hasdifferent spatial distribution, since the sources of different peaks are located differently in the brain. Some of them can be described with high accuracy by the current dipole model.

    Thus , the action potential can be described as a stream of positively charged sodium ions penetrating the membrane into the neuron and moving along the axon. The main advantage of the electrical impulse path is that the signal propagates over long distances quickly and without attenuation. Reaching the end of the axon , the depolarization wave causes the release of mediator molecules from the synaptic vesicles, and the mechanism for transmitting the nerve impulse again acquires a subtle chemical nature. Neuron quickly restores electrochemical balanceand returns to a state with a negative potential inside the cell until the next signal. Thus , the extremely short duration of synaptic conjugation between neighboring neurons makes it possible to transmit via  si . It is solved primarily with the help of ES. Neurons of different levels that control the electrical organ are connected between

     ES are themselves and therefore are discharged almost simultaneously. In electric fish, reflex circuits have been found that lead to the excitation of an electric organ in these circuits , signals are sequentially transmitted between three types of neurons , and signals are transmitted only through ES. However, the simultaneous excitation of the neurons acting on the cells of the electrical organ is not yet sufficient for the simultaneous excitation of these cells themselves, since hundreds and thousands of column cells are located at different distances from the neurons. The axons of these neurons have different conduction rates to moredistant cells receive a signal at a faster rate.

    Usually, action potentials originate at the base of the axon and are then transmitted along its entire length. In order to understand the mechanism of this transmission, it is useful to first consider how electrical excitation propagates through the nerve cell in the absence of action potentials. As already mentioned, such passive propagation is a very common phenomenon, especially in neurons in which axons are very short or none at all. Such cells often have no or almost no voltage-dependent Ka-channels and for signal transmissionuse only passive propagation associated with smoothly changing local potentials. 

    The simplest way to transmit a signal from neuron to neuron is direct electrical coupling through gap junctions . The main advantage of such electrical synapses is that that signals are transmitted without delay. On the other hand, these synapses are far less capable of regulation and adaptation than the chemical synapses , which make up most of the connections between neurons. Electrical connection through gap junctions wasconsidered in ch. 14 (sections 14.1.5-14.1.8), but here we will deal only with chemical synapses. 

    As shown by three simple observations, synaptic transmission requires an influx of calcium ions into the end of the axon . First, if these ions are absent in the extracellular environment around the end of the axon at the time of arrival of the nerve impulse , then the mediator is not released and signal transmission does not occur. Secondly, if Ca is artificially introduced through a micropipette into the cytoplasm of the nerve ending, the release of the neurotransmitter occurs immediately even without electrical stimulation of the axon (this is difficult to implement at the neuromuscular junction due to the small size of the axon ending , therefore, such an experiment was carried out at the synapse between giant squid neurons ) Thirdly, artificial depolarization of the axon ending (also in synapse between giant neurons ) without a nerve impulse and under conditions of blockade of sodium and potassium channels by specific toxins. 

    Endocrine and nerve cells are specially designed for chemical signaling, they jointly coordinate the various activities of the billions of cells that make up the body of the higher animal. Nerve cells transmit information much faster than endocrine cells, since in order to transmit a signal over long distances, they do not need diffusion and blood flow, the signal is quickly transmitted along the nerve fiber by electrical impulses . It is only at the nerve endings where the neurotransmitter is released that these impulses are converted into chemical signals. The neurotransmitter reaches the target cell by diffusion over a microscopically small distance , which takes less than a millisecond (Fig. 13-2). While hormones in the bloodstreamare highly diluted and must be able to act at extremely low concentrations (usually <10 M), the dilution of neurotransmitters is negligible, and their concentration in certain areas of target cells can be high. For example, the concentration of the neurotransmitter acetylcholine in the synaptic cleft of the neuromuscular junction can be as high as 5-M. In other respects , however, the mechanisms of chemical signaling by hormones and neurotransmitters are broadly similar, and many of the signaling molecules used by endocrine cells, are also used by nerve cells (neurons). 

    Let me illustrate this thesis. The true function of a neuron is signaling. However, we will see (Chapter 5) that there are only two types of signals in the nervous system , electrical and chemical. It is important to note that the signal itself contains very little information. Its specificity depends on the places of origin and reception, i.e., on the cells of the organs between which it is transmitted. So, for example, the reason that we hear and do not see sound lies not in the electrical or chemical code of the nerve impulse , but in the fact that the visual cortex of the occipital lobe of the brain is connected to retinal neuronsand not an ear. With electrical or mechanical, and not optical, action on the retina, we will also see. Anyone who has had sparks in their eyes after a hard blow can attest to this. Consequently, the qualitative information transmitted by a neuron depends solely on the specificity of its connection, and only a quantitative characteristic is contained, apparently, in the signal itself, a strong stimulator sends more nerve impulses from the receptor to the perceiving organ than a weak one. Again, nerve impulses , say, in the optical or acoustic region of our nervous system are practically indistinguishable from nerve impulses in a completely different way.other systems , such as more primitive life forms . By themselves, these impulses are very little informative even for a narrow specialist . Thus , a neurochemist who studies the biochemistry of neurons can only find out the mechanism of the origin and transmission of signals, the specific content (meaning) of signals is inaccessible to his methods. He can study the general molecular reactions that underlie signal processing, but not the results of this processing, i.e., information. 

    THESE signals, their nature is the same in all cases and consists in a change in the electricalpotential on the plasma membrane of the neuron. Signal transmission is based on the fact that an electrical disturbance that has arisen in one part of the cell spreads to other parts. If there is no additional amplification, these perturbations decay with distance from their source. At short distances, the attenuation is negligible, and many neurons conduct signals passively, without amplification. However, for long-distance communication, such passive signal propagation is not enough. Therefore, in the course of evolution , neurons with long processes have developed an active signaling mechanism, which is one of the most amazing and characteristic properties.neuron. An electrical stimulus, the strength of which exceeds a certain threshold value , causes a burst of electrical activity , propagating at high speed along the plasma membrane of the neuron. This rushing wave of excitation is called the action potential or nerve impulse . The action potential transmits information from one end of the neuron to the other without attenuation at a speed of up to 10 m / s, and in some neurons even faster.

Rice. 18-3. Diagram of a typical synapse. An eleggric signal arriving at the end of the axon of cell A leads to the release of a chemical messenger into the synaptic cleft (ieromednator X, which causes an electrical change in the dehydrate membrane of cell B. A wide arrow indicates the direction of signal transmission, the axon of one neuron, such as shown in Fig. 18- 2, sometimes forms thousands of output synaptic connections with other cells ... Conversely, a neuron can receive signals through thousands of input synaptic connections located on its dendrites and body.

    The simplest way to transmit a signal from neuron to neuron is direct electrical interaction through gap junctions. Such electrical crosslinks between neurons are found in some parts of the nervous system in many animals, including vertebrates. The main advantage of electrical synapses is that the signal is transmitted without delay. On the other hand, these synapses are not adapted to perform some functions and cannot be regulated as finely as chemical synapses, through which most connections between neurons are carried out.

    Two simple observations show that synaptic transmission requires an influx of Canons into the axon terminal. First, if there is no Ca in the extracellular environment, the mediator is not released and signal transmission does not occur. Secondly, if Ca is artificially introduced into the cytoplasm of the nerve ending using a micropipette, the release of the neurotransmitter occurs even without electrical stimulation of the axon, which is difficult to achieve at the neuromuscular junction due to the small size of the axon ends. Therefore, such an experiment was carried out on a synapse between giant squid neurons .) These observations made it possible to reconstruct the sequence of events occurring at the end of an axon, which is described below. 

    Any signal received by the nervous system must first of all turn into an electrical one. The transformation of a signal of one type into another is called a transformation, so all sensory cells are transducers. In a more general sense, almost every neuron is a transducer, receiving chemical signals at synapses. signals, it converts them into electrical ones. Although some sensory cells respond to light, others to temperature, still others to certain chemicals, still others to mechanical force or movement, etc., in all these cells, the transformation is based on a number of basic principles that have already been discussed in the discussion of synaptic transmission through neurotransmitters. In some sense organs, the transducer is part of the sensory neuron that conducts impulses, while in others it is part of the sensory cell, specially adapted for signal conversion, but not participating in the implementation of long-distance communication, such a cell transmits its signals to the neuron associated with it) through the synapse (Fig.). But in both cases, exposure to an external stimulus causes an electrical shift in the transducer cell called a receptor potential, which is analogous to the postsynaptic potential and also ultimately serves to regulate the release of a neurotransmitter from another part of the cell.

    As already noted, approximately 50% of the motor neurons of the embryo die soon after the formation of synaptic contacts with muscle cells. Such death of extra neurons can be prevented by blocking neuromuscular transmission (for example, with α-bungarotoxin), or, conversely, enhanced by subjecting the muscle to direct electrical stimulation. This suggests that the electrical activity of the muscle regulates the formation of neurotrophic factors in the muscle, which is necessary for the survival of embryonic motor neurons. This factor is probably identical to the one believed to cause the growth of axon endings towards the denervated muscle. When the muscle is inactive as a result of blocking synaptic transmission or due to the lack of innervating axons, this factor is formed in large quantities as a signal that the cell needs innervation. Electrical activation of the muscle under the action of artificial stimuli or as a result of spontaneous excitation of the motor neurons innervating it suppresses the formation of the factor, and some of the immature motor neurons of the embryo die in competition for its remaining small amounts.

    The division of synapses into chemical, electrical and mixed ones. The greater the degree of evolutionary organization of the nervous system, the greater the diversity of the nature of chemical synapses. This is especially true of the brain of higher mammals, including humans. Obviously, chemical synapses turned out to be evolutionarily more advantageous for the transmission of discrete signals compared to other types of intercellular contacts, since on their basis not only signal transmission is possible but also its various modulation, including by humoral factors. Receptors are the basis for the perception of a chemical signal in a synapse by a neuron, as well as a number of modulating influences. 

Types of electrical signals in nerve cells. Propagation of nerve impulses along with the fiber.

To analyze events in the outside world or inside our body, to transmit information from cell to cell, neurons use electrical and chemical signals. The signal transmission distance can be large: from the tips of the toes to the spinal cord. Different signals are perfectly represented all in the same retina. At the time when Ramon iCajal drew arrows, there was almost no information about these signals, which makes his achievements even more remarkable.

The steps of information processing can be traced sequentially: light falls on photoreceptors and generates electrical signals that act on bipolar cells. Signals are transmitted from bipolar cells to ganglion cells and from them to the higher centers of the brain, which carry out the perception of the outside world. The following sections discuss the properties of signals and ways of processing information.

Classes of electrical signals

The electrical signals of nerve cells can be divided into two main classes. First, these are local gradient potentials, which are caused by such external stimuli as light falling on the photoreceptors of the eye, a sound wave that deforms the hair cells of the ear, or a touch that mechanically displaces the process of a sensory cell in the skin. Similar in characteristics, but significantly different in origin, are the signals generated at synapses, the connections between cells that we will discuss later. All these signals are gradual and tied to the place of origin, and their distribution depends on the passive characteristics of the nerve cells.

Action potentials constitute the second main category. Action potentials are evoked by local gradient potentials. Unlike local potentials, they quickly spread over long distances - for example, from the eye to higher centers along the fibers of the ganglion cells that make up the optic nerve, or from motor cells in the spinal cord to the muscles of the leg. The second difference between action potentials is that they are fixed in amplitude and duration, like dots in Morse code. It is extremely important to understand that action potentials traveling along the fibers of the optic nerve are not epiphenomena present only in our ideas about the work of the brain. They are the only form of signaling that supplies the brain with information about the outside world.

Signal transmission from the retina can be represented by the following simplified diagram:

An important property of electrical signals is that they are virtually identical in all nerve cells in the body, whether they trigger movement, convey information about colors, shapes, or painful stimuli, or connect different areas of the brain. The second important property of the signals is that they are so similar in different animals that even an experienced researcher is not able to accurately distinguish the recording of an action potential from the nerve fiber of a whale, mouse, monkey or professor. In this sense, action potentials can be considered stereotypical units. They are a universal standard for the exchange of information in all studied nervous systems. In the brain, there are not types of signals, but a huge number of cells (from 10 10 to 10 12 neurons) and a variety of connections ensure the complexity of the tasks performed.

This idea was put forward in 1868 by the German physicist and biologist Hermann von Helmholtz. Taking hypothetical principles as a basis, long before the discovery of now known facts, he wrote: “Nerve fibers are often compared to telegraph wires traversing the area, and this comparison is well adapted to illustrate the amazing and important features of their mode of operation. In the telegraph network everywhere we find those but copper or steel wires, carrying only one kind of movement, the flow of electricity, but producing very different results at different stations according to the additional apparatus to which the wires are connected.In one station the effect is to ring a bell, to another the signal is simply passed on, at the third, the recording apparatus comes into play... In short, each of the... various actions, caused by electricity, can be called up and transmitted by wire to any desired point. With this β wire, the same process occurs, leading to a variety of consequences .... The difference that we see in the excitation of various nerves lies only in the difference in the organs themselves to which the nerve is attached and to which the state of excitation is transmitted.

In fact, as will be shown in Chapter 6, slight differences in amplitude and duration are evident in the action potentials of different neurons. Saying that all action potentials are the same is tantamount to saying that all oaks are the same.

Technique for recording signals from neurons using electrodes

To solve some problems, it is essential to record the activity of one neuron or even one ion channel, while other tasks require the total activity of many neurons. Below is a brief summary of the basic techniques for recording the activity of neurons, used for discussion in the following chapters.

For the first time, recording of action potentials from a nerve was made from peripheral nerves with extra cellular and electrodes. Passing a current between a pair of silver conductors evoked an action potential, while a second pair of similar electrodes recorded a response at some distance. In the central nervous system, registration from a neuron or a group of neurons is carried out by an extracellular electrode, which consists of a conductor in an insulating sheath or a glass capillary filled with a conductive saline solution.

With the help of an intracellular microelectrode, we can directly measure the potential difference between the external and internal environment of the cell, as well as excitation, inhibition and the occurrence of impulses. A glass microelectrode filled with saline and with a tip less than 0.1 mm in diameter is inserted into the cell using a micromanipulator. Microelectrodes are also used to pass current through the membrane or to intracellularly inject molecules into the cytoplasm.

A technique often used to measure membrane potential is known as the whole cell patch clamp. A glass pipette with a relatively large polished tip is advanced to the surface of the cell, where it adheres to the membrane and forms a strong bond. After the integrity of the membrane inside the pipette is broken, the liquid in the pipette comes into direct contact with the intracellular fluid.

Non-invasive methods for recording neural activity

Using the method of optical registration, it is possible to trace the transmission of information in some brain preparations without the use of electrodes. Specially designed dyes that bind to the cell membrane change the absorption of transmitted light or fluorescence with changes in the cell's membrane potential, which can be objectively recorded. There are also non-invasive methods such as positron emission tomography and magnetic resonance imaging (MRI) that can determine which areas of the brain of an awake person are activated when presented with stimuli or when moving. The image obtained using MRI shows the areas that are activated upon presentation of a visual stimulus.

The retinogram reflects the total activity of the retina, the electroencephalogram - the total activity of the brain. These methods are mainly used to diagnose brain dysfunctions.

Distribution of local gradient potentials and passive electrical properties of neurons.

Ramon y Cajal's diagrams of the cellular structure of the brain show through the idea that illumination of the retina changes the activity of photoreceptors, and these changes are reflected in the activity of nerve fibers exiting the eye. For such transmission of information, signals must propagate not only from cell to cell, but also along the cell, from one end to the other. How, for example, does an electrical signal generated at the end of a bipolar cell contacting the photoreceptor propagate along the neuron and reach the terminal, which is located near the ganglion cell?

In order to answer this question, it is useful to consider the relevant structures that transmit signals. A bipolar cell can be viewed as a long cylinder filled with an aqueous solution of salts (dissociated into positively and negatively charged ions) and proteins, separated from the extracellular solution by a membrane. Intracellular and extracellular solutions are osmotically the same, but have different ionic composition. Ions move along special ion channels, which are formed by protein molecules penetrating the membrane. Electrical and chemical stimuli cause channels for calcium, sodium, potassium, and chloride ions to open or close.

As a result of differences in the concentration of ions on both sides of the membrane and due to the selectivity of channels for certain ions, the resting potential of the cell is formed. At rest, the inner contents of the cell are negatively charged with respect to the outer environment. The structure and properties of a neuron determine the ability to conduct electrical signals. First, the intracellular fluid, the cytoplasm (axoplasm in the outgrowth of the cell, axon) is approximately 10 7 conducts electricity worse than a metal conductor. One of the reasons is that the density of charge carriers, ions, is several times less than that of electrons in a metal; in addition, the ion mobility is low. Secondly, the flow of current along the axon over a long distance is complicated by the fact that the membrane is not an ideal insulator. Accordingly, the amount of current flowing along the fiber rapidly decreases due to leakage through the ion channels of the membrane. The fact that nerve fibers are very small (typically no more than 20 microns (µm) in diameter in vertebrates) further reduces the amount of current being conducted. Alan Hodgkin gave an interesting illustration of these properties of electrical signal propagation.

If an electrical engineer looks at the nervous system, he will immediately see that signal transmission under the nerve fiber is a huge problem. The diameter of the axon in the nerve varies from 0.1 to 20 microns. The internal content contains ions and is a good conductor of electricity. However, the fiber is small and its longitudinal resistance is very high. A simple calculation shows that in a fiber with a diameter of 1 micron and a resistance of 100 ohm/cm, the resistivity will be about 10 10 ohm/cm. This means that the electrical resistance of a small nerve fiber 1 meter long is equal to the resistance of 10 10 miles of 0.2 mm copper wire, that is, a wire ten times longer than from Earth to the planet Saturn.

Thus, passive conduction of electrical signals is difficult and limited to a distance of 1–2 mm. Also, when such a signal is short, its shape can be severely distorted and its amplitude further reduced by the capacitance of the cell membrane. However, local potentials are very important for evoking and conducting a propagating signal.

Propagation of potential changes in bipolar cells and photoreceptors

Photoreceptors and bipolar cells are small in length, so the local gradient signal can efficiently propagate from one end of the cell to the other. The electrical signal that reflects light hitting the photoreceptor is generated in the outer segment of the rods or cones. From there, the signal passively propagates along with the cell to the terminal on the bipolar cell. If the receptor or bipolar cell were longer (several millimeters in length), then the local potential would not reach the terminal due to strong attenuation and could not affect the next cell in the chain. Bipolar cells and photoreceptors are an exception to the general rule that action potentials are needed to carry information along a neuron. Ganglion cells have a long (several centimeters) axon and therefore must generate action potentials for efficient signal propagation to the optic nerve. Activity records are made from cell bodies. Local potentials arise on the dendrites as a result of synaptic influences and passively propagate to the site of abstraction.

Properties of action potentials

One of the main properties of the action potential is that it is an explosive, threshold event that occurs according to the all-or-nothing law. An action potential occurs in a ganglion cell when the signals coming from bipolar and amacrine cells reach a certain critical level (threshold) of the membrane potential. The action potential (AP) has a well-defined threshold, after reaching which the amplitude and duration of the AP do not depend on the stimulation parameters. Large amplitude stimuli do not cause large-amplitude APs, just as long stimuli do not lead to the appearance of longer APs. All PD phases must be fully completed before the next PD occurs. After each AP, there is a period of forced silence (refractory period), during which the initiation of AP is impossible.

Each impulse causes electrical currents to propagate passively ahead of it along the axon. Although the resulting local potential decays rapidly with distance, it still exceeds the threshold. Thus, AP produces an electrical stimulus to the axonal region to which it will propagate. The fastest AP propagates along large-diameter fibers at a speed of about 120 meters per second (430 km/h), which determines the possibility of fast information transmission over large distances compared to the size of the cell body.

PD as a neural code

Considering that each AP has a fixed amplitude, it is not clear how the magnitude of the stimulus is reflected. The intensity is encoded by the PD frequency. A more effective visual stimulus causes greater depolarization and, as a result, a higher frequency of AP generation in the ganglion cell. This generalization was first made by E. Adrian, who showed that the frequency of AP in the sensory ending of the cutaneous nerve depends on the intensity of the stimulus. In addition, he found that a stronger stimulus activated more sensory fibers.

Synapses: areas of intercellular communication

Photoreceptors affect bipolar cells, which affect ganglion cells and so on, resulting in visual image perception. The structure through which one cell communicates information to another is known as the synapse. The mechanism of synaptic transmission is a major research topic in modern neuroscience. Through synaptic interactions, neurons like a ganglion cell integrate information about signals in many photoreceptors, producing a new information signal of their own for the neural network.

Chemically mediated synaptic transmission

The presynaptic terminal of the photoreceptor is separated from the bipolar cell by a slit filled with extracellular fluid. This space is too large for the currents generated by the photoreceptor to pass through. Instead, the photoreceptor terminal releases a neurotransmitter (aka transmitter or neurotransmitter) that is stored in presynaptic vesicles. The mediator (in this case, glutamate) diffuses through the synaptic cleft and reacts with specific protein molecules (receptors) that are located in the postsynaptic membrane of the bipolar cell. A distinction should be made between "chemoreceptors", which respond to molecules, and "sensory receptors", which respond to external stimuli, such as a photoreceptor. Neuronally synthesized and released mediators and membrane receptors can be identified and visualized by several techniques, including antibody labeling.

Activation of bipolar cell receptor molecules by glutamate leads to the appearance of a gradual local potential that propagates throughout the neuron. The more the mediator is released, the higher its concentration in the gap, the more receptors are activated and the greater the local potential. All these events occur quickly, in about 1 ms. The basic principles of synaptic transmission were first described by Katz, Kuffler et al., who used muscle receptor responses as a very sensitive biomodel with good time resolution to measure transmitter release.

Excitation and inhibition

A feature of synaptic transmission, demonstrated by the example of the interaction between a photoreceptor and a bipolar cell, is the possibility of inhibition or excitation, depending on the set of receptors in the postsynaptic cell. For example, one type of glutamate receptor on a bipolar cell responds to glutamate with an excitation (depolarization) that propagates to terminals at the other end of the cell and results in the release of a mediator. Another class of bipolar cells contains a different kind of glutamate receptor that responds by inhibition. In this case, events occur in the same sequence, but lead to a decrease in the mediator release.

In all neurons of the nervous system, the ratio of excitatory and inhibitory inputs determines the possibility of reaching the threshold of action potential initiation. For example, a ganglion cell receives both excitatory and inhibitory inputs. If the threshold is overcome, then a new signal in the form of PD will be sent to higher centers, if not, then there will be no signal. In the motor cells of the spinal cord, for example, excitatory and inhibitory influences from different fibers determine whether or not a movement controlled by these motor neurons will be produced. Such motor neurons receive about 10,000 inputs from fibers. These fibers release mediators that bring the membrane potential closer or further away from the threshold of AP occurrence. Individual cells in the cerebellum receive over 100,000 inputs.

Electrical transmission

Although the main mode of information transmission is through chemical transmission, some cells in the retina and other areas of the nervous system are connected by specialized junctions in which electrical transmission of information takes place. Pre- and postsynaptic membranes in such junctions are closely spaced and connected by channels that connect the intracellular contents of two cells. Such a connection allows local potentials and even action potentials to propagate directly from cell to cell without a chemical transmitter. Metabolic products and dyes can also spread from cell to cell. There are so-called horizontal cells in the retina that are electrically connected in this way. Due to this property, gradient potentials can propagate from one horizontal cell to another, strongly influencing the processing of visual information in the retina. Electrical synapses have also been found between other cells of the body, such as between epithelial cells, muscle fibers of the intestines and heart.

Chemically mediated synaptic transmission of information is very labile. The main changes occur in the amount of mediator released when the presynaptic terminal is reached by an action potential or a gradual potential. The retinal photoreceptor is an example: the amount of the glutamate neurotransmitter released by a rod or cone in response to a standard light stimulus can be increased or decreased depending on terminal feedback from horizontal cells that receive inputs from photoreceptors. This feedback loop plays a critical role in adapting the eye to different levels of illumination.

Other mechanisms that affect the amount of mediator release depend on the prehistory of impulse activity. During or after a volley of impulses in a neuron, the amount of mediator secreted by it can significantly increase or decrease depending on the frequency and duration of the preceding impulse activity. Efficiency modulation can also occur in the postsynapse. Long-term and short-term plasticity is the focus of attention of neuroscientists.

Integrative Mechanisms

Each neuron in the central nervous system takes into account all incoming influences and, on their basis, creates its own impulse “message” with a new meaning. The term integration was first used by C. Sherrington, who also coined the term "synapse".

Ganglion cells of the retina again can serve as an example of the ability to integrate. S. Kuffler was the first to show that ganglion cells respond most strongly to a small light or dark spot the size of several receptors in a certain area of the retina. Such a spot causes a distinct volley of action potentials. A large spot that illuminates the same area of the retina is less effective. This is because another group of photoreceptors located around the activated ones also responds to light. The action of these photoreceptors inhibits the activity of ganglion cells. The summation of the excitatory action of a small spot and the inhibitory effect of the receptors located around it lead to the fact that ganglion cells are relatively weakly sensitive to diffuse light.

Thus, the value of the ganglion cell signal not only reflects “light” or “darkness”, but also correlates with the contrast pattern of the light stimulus in the visual field. Such a complex signal arises from the fact that each ganglion cell receives signals from many photoreceptors. Specific connections mediated by bipolar, horizontal, and amacrine cells determine a specific light stimulus pattern that optimally activates each specific ganglion cell.

The complexity of the information conveyed by action potentials

Even more complex information about visual stimuli is carried by the APs of nerve cells in the neocortex, which receive signals three times after the retina. The appearance of AP in cortical neurons depends on the retinal illumination pattern, which can be specific to different cells. For example, one type of cell selectively responds to a streak of light of a specific orientation (vertical, horizontal, or oblique) that moves in a specific direction in a specific part of the visual field. The parameters of the discharges of such a cell do not depend on diffuse illumination or on the appearance of a strip of non-optimal orientation, or on movement in the wrong direction. Thus, APs in such a neuron provide accurate information about the visual stimulus to the higher centers of the brain. Such detailing of meaning conveyed by stereotyped PDs

Information processing can be presented in the following form:

the photoreceptor signal carries information about the change in the intensity of illumination in a given area of the field of view;

the ganglion cell signal carries information about the contrast;

the signal of the cortical neuron carries information about the presence of an oriented strip of light.

+Complex integration of information occurs in other sensory systems. For example, the direction and localization of a mechanical stimulus at the fingertips serve as a selective stimulus for specific neurons in the area of the neocortex in which information about the direction of tactile stimulation is processed.

Electrical properties of neurons

Physiologist Vyacheslav Dubynin on the properties of neurons, action potentials and the cytoplasm of nerve cells

Between neurons, the signal is transmitted in special structures called synapses. The transmission of information in synapses is due to the release of chemicals, that is, according to the chemical principle. As long as the information remains inside the nerve cell, the transmission proceeds electrically due to the fact that special electrical impulses, action potentials, propagate along the membrane of the nerve cells. These are short steps of electric current, they are roughly triangular in shape and run along the dendritic membrane, along the body of the neuron to the axon, and eventually reach the synapses.

You can compare action potentials with the binary code of a computer. In a computer, as you know, all information is encoded by a sequence of zeros and ones. Action potentials are essentially units that encode all of our thoughts, feelings, sensory experiences, movements, and so on. By connecting to the right place of the neural network and applying electrical impulses of this kind to nerve cells, we can make a person feel, for example, positive or negative emotions, or cause some kind of sensory illusions, or control the work of internal organs. This, of course, is a very promising branch of modern neurophysiology and neuromedicine.

In order to manage action potentials, you need to understand where they come from. In principle, action potentials can be compared to the situation when you use an electric flashlight to signal your friend on the other side of the river. That is, you press the button, the flashlight flashes, and then you transmit something with some secret code. In order for your flashlight to work, you need a battery inside, that is, a certain charge of energy. Nerve cells, in order to generate an action potential, must also have such a charge of energy, and this charge is called the resting potential. It exists, it is inherent in all nerve cells and is approximately -70 mV, that is, -0.07 V.

The study of the electrical properties of neurons began a long time ago. The fact that electricity is present in living organisms was understood back in the Renaissance, when they noticed that the frog's leg twitches from electric shocks, when they realized that the electric ramp radiates energy flows. Then there was a search for those technical methods that would allow us to seriously approach the nerve cells and see what electrical processes were taking place there. Here we have to thank the squid, because the squid is such a wonderful animal that has very thick axons. This is due to the peculiarities of his lifestyle: he has a fold-mantle, which contracts and throws out water, a reactive impulse arises, and the squid moves forward. In order for many muscles of the mantle to contract vigorously and simultaneously, a powerful axon is needed, which would immediately transmit impulses to all this muscle mass. The axon is 1–1.5 mm thick. Back in the middle of the 20th century, they learned how to isolate it, insert thin electrical wires inside, measure and record those electrical processes that occur. Then it became already clear that there is a resting potential and an action potential.

A fundamental breakthrough occurred at the moment when glass microelectrodes were invented, that is, they learned how to make very thin glass tubes that are filled inside with a salt solution, say KCl. If such a tube is very carefully (this must, of course, be done under a microscope) brought to the nerve cell and pierced the membrane of the neuron, then the neuron, a little indignant, continues to work normally, and you can see what charge it has inside and how this charge changes when information is being transferred. Glass microelectrodes are the basic technology that is still in use today.

Toward the end of the 20th century, another method appeared, it is called patch-clamp , when a glass microelectrode does not pierce the membrane, but is very carefully brought to it, a piece of the membrane is sucked, while a very small area of the cell membrane is analyzed, and you can watch how they work , for example, individual protein molecules such as various ion channels.

The use of all these technologies made it possible to begin with understanding where the resting potential comes from, where the charge inside the nerve cells comes from. It turned out that the resting potential is associated primarily with the accumulation of potassium ions. Electrical processes in living organisms differ from those electrical processes that occur in a computer, because physical electricity is mainly the movement of electrons, and in living systems it is the movement of ions, that is, charged particles, primarily sodium, potassium, chlorine, calcium ions. . This four mainly provides various electrical phenomena in our body: in the nervous system, and in the muscles, and in the heart - this is a very important section of modern physiology.

When they began to analyze the composition of the cytoplasm of nerve cells, it turned out that in comparison with the external environment, the cytoplasm of neurons contains a lot of potassium and little sodium. This difference arises due to the work of a special protein molecule - the sodium-potassium pump (or sodium-potassium ATPase). It must be said that the sodium-potassium pump is located on the membranes of all cells, because living cells are arranged in such a way that they need an excess of potassium inside the cytoplasm, for example, in order for many proteins to work normally. Cells exchange intracellular sodium for extracellular potassium, pump in potassium, remove sodium from the cytoplasm, but at the same time the charge does not change, because the exchange is more or less equivalent. An ordinary cell, not a nervous one, has an excess of potassium inside, but there is no charge: how many positively charged particles, so many negatively charged ones; there are, for example,

In order for this system to acquire a negative charge, the following occurs. At some point in the maturation of a neuron, permanently open channels for potassium appear on its membrane. These are protein molecules, and in order for them to appear, the corresponding genes must work, constantly open channels for potassium allow potassium to leave the cytoplasm, and it comes out, because inside it is about 30 times more than outside. The well-known law of diffusion works: particles (in this case, potassium ions) come out from where there are a lot of them, to where they are few, and potassium begins to “escape” from the cytoplasm through these constantly open channels, specially adapted for this.

The banal answer to the question “How long will he run away?” It would seem that it should sound: “Until the concentration equalizes,” but everything is somewhat more complicated, because potassium is a charged particle. When one potassium escapes, its lone pair remains inside in the cytoplasm, and the cytoplasm acquires a charge of -1. The second potassium ran away - the charge is already -2, -3 ... As potassium escapes through diffusion, the internal charge of the cytoplasm grows, and this charge is negative. Pluses and minuses attract, therefore, as the negative charge of the cytoplasm increases, this charge begins to restrain the diffusion of potassium ions, and it becomes more and more difficult for them to leave, and at some point an equilibrium arises: how much potassium escapes due to diffusion, the same amount enters due to attraction to negative charge in the cytoplasm. This equilibrium point is approximately -70 mV, the same resting potential. The nerve cell has charged itself and is now ready to use this charge in order to generate action potentials.

When they began to study where the action potential comes from, they noticed that in order to awaken the cell, so that it generates an impulse, it is necessary to stimulate it with a fairly certain force. The stimulus, as a rule, should raise the charge inside the nerve cell to a level of about -50 mV, that is, the resting potential is -70 mV, and the so-called threshold for triggering an action potential is somewhere around -50 mV. If you raise the charge to this level, the neuron seems to wake up: suddenly a very large positive charge arises in it, which reaches a level of about +30 mV, and then quickly drops to about the level of the resting potential, that is, from 0 to 1, and then again to 0. Here it is, the current step, which is further capable of transmitting information.

Where does it come from? Why did the neuron suddenly wake up and give out this impulse? It turned out that other ion channels work here - not permanently open, but ion channels with valves. At that moment, when the charge in the nerve cell reaches the level of -50 mV, these valves begin to open, and the movement of ions begins. First, the sodium channel opens, for about half a millisecond, a portion of sodium ions has time to enter the neuron. Sodium enters because, firstly, there is little of it in the cytoplasm - about 10 times less than outside, and, secondly, it is positively charged, and the cytoplasm is negatively charged, that is, plus is attracted to minus. Therefore, the entry is very fast, total, and we observe the ascending phase of the action potential. Then sodium channels (thousands of channels work at the same time) close, and potassium channels open, electrosensitive and also with shutters. These are not those that are constantly open, but these are channels that have a special protein loop (a channel is a cylinder with a passage inside) that opens like a tourniquet, and potassium ions get the opportunity to leave the cytoplasm and carry a large amount of positive charge , and in general the charge in the neuron drops to the level of the resting potential. At this moment, potassium powerfully comes out, because we are at the top of the action potential, there is no longer -70 mV, there is a lot of potassium inside, but little outside, it comes out, takes out a positive charge, and the system is recharged. and potassium ions get the opportunity to leave the cytoplasm and take out a large amount of positive charge, and in general the charge in the neuron drops to the level of the resting potential. At this moment, potassium powerfully comes out, because we are at the top of the action potential, there is no longer -70 mV, there is a lot of potassium inside, but little outside, it comes out, takes out a positive charge, and the system is recharged. and potassium ions get the opportunity to leave the cytoplasm and take out a large amount of positive charge, and in general the charge in the neuron drops to the level of the resting potential. At this moment, potassium powerfully comes out, because we are at the top of the action potential, there is no longer -70 mV, there is a lot of potassium inside, but little outside, it comes out, takes out a positive charge, and the system is recharged.

The membrane of a nerve cell is organized in such a way that if such an impulse arises at one point - and it mainly occurs in the zone of synapses, where the neurotransmitter excited the nerve cell - then this impulse is able to propagate along the membrane of the nerve cell, and this is transmission. The propagation of an impulse along the membrane of a neuron is a separate process. Unfortunately, it happens quite slowly - a maximum of 100 m/s, and at this level, of course, we are inferior to computers, because the electrical signal propagates through the wires at the speed of light, and we have a maximum of 100–120 m/s, which is not much. Therefore, we are rather slow organisms compared to computer systems.

In order to study the work of ion channels, physiologists use special toxins that block these channels. The best known of these toxins is tetrodotoxin, the poison of puffer fish. Tetrodotoxin turns off the electrosensitive sodium channel, sodium does not enter, the action potential does not develop, and signals do not propagate through neurons at all. Therefore, poisoning with puffer fish causes gradually developing paralysis, because the nervous system stops transmitting information. Local anesthetics like novocaine, which are used in medicine to very locally stop the transmission of impulses and not trigger pain signals, have a similar effect, only milder. Animal models are used to study neurons; human nerve cells can only be recorded on very special occasions. During neurosurgical operations, there are situations when it is not only acceptable, but also necessary. For example, in order to accurately reach the area that needs to be destroyed, say, with some kind of chronic pain.

There are ways to record the electrical activity of the human brain more totally. This is done during the registration of an electroencephalogram, where the total action potentials of millions of cells are simultaneously recorded. There is another technology, it is called the technology of evoked potentials. These technologies complement what tomographic studies give us and allow us to fully represent the picture of electrical processes that takes place in the human brain.

Scientists reveal how electrical signals control muscles

A detailed analysis of the communication network between nerves and muscle cells in turtles sheds light on how movements are controlled and maintained. Writes about this " Interlocutor ".

Finding out how motor nerves and muscle cells communicate with each other could help scientists better understand the underlying mechanisms behind ALS or spinal injuries. The study was conducted by scientists from the University of Copenhagen.

To move a leg or arm, the central nervous system must send a message to certain muscles in order for them to perform the desired action. And this process requires complex communication mechanisms from the brain and spine to the muscles. For example, for one single movement, it is necessary to trigger flexor and extensor signals, which cause opposite muscle responses. This particular aspect raised certain questions, such as how the network of nerve cells could generate electrical impulses for the contractions involved in this process.

In experiments with turtles, the Danish researchers used electrodes to measure electrical signals from paired nerves in the spine when the turtles scratched themselves with one foot. Overall, the information collected suggests that the neuro motor pathway probably consists of a large and complex network of different cells, each of which transmits signals to just a few other cells. The scientists were able to confirm this discovery in a simpler nervous system using a computer simulation technique. This will allow a better understanding of the mechanism of muscle movements, which is expected to help in the treatment of severe diseases.