Motor neuron.

The contractile activity of the muscle is controlled using a large number motor neurons- nerve cells, the bodies of which lie in the spinal cord, and long branches - axons as part of the motor nerve they approach the muscle. Having entered the muscle, the axon branches into many branches, each of which is connected to a separate fiber, like electrical wires connected to houses. Thus, one motor neuron controls a whole group of fibers (the so-called neuromotor unit), which works as a single unit.

A muscle consists of many neuromotor units and is capable of working not with its entire mass, but in parts, which allows you to regulate the strength and speed of contraction.

Let's look at the more detailed structure of a neuron cell.

The structural and functional unit of the nervous system is the nerve cell - neuron.

Neurons– specialized cells capable of receiving, processing, transmitting and storing information, organizing a reaction to stimulation, and establishing contacts with other neurons and organ cells.

A neuron consists of a body with a diameter of 3 to 130 µm, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough endoplasmic reticulum with active ribosomes, the Golgi apparatus), as well as processes. There are two types of processes: dendrites and axons. The neuron has a developed and complex cytoskeleton that penetrates its processes. The cytoskeleton maintains the shape of the cell; its threads serve as “rails” for the transport of organelles and substances packaged in membrane vesicles (for example, neurotransmitters).

Dendrites- branching short processes that receive signals from other neurons, receptor cells, or directly from external stimuli. The dendrite conducts nerve impulses to the body of the neuron.

Axons– a long process for conducting excitation from the neuron body.

The unique abilities of the neuron are:

- ability to generate electrical charges
- convey information using specialized endings –synapses.

Nerve impulse.

So, how does nerve impulse transmission occur?
If the stimulation of a neuron exceeds a certain threshold value, then a series of chemical and electrical changes occur at the point of stimulation that spread throughout the neuron. Transmitted electrical changes are called nerve impulse.

Unlike a simple electrical discharge, which, due to the resistance of the neuron, will gradually weaken and will be able to cover only a short distance, a much slower “running” nerve impulse is constantly restored (regenerated) in the process of propagation.
The concentrations of ions (electrically charged atoms) - mainly sodium and potassium, as well as organic substances - outside the neuron and inside it are not the same, so the nerve cell at rest is negatively charged from the inside and positively charged from the outside; As a result, a potential difference appears on the cell membrane (the so-called “resting potential” is approximately –70 millivolts). Any change that reduces the negative charge within the cell and thereby the potential difference across the membrane is called depolarization.
The plasma membrane surrounding the neuron is a complex formation consisting of lipids (fats), proteins and carbohydrates. It is practically impenetrable to ions. But some of the protein molecules in the membrane form channels through which certain ions can pass. However, these channels, called ion channels, are not constantly open, but, like gates, can open and close.
When a neuron is stimulated, some of the sodium (Na+) channels open at the point of stimulation, allowing sodium ions to enter the cell. The influx of these positively charged ions reduces the negative charge of the inner surface of the membrane in the channel area, which leads to depolarization, which is accompanied by a sharp change in voltage and discharge - the so-called. “action potential”, i.e. nerve impulse. The sodium channels then close.
In many neurons, depolarization also causes potassium (K+) channels to open, causing potassium ions to leave the cell. The loss of these positively charged ions again increases the negative charge on the inner surface of the membrane. The potassium channels then close. Other membrane proteins also begin to work - the so-called. potassium-sodium pumps that move Na+ out of the cell and K+ into the cell, which, along with the activity of potassium channels, restores the original electrochemical state (resting potential) at the point of stimulation.
Electrochemical changes at the point of stimulation cause depolarization at an adjacent point on the membrane, triggering the same cycle of changes in it. This process is constantly repeated, and at each new point where depolarization occurs, an impulse of the same magnitude is born as at the previous point. Thus, along with the renewed electrochemical cycle, the nerve impulse spreads along the neuron from point to point.

We have figured out how a nerve impulse travels through a neuron, now let’s figure out how the impulse is transmitted from the axon to the muscle fiber.

Synapse.

The axon is located in the muscle fiber in peculiar pockets, formed from protrusions of the axon and the cytoplasm of the cell fiber.
A neuromuscular synapse is formed between them.

Neuromuscular junction– nerve ending between the axon of a motor neuron and the muscle fiber.

1. Axon.
2. Cell membrane.
3. Axon synaptic vesicles.
4. Receptor protein.
5. Mitochondria.

A synapse consists of three parts:
1) presynaptic (giving off) element containing synaptic vesicles (vesicles) with a transmitter
2) synaptic cleft (transmission cleft)
3) a postsynaptic (perceiving) element with receptor proteins that ensure the interaction of the transmitter with the postsynaptic membrane and enzyme proteins that destroy or inactivate the transmitter.

Presynaptic element- an element that gives off a nerve impulse.
Postsynaptic element- an element that receives a nerve impulse.
Synaptic cleft- the interval in which the transmission of nerve impulses occurs.

When a nerve impulse in the form of an action potential (a transmembrane current caused by sodium and potassium ions) “arrives” at the synapse, calcium ions enter the presynaptic element.

Mediator– a biologically active substance secreted by nerve endings and transmitting a nerve impulse at the synapse. A mediator is used to transmit impulses to muscle fibers – acetylcholine.

Calcium ions ensure the rupture of the vesicles and the release of the transmitter into the synaptic cleft. Having passed through the synaptic cleft, the transmitter binds to receptor proteins on the postsynaptic membrane. As a result of this interaction, a new nerve impulse arises on the postsynaptic membrane, which is transmitted to other cells. After interaction with receptors, the mediator is destroyed and removed by enzyme proteins. Information is transmitted to other nerve cells in encoded form (frequency characteristics of potentials arising on the postsynaptic membrane; a simplified analogue of such a code is a barcode on product packaging). “Deciphering” occurs in the corresponding nerve centers.
The mediator that is not bound to the receptor is either destroyed by special enzymes or captured back into the vesicles of the presynaptic ending.

A fascinating video about how a nerve impulse travels:

Even more beautiful video

Synapse

How a nerve impulse is conducted (slide show)

Mechanisms of interaction between nerve cells

Nerve cells function in close cooperation with each other.

The meaning of nerve impulses. All interactions between nerve cells are carried out due to two mechanisms: 1) the influence of the electric fields of nerve cells (electrotonic influences) and 2) the influence of nerve impulses.

The former spread to very small areas of the brain. The electrical charge of a nerve cell creates an electric field around it, the oscillations of which cause changes in the electric fields of nearby neurons, which leads to changes in their excitability, lability and conductivity. The electric field of a neuron has a relatively small extent - about 100 microns; it quickly decays as it moves away from the cell and can only affect neighboring neurons.

The second mechanism provides not only immediate interactions, but also the transmission of neural influences over long distances. It is with the help of nerve impulses that remote and isolated areas of the brain are united into a common, synchronously working system, which is necessary for complex forms of body activity. The nerve impulse is therefore the main means of communication between neurons. The high speed of impulse propagation and their local impact on a selected point in the brain contribute to the rapid and accurate transmission of information in the nervous system. In interneuron interactions, a frequency code is used, i.e. changes in the functional state and nature of responses of one nerve cell are encoded by changes in the frequency of impulses (action potentials) that it sends to another nerve cell. The total number of impulses sent by a nerve cell per unit of time, or its total impulse activity, is an important physiological indicator of the activity of a neuron.

The main elements of a chemical synapse: synaptic cleft, vesicles (synaptic vesicles), neurotransmitters, receptors.

Synapse(Greek σύναψις, from συνάπτειν - hug, clasp, shake hands) - the place of contact between two neurons or between a neuron and the effector cell receiving the signal. It serves to transmit a nerve impulse between two cells, and during synaptic transmission the amplitude and frequency of the signal can be adjusted. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.

The term was introduced in 1897 by the English physiologist Charles Sherrington. However, Sherrington himself claimed to have received the idea for the term in a conversation from physiologist Michael Foster.

Classifications of synapses

The main elements of an electrical synapse (ephaps): a - connexon in a closed state; b - connecton in the open state; c - connexon embedded in the membrane; d - connexin monomer, e - plasma membrane; f - intercellular space; g - a gap of 2-4 nanometers in the electrical synapse; h - hydrophilic connexon channel.

According to the mechanism of nerve impulse transmission

chemical is a place of close contact between two nerve cells, for the transmission of a nerve impulse through which the source cell releases into the intercellular space a special substance, a neurotransmitter, the presence of which in the synaptic cleft excites or inhibits the receiver cell.

electrical (ephaps) - a place of closer contact between a pair of cells, where their membranes are connected using special protein formations - connexons (each connexon consists of six protein subunits). The distance between cell membranes in the electrical synapse is 3.5 nm (usual intercellular distance is 20 nm). Since the resistance of the extracellular fluid is low (in this case), impulses pass through the synapse without delay. Electrical synapses are usually excitatory.

mixed synapses - The presynaptic action potential produces a current that depolarizes the postsynaptic membrane of a typical chemical synapse where the pre- and postsynaptic membranes are not tightly adjacent to each other. Thus, at these synapses, chemical transmission serves as a necessary reinforcing mechanism.

The most common are chemical synapses. Electrical synapses are less common in the mammalian nervous system than chemical ones.

By location and affiliation with structures[edit | edit wiki text]

peripheral

• neuromuscular

  neurosecretory (axo-vasal)

  receptor-neuronal

central

• axo-dendritic- with dendrites, including

  • axo-spinous- with dendritic spines, outgrowths on dendrites;

axo-somatic- with the bodies of neurons;

axo-axonal- between axons;

dendro-dendritic- between dendrites;

Various locations of chemical synapses

By neurotransmitter

aminergic, containing biogenic amines (for example, serotonin, dopamine);

• including adrenergic containing adrenaline or norepinephrine;

cholinergic, containing acetylcholine;

purinergic, containing purines;

peptidergic, containing peptides.

At the same time, only one transmitter is not always produced at the synapse. Usually the main pick is released along with another one that plays the role of a modulator.

By action sign

stimulating

brake.

If the former contribute to the occurrence of excitation in the postsynaptic cell (in them, as a result of the arrival of an impulse, depolarization of the membrane occurs, which can cause an action potential under certain conditions), then the latter, on the contrary, stop or prevent its occurrence and prevent further propagation of the impulse. Typically inhibitory are glycinergic (mediator - glycine) and GABAergic synapses (mediator - gamma-aminobutyric acid).

Inhibitory synapses are of two types: 1) a synapse, in the presynaptic endings of which a transmitter is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential; 2) axo-axonal synapse, providing presynaptic inhibition. Cholinergic synapse (s. cholinergica) - a synapse in which acetylcholine is the mediator.

Present at some synapses postsynaptic condensation- electron-dense zone consisting of proteins. Based on its presence or absence, synapses are distinguished asymmetrical And symmetrical. It is known that all glutamatergic synapses are asymmetric, and GABAergic synapses are symmetrical.

In cases where several synaptic extensions are in contact with the postsynaptic membrane, multiple synapses.

Special forms of synapses include spinous apparatus, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite contact the synaptic extension. Spine apparatuses significantly increase the number of synaptic contacts on a neuron and, consequently, the amount of information processed. Non-spine synapses are called sessile synapses. For example, all GABAergic synapses are sessile.

Located in the cell membrane Na + , K + –ATPases, sodium and potassium channels.

Na + , K + –ATPase Due to energy, ATP constantly pumps Na + out and K + in, creating a transmembrane gradient of the concentrations of these ions. The sodium pump is inhibited by ouabain.

Sodium and potassium channels can pass Na + and K + along their concentration gradients. Sodium channels are blocked by novocaine, tetrodotoxin, and potassium channels by tetraethylammonium.

The work of Na + ,K + –ATPase, sodium and potassium channels can create a resting potential and an action potential on the membrane .

Resting potential is the potential difference between the outer and inner membranes under resting conditions, when the sodium and potassium channels are closed. Its value is -70 mV, it is created mainly by the concentration of K + and depends on Na + and Cl -. The concentration of K + inside the cell is 150 mmol/l, outside 4-5 mmol/l. The concentration of Na + inside the cell is 14 mmol/l, outside 140 mmol/l. A negative charge inside the cell is created by anions (glutamate, aspartate, phosphates), for which the cell membrane is impermeable. The resting potential is the same throughout the fiber and is not a specific feature of nerve cells.

Stimulation of a nerve can result in an action potential.

Action potential- this is a short-term change in the potential difference between the outer and inner membrane at the moment of excitation. The action potential depends on the Na + concentration and occurs on an all-or-none basis.

The action potential consists of the following stages:

1. Local response . If, during the action of a stimulus, the resting potential changes to a threshold value of -50 mV, then sodium channels open, which have a higher capacity than potassium channels.

2.Depolarization stage. The flow of Na + into the cell first leads to membrane depolarization to 0 mV, and then to polarity inversion to +50 mV.

3.Repolarization stage. Sodium channels close and potassium channels open. The release of K+ from the cell restores the membrane potential to the resting potential level.

The ion channels open for a short time and after they close, the sodium pump restores the original distribution of ions along the sides of the membrane.

Nerve impulse

In contrast to the resting potential, the action potential covers only a very small area of ​​the axon (in myelinated fibers - from one node of Ranvier to the neighboring one). Having arisen in one section of the axon, an action potential due to the diffusion of ions from this section along the fiber reduces the resting potential in the adjacent section and causes the same development of the action potential here. Thanks to this mechanism, the action potential propagates along the nerve fibers and is called nerve impulse .

In myelinated nerve fibers, sodium and potassium ion channels are located at the unmyelinated sites of the nodes of Ranvier, where the axon membrane contacts the intercellular fluid. As a result, the nerve impulse moves “in leaps”: Na + ions entering the axon when channels open in one interception diffuse along the axon along a potential gradient until the next interception, reduce the potential here to threshold values ​​and thereby induce an action potential. Thanks to this device, the speed of impulse behavior in a myelinated fiber is 5-6 times greater than in unmyelinated fibers, where ion channels are located evenly along the entire length of the fiber and the action potential moves smoothly rather than abruptly.

Synapse: types, structure and functions

Waldaer in 1891 formulated neural theory , according to which the nervous system consists of many individual cells - neurons. The question remained unclear: what is the mechanism of communication between single neurons? C. Sherrington in 1887 to explain the mechanism of interaction between neurons, he introduced the terms “synapse” and “synaptic transmission”.

No one will argue that nature's greatest achievement is the human brain. Nerve impulses running along nerve fibers are the quintessence of our essence. The work of the heart, stomach, muscles and the spiritual world - all this is in the hands of the nerve impulse. What is a nerve impulse, how does it arise and where does it disappear, we will consider in this article.

Neuron as a structural unit of the system

The evolution of the nervous system of vertebrates and humans followed the path of the emergence of a complex information network, the processes in which are based on chemical reactions. The most important component of this system is specialized cells called neurons. They consist of a body with a nucleus and important organelles. Two types of processes extend from the neuron: several short and branched dendrites and one long axon. Dendrites are receivers of signals from sensory receptors or other neurons, and the axon transmits signals in the nervous network. To understand the transmission of nerve impulses, it is important to know about the myelin sheath around the axon. These are specific cells; they form an axon sheath, but not continuous, but with interruptions (constrictions of Ranvier).

Transmembrane gradient

All living cells, including neurons, have electrical polarity, which arises as a result of the work of potassium-sodium pumps in the membrane. Its inner surface has a negative charge relative to the outer one. An electrochemical gradient equal to zero arises and dynamic equilibrium is established. The resting potential (potential difference inside and outside the membrane) is 70 mV.

How does a nerve impulse occur?

When a nerve fiber is exposed to an irritant, the membrane potential in this place is sharply disrupted. At the beginning of excitation, the permeability of the membrane for potassium ions increases, and they rush into the cell. In 0.001 seconds, the inner surface of the neuronal membrane becomes positively charged. This is what a nerve impulse is - a short-term recharging of a neuron or an action potential equal to 50-170 mV. A so-called action potential wave occurs, which propagates along the axon like a flow of potassium ions. The wave depolarizes sections of the axon, and the action potential moves with it.

On this topic: Korean Zen. Patriarch of the present century

Transmission between axon and another neuron

After reaching the end of the axon, it becomes necessary to transmit the nerve impulse to one or more axons. And here we need another mechanism, different from the action potential wave. The end of the axon is the synapse, the point of contact with the synaptic cleft and presynaptic sacs of the axon. The action potential in this case activates the release of neurotransmitters from the presynaptic sacs into the synaptic cleft. Neurotransmitters interact with the membrane of underlying neurons, causing ionic imbalance in them. And the story with the sodium-potassium pump is repeated in another neuron. Having completed their function, neurotransmitters either diffuse or are captured back into the presynaptic sacs. In this situation, to the question of what a nerve impulse is, the answer will be: the transmission of excitation through chemical agents (neurotransmitters).

Myelin and impulse speed

In the constrictions of the myelin sheaths, which wrap the axon like a coupling, the ionic current easily flows into the medium and back. In this case, the membrane is irritated and an action potential is formed. Thus, the nerve impulse moves along the axon in jumps, causing the formation of an action potential only at the nodes of Ranvier. It is this spasmodic flow of the action potential that greatly increases the speed of the nerve impulse. For example, in thick myelinated fibers the impulse speed reaches values ​​of 70-120 m/sec, while in thin nerve fibers without a myelin sheath the impulse speed is less than 2 m/sec.

Galvanization and nerve impulse

In semi-liquid colloidal protoplasm, the current is galvanic - it is carried by atoms with an electric charge (ions). But galvanic current cannot travel over fairly large distances, but a nerve impulse can. Why? The answer is simple. When an action potential wave travels along an axon, it forms a galvanic element inside the neuron. In a nerve, as in any galvanic element, there is a positive pole (the outer side of the membrane) and a negative pole (the inner side of the membrane). Any external influence disrupts the balance of these poles, the permeability of a particular section of the membrane changes, and a change in permeability is initiated in the neighboring section. That's it, the impulse went further along the length of the axon. And the initial section from which the excitation began has already restored its integrity, found its zero gradient and is ready to launch an action potential in the neuron again.

On this topic: Contactless washing: future technologies in action

The neuron is not just a conductor

Neurons are living cells, and their protoplasm is even more complex than in the cells of other tissues. In addition to the physical processes associated with the initiation and conduction of a nerve impulse, complex metabolic processes occur in the neuron. It has been experimentally established that when a nerve impulse passes through a neuron, the temperature in it increases (even by millionths of a degree). And this means only one thing - all metabolic processes in it accelerate and proceed more intensively.

Nerve impulses are of the same type

The main property of a neuron is the ability to generate a nerve impulse and conduct it quickly. Information about the quality and strength of stimulation is encoded in changes in the frequency of transmission of nerve impulses to and from neurons. This frequency varies from 1 to 200 per second. This frequency code assumes different pulse repetition periods, combining them into groups with different numbers and patterns of movement. This is exactly what an encephalogram records - a complex spatial and temporal sum of nerve impulses in the brain, its rhythmic electrical activity.

Neuron chooses

What causes a neuron to “fire” and initiate an action potential is still an open question today. For example, neurons in the brain receive transmitters sent by thousands of their neighbors and send thousands of impulses to the nerve fibers. In the neuron, the process of processing impulses and making a decision occurs - to initiate an action potential or not. The nerve impulse will fade away or be sent further. What is it that causes the neuron to make this choice and how does it make the decision? We know almost nothing about this fundamental choice, although it is what controls the activity of our brain.

NERVOUS IMPULSE- a wave of excitation, which spreads along the nerve fiber and serves to transmit information from the peripheral. receptor (sensitive) endings to the nerve centers, inside the center. nervous system and from it to the executive apparatus - muscles and glands. Passage of N. and. accompanied by transitional electrical processes that can be recorded with both extracellular and intracellular electrodes.