NERVE ENDINGS
[
terminationes nervorum
(LNH)] - specialized terminal devices of nerve fibers.
Depending on the structure and function of H. o. divided into several types: 1) afferent (sensitive), or receptors; 2) efferent; 3) interneuronal (Fig. 1). Rice.
1. Schematic representation of the main forms of nerve endings: a - ultrastructure of the synapse in the central nervous system (1 - axon with synaptic vesicles, 2 - dendrite); b — ultrastructure of the neuromuscular synapse (1 — axon, 2 — neurolemmocytes, 3 — muscle fibers); c - microstructure of the sensitive nerve ending in the tissue; The synapse area is highlighted with a thick line. Studying various types of N. o. in the peripheral nervous system the works of researchers of the late 19th - first half of the 20th century are devoted: A. E. Smirnov, A. S. Dogel, S. E. Mikhailova, N. G. Kolosova, B. A. Dolgo-S Aburova, E K. Plechkova. B. I. L. Avrentyeva, T. A. Grigorieva, etc.
In c. n. With. interneuronal N. o. studied by Bodian (D. Bodian, 1942), A. D. Zurabashvili (1947). S. A. Sarkisov (1948), S. Ramon-i-K ahal (1954), etc.
Reflex arc. Receptor, conductor and efferent neuron
Simple reflex arc
consists of at least two neurons, one of which is connected to some sensitive surface (for example, skin), and the other, with the help of its neurite, ends in a muscle (or gland).
When a sensitive surface is irritated, excitation travels along the neuron associated with it in a centripetal direction (centripetal) to the reflex center
, where the connection (synapse) of both neurons is located.
Here the excitation passes to another neuron and goes centrifugally (centrifugally)
to the muscle or gland.
The result is muscle contraction or a change in gland secretion. Often a simple reflex arc includes a third interneuron, which serves as a transfer station from the sensory to the motor pathway
.
In addition to a simple (three-membered) reflex arc
, there are complex
multineuronal reflex arcs
passing through different levels of the brain, including its cortex.
In higher animals and humans, against the background of simple and complex reflexes, temporary reflex connections of a higher order are also formed through neurons, known as conditioned reflexes
(I. P. Pavlov).
Thus, the entire nervous system can be imagined as consisting functionally of three types of elements.
1. Receptor (perceptor),
transforming the energy of external stimulation into a nervous process; it is associated with an afferent (centripetal, or receptor) neuron, spreading the incipient excitation (nerve impulse) to the center; analysis begins with this phenomenon (I. P. Pavlov).
2. Conductor (conductor),
intercalary, or associative, neuron that performs closure, i.e. switching excitation from a centripetal neuron to a centrifugal one. This phenomenon is a synthesis, which “obviously represents the phenomenon of nervous closure” (I.P. Pavlov). Therefore, I.P. Pavlov calls this neuron a contactor, a contactor.
3. Efferent (centrifugal) neuron,
carrying out a response (motor or secretory) due to the conduction of nervous excitation from the center to the periphery,
to the effector
.
An effector
is the nerve ending of an efferent neuron that transmits a nerve impulse to the working organ (muscle, gland).
Therefore, this neuron is also called an effector neuron. Receptors are excited from three sensitive surfaces, or receptor fields, of the body: 1) from the outer, skin, surface of the body (exteroceptive field)
through the sensory organs genetically associated with it, receiving irritation from the external environment;
2) from the inner surface of the body (interoceptive field)
, which receives irritation mainly from chemicals entering the internal cavities, and 3) from the thickness of the walls of the body itself
(proprioceptive field)
, which contains bones, muscles and other organs that produce irritation , perceived by special receptors.
Receptors from these fields are connected to afferent neurons, which reach the center and there switch through a sometimes very complex system of conductors to various efferent conductors; the latter, connecting with the working bodies, give one or another effect.
Reflex Arc Summary
The activity of the nervous system is reflexive in nature, and the nervous system itself is built on the principle of reflex arcs. A reflex is the body’s reaction to a particular irritation, which occurs with the participation of the nervous system. In it, nerve cells, contacting each other through synapses, form chains of varying lengths and complexity. A chain of neurons that necessarily includes the first neuron (sensitive) and the last neuron (motor or secretory) is called a reflex arc.
The reflex arc includes an afferent neuron with its sensory endings - receptors, one or more interneurons located in the central nervous system, and an efferent neuron, whose effector endings end on the working organs (muscles, etc.). The simplest reflex arc consists of three neurons - sensitive, intercalary and motor (or secretory).
The body of the first neuron (afferent) is located in the spinal ganglion (or sensory ganglion of the cranial nerve). The dendrites of these cells are sent as part of the corresponding spinal or cranial nerve to the periphery, where they end in a receptor apparatus that perceives irritation. In the receptor, the energy of external or internal stimulation is processed into a nerve impulse, which is transmitted along the nerve fiber to the body of the nerve cell, and then along the axon, which, as part of the posterior (sensitive) root of the spinal or cranial nerve, follows into the spinal cord or brain to the corresponding sensitive nucleus .
In the gray matter of the dorsal horn of the spinal cord or the sensory nuclei of the brain, the endings form synapses with the bodies of the second interneuron. The axon of this neuron within the spinal cord or brain ends on the cells of the third (motor) neuron. The processes of the cells of the third neuron leave the brain as part of the spinal or corresponding cranial nerve and are directed to the organ.
A monosynaptic arc consists of several neurons: an afferent, one or more interneurons, and an efferent. The reflex arc most often consists of many neurons. Between the afferent (sensitive) and efferent (motor or secretory) neurons there are several interneurons. In such a reflex arc, excitation from the sensitive neuron is transmitted along the central process to interneurons located successively one after another. Most reflexes are carried out by “multi-story” reflex arcs, in which nerve centers of various parts of the central nervous system participate.
Instructional Video - Somatic Reflex Arc
Editor: Iskander Milevski. Date of last update of the publication: 20.8.2020
Source
Physiology
But. participate in the transmission of impulses from one nerve cell to another, as well as in ensuring the regulatory influence of nerve cells on the activity of other elements of nervous tissue, muscle and glandular cells.
Afferent N. o., located in various tissues of the body, are receptors. Efferent N. o., forming synapses on muscle elements, regulate the activity of skeletal and smooth muscles. N. o., forming contacts with other nerve cells, participate in the mechanisms of interaction of neurons, ensuring the transmission of excitation to the c. n. With. from afferent nerve cells to efferent ones.
In N. o. processes of accumulation and release of mediators are carried out (see). The process of excitation, spreading into the N. region, causes depolarization of the presynaptic membrane and the release of one or another mediator. In addition to mediators in N. o. found in a significant amount of macromolecules coming from the body of the nerve cell due to the movement of axoplasm (axocurrent) in the nerve fibers (see): neurophysins, chromogranins. These macromolecules influence the processes of accumulation and storage of mediators in the NO.
With axotoc in N. o. enzymes arrive. So, in N. o. In the cerebral cortex, up to 30-40 different enzymes were discovered. In N. o. A system of contractile proteins has been identified, possibly also involved in the secretion of mediators. The ability of local synthesis in N. o. has been demonstrated. certain biologically active substances (for example, proteins and RNA), which enter the body of the nerve cell with a reverse axocurrent (movement of axoplasm from the N. region to the body of the nerve cell).
But. not only participate in the process of releasing mediators into the synaptic space, but also remove excess mediators and their decay products. Through N. o. the trophic influence of nerve cells on the executive organs is ensured (see Trophics). The trophic role of N. o. is especially clear. manifests itself in conditions of denervation (see), as a result, the cut organs and tissues acquire increased sensitivity to the action of chemicals. substances.
Nervous tissue
Nervous tissue is the main tissue that forms the nervous system and creates the conditions for the implementation of its many functions. Nervous tissue is of ectodermal origin; it is not customary to divide nervous tissue into any types of tissue. It has two main properties: excitability and conductivity.
Neuron
The structural and functional unit of nervous tissue is a neuron (from ancient Greek νεῦρον - fiber, nerve) - a cell with one long process - axon (Greek axis - axis), and one / several short ones - dendrites (Greek dendros - tree ).
I hasten to inform you that the idea that a short process of a neuron is always a dendrite, and a long process is always an axon, is fundamentally wrong. From a physiological point of view, it is more correct to give the following definitions: dendrite - a process of a neuron along which a nerve impulse moves to the body of a neuron, axon - a process of a neuron along which an impulse moves from the body of a neuron.
Neurons have 4 properties:
- Reception (lat. receptio - acceptance) - capable of perceiving incoming signals (dendrites)
- In response to signals, they are able to switch to a state of excitation or inhibition
- Conduction of excitation (from the dendrite to the neuron body, then to the end of the axon)
- Transmission of a signal to other objects - a neuron or an effector organ
In physiology, an effector organ (from the Latin efferes - efferent) is often called an executive organ or a target organ (muscles, glands). The effector organ carries out certain “orders” of the central nervous system (CNS) or endocrine glands
The processes of neurons conduct nerve impulses and transmit them to other neurons, effectors, due to which muscles contract or relax, and the secretion of glands increases or decreases.
Myelin sheath
Nerve fibers are divided into myelinated and unmyelinated. A nerve fiber is one or more processes of neurons (can be either axons or dendrites) with a surrounding sheath.
Unmyelinated nerve fibers are found predominantly in the autonomic nervous system (conduction speed 1-2 m/s). Myelin - form the white matter of the brain and spinal cord, nerve fibers of the somatic nervous system (5-120 m/s).
In myelinated nerve fibers, the processes of neurons are covered with a myelin sheath (70-75% composed of lipids (fats)), which ensures isolated transmission of the nerve impulse along the nerve. If there were no myelin sheath (imagine!), nerve impulses would propagate chaotically, and when we wanted to move our arm, our leg would move along with our arm.
There is a disease in which one’s own antibodies destroy the myelin sheath of the nerve fibers of the brain and spinal cord (such malfunctions of the body also occur). This disease - multiple sclerosis, as it progresses, leads to the destruction of not only the myelin sheath, but also the nerves - which means muscle atrophy occurs and the person gradually becomes immobilized.
The myelin layer is represented by several layers of glial cell membrane (lemmocyte, Schwann cell), which twist around the axial cylinder (neuron process). This twisting is clearly visible in the picture of a healthy nerve, just above
