Reflexes. Reflex arc: the most important element of the nervous system

The reflex arc consists of:

• receptor - a nerve link that perceives irritation;
• afferent link - centripetal nerve fiber - processes of receptor neurons that transmit impulses from sensory nerve endings to the central nervous system;
• the central link is the nerve center (an optional element, for example, for an axon reflex);
• efferent link - carry out transmission from the nerve center to the effector;
• effector - an executive body whose activity changes as a result of a reflex.

Distinguish:

• monosynaptic, two-neuron reflex arcs;
• polysynaptic reflex arcs (include three or more neurons).

The simplest reflex arc in humans is formed by two neurons - sensory and motor (motor neuron). An example of the simplest reflex is the knee reflex. In other cases, three (or more) neurons are included in the reflex arc - sensory, intercalary and motor. In a simplified form, this is the reflex that occurs when a finger is pricked with a pin. This is a spinal reflex, its arc passes not through the brain, but through the spinal cord. The processes of sensory neurons enter the spinal cord as part of the posterior root, and the processes of motor neurons exit the spinal cord as part of the anterior root. The bodies of sensory neurons are located in the spinal node of the posterior root (in the dorsal ganglion), and the intercalary and motor neurons are located in the gray matter of the spinal cord.

The simple reflex arc described above allows a person to automatically (involuntarily) adapt to changes in the environment, for example, withdraw his hand from a painful stimulus, change the size of the pupil depending on the lighting conditions. It also helps to regulate the processes occurring inside the body. All this contributes to maintaining the constancy of the internal environment, that is, maintaining homeostasis.

In many cases, a sensory neuron transmits information (usually through several interneurons) to the brain. The brain processes incoming sensory information and stores it for later use. Along with this, the brain can send motor nerve impulses along the descending path directly to the spinal motor neurons; spinal motor neurons initiate the response

reflexes- this is the body's response to irritation of sensitive nerve formations - receptors, realized with the participation of the nervous system.

Types of reflexes conditional and unconditional

reflex arc

With the help of a reflex, excitation spreads along reflex arcs and the process of inhibition is carried out.

reflex arc- this is the path along which nerve impulses are conducted during the implementation of the reflex.

Reflex arc diagram

5 links of the reflex arc:

1. Receptor - perceives irritation and converts it into a nerve impulse.

2. Sensitive (centripetal) neuron - transmits excitation to the center.

3. Nerve center - excitation switches from sensory to motor neurons (there is an intercalary neuron in the three-neuron arc).

4. Motor (centrifugal) neuron - carries excitation from the central nervous system to the working organ.

5. Working body - reacts to the received irritation.

Information from the receptors of the working organ enters the nerve center to confirm the effectiveness of the reaction and, if necessary, coordinate it.

Scheme of the reflex arc of the knee jerk (a simple arc of two neurons)

Scheme of the reflex arc of the flexion reflex (a complex arc of several neurons)

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The source of information:

Biology in tables and diagrams. / Edition 2e, - St. Petersburg: 2004.

Rezanova E.A. Human biology. In tables and diagrams./ M.: 2008.

The main form of nervous activity is the reflex. Reflex - a causal reaction of the body to changes in the external or internal environment, carried out with the obligatory participation of the central nervous system in response to irritation of the receptors. Due to reflexes, the occurrence, change or cessation of any activity of the body occurs.

The nerve pathway along which excitation spreads during the implementation of reflexes is called reflex arc.

Reflex arcs consist of five components: 1) receptor; 2) afferent nerve pathway; 3) reflex center; 4) efferent nerve pathway; 5) effector (working body).

Receptor- This is a sensitive nerve ending that perceives irritation. In receptors, the energy of the stimulus is converted into the energy of a nerve impulse. Distinguish: 1) exteroreceptors- are excited under the influence of irritations from the environment (receptors of the skin, eyes, inner ear, nasal and oral mucosa); 2) interoreceptors- perceive irritations from the internal environment of the body (receptors of internal organs, blood vessels); 3) proprioreceptors- react to a change in the position of individual parts of the body in space (receptors of muscles, tendons, ligaments, articular bags).

Afferent neural pathway represented by processes of receptor neurons that carry excitations to the central nervous system.

reflex center consists of a group of neurons located at different levels of the central nervous system and transmitting nerve impulses from the afferent to the efferent nerve pathway.

efferent neural pathway conducts nerve impulses from the central nervous system to the effector.

Effector- an executive organ, the activity of which changes under the influence of nerve impulses that come to it through the formations of the reflex arc. The effectors can be muscles or glands.

reflex arcs can be simple or complex. A simple reflex arc consists of two neurons - perceiving and effector, between which there is one synapse. A diagram of such a two-neuron reflex arc is shown in fig. 71.

Rice. 71. Scheme of a two-neuron reflex arc of the spinal reflex. 1 - receptor; 2 - effector (muscle); P - receptor neuron; M - effector neuron (motor neuron)

An example of a simple reflex arc is the tendon reflex arcs, such as the patellar reflex arc.

The reflex arcs of most reflexes include not two, but a larger number of neurons: receptor, one or more intercalary and effector. Such reflex arcs are called complex, multi-neuronal. A diagram of a complex (three-neuron) reflex arc is shown in fig. 72.

Rice. 72. Scheme of a three-neuron reflex arc of the spinal reflex. P - receptor neuron; B - intercalary neuron; M - motor neuron

It has now been established that during the response of the effector, numerous nerve endings present in the working organ are excited. Nerve impulses now from the effector again enter the central nervous system and inform it about the correct response of the working organ. Thus, the reflex arcs are not open, but ring formations.

Reflexes are very diverse. They can be classified according to a number of characteristics: 1) according to biological significance (food, defensive, sexual); 2) depending on the type of irritated receptors: exteroceptive, interoceptive and proprioceptive; 3) by the nature of the response: motor or motor (executive organ - muscle), secretory (effector - iron), vasomotor (constriction or expansion of blood vessels).

All reflexes of the whole organism can be divided into two large groups: unconditioned and conditioned. The differences between them will be dealt with in Chapter XII.

The concept of nerve centers

From the receptors, nerve impulses travel along afferent pathways to the nerve centers. It is necessary to distinguish between the anatomical and physiological understanding of the nerve center.

Anatomical definition of the nerve center. The nerve center is a collection of neurons located in a specific section of the central nervous system. Due to the work of such a nerve center, a simple reflex activity is carried out, for example, a knee jerk. The nerve center of this reflex is located in the lumbar spinal cord (II-IV segments).

Physiological understanding of the nerve center. The nerve center is a complex functional association of several anatomical nerve centers located at different levels of the central nervous system and causing the most complex reflex acts due to their activity. For example, many organs (glands, muscles, blood and lymph vessels, etc.) are involved in the implementation of food reactions. The activity of these organs is regulated by nerve impulses coming from nerve centers located in various parts of the central nervous system. During food reactions, various anatomical nerve centers are functionally combined to obtain a certain beneficial result. A. A. Ukhtomsky called these functional associations "constellations" of nerve centers.

Physiological properties of nerve centers. Nerve centers have a number of characteristic functional properties that depend on the presence of synapses and a large number of neurons that make up them. The main properties of the nerve centers are: 1) unilateral conduction of excitation; 2) delay in excitation; 3) summation of excitations; 4) transformation of the rhythm of excitations; 5) reflex aftereffect; 6) fast fatigue.

Unilateral conduction of excitation. In the central nervous system, excitation spreads in only one direction - from the receptor neuron to the effector one. This is due to the presence of synapses in the nerve centers, in which the transmission of excitation is possible only in one direction - from the nerve ending that releases the mediator to the postsynaptic membrane.

Delay in the conduction of excitation in the nerve centers also associated with the presence of a large number of synapses. The release of the mediator, its diffusion through the synaptic cleft, and the excitation of the postsynaptic membrane require more time than the propagation of excitation along the nerve fiber.

Summation of excitations in the nerve centers occurs either when applying weak, but repetitive (rhythmic) stimuli, or with the simultaneous action of several subthreshold stimuli. The mechanism of this phenomenon is associated with the accumulation of a mediator on the postsynaptic membrane and an increase in the excitability of the cells of the nerve center. An example of the summation of excitation is the sneeze reflex. This reflex occurs only with prolonged irritation of the receptors of the nasal mucosa. For the first time, the phenomenon of summation of excitations in the nerve centers was described by I. M. Sechenov in 1863.

Transformation of the rhythm of excitations. The central nervous system responds to any rhythm of stimulation, even a slow one, with a volley of impulses. The frequency of excitations coming from the nerve centers to the periphery to the working organ ranges from 50 to 200 per 1 s. This feature of the central nervous system explains the fact that all skeletal muscle contractions in the body are tetanic.

reflex aftereffect. Reflex acts end not simultaneously with the cessation of the stimulus that caused them, but after a certain, sometimes relatively long period. This phenomenon is called the reflex aftereffect. Two mechanisms responsible for the aftereffect have been established. The first is due to the fact that excitation in nerve cells does not disappear immediately after the cessation of irritation. For some time (hundredths of a second), nerve cells continue to give rhythmic discharges of impulses. This mechanism can cause only a relatively short aftereffect. The second mechanism is the result of the circulation of nerve impulses through closed neural circuits of the nerve center and provides a longer aftereffect. On fig. 73 shows such a closed circuit of neurons.

Figure 73. Ring connections of neurons in the nerve center

The excitation of one of the neurons is transmitted to the other, and along the branches of its axon it returns again to the first nerve cell, etc. The circulation of nerve impulses in the nerve center will continue until one of the synapses is fatigued or the activity of the neurons is suspended arrival of inhibitory impulses.

Fatigue of the nerve centers. Nerve centers, unlike nerve fibers, are easily fatigued. With prolonged stimulation of afferent nerve fibers, fatigue of the nerve center is manifested by a gradual decrease, and then a complete cessation of the reflex response.

This feature of the nerve centers is proved as follows. After the cessation of muscle contraction in response to irritation of the afferent nerves, the efferent fibers that innervate the muscle begin to irritate. In this case, the muscle contracts again. Consequently, fatigue did not develop in efferent pathways; but in the nerve center.

Numerous studies have found that the most fatigued are the perceiving neurons (sensory and intermediate) compared to the efferent nerve cells of the reflex arc. At present, it is believed that the fatigue of the nerve centers is associated primarily with a violation of the transmission of excitation in the synapses. Such a violation may be due to a decrease in the stores of the neurotransmitter or a decrease in the sensitivity of the postsynaptic membrane of the nerve cell to the mediator.

Reflex tone of nerve centers. In a state of relative rest, without causing additional irritations from the nerve centers to the periphery, discharges of nerve impulses arrive at the corresponding organs and tissues. At rest, the frequency of discharges and the number of simultaneously working neurons are very small. Rare impulses, continuously coming from the nerve centers, determine the tone (moderate tension) of the skeletal muscles, smooth muscles of the intestines and blood vessels. Such constant excitation of the nerve centers is called the tone of the nerve centers. It is supported by afferent impulses continuously coming from receptors (especially proprioreceptors) and various humoral influences (hormones, carbon dioxide, etc.).

Similar information.

The simplest response of the nervous system is reflex. It is a fast, automatic, stereotyped response to irritation, it is called involuntary act because it is not under conscious control. The neurons that form the path of nerve impulses during a reflex act are reflex arc. The simplest reflex arc in animals includes one neuron and has the following form:

Neuron Stimulus → Receptor - Effector → Reaction

This level of organization is characteristic of the nervous system of the coelenterates. The reflex arcs of all groups of animals with a higher level of structural and functional organization consist of at least two neurons - afferent, or sensory(sensitive), conducting impulses from the receptor, and efferent, or motor(motor), transmitting impulses to the effector. Between these two neurons there may also be intercalary neurons located in a cluster of nerve cells - a ganglion, a nerve chain or a central nervous system (Fig. 16.13). There is a huge variety of reflexes of varying structural and functional complexity, but they can all be divided into the following four groups:

1. monosynaptic reflexes. These are reflexes with the simplest arc found in vertebrates. The sensory neuron directly contacts the body of the motor neuron. In such an arc, only one synapse is involved, located in the central nervous system. Such reflexes are very common in all vertebrates; they are involved in the regulation of muscle tone and posture (such, for example, the knee reflex - extension of the leg at the knee joint). In these reflex arcs, neurons do not reach the brain, and reflex acts are carried out without its participation, since they are stereotyped and do not require reflection or conscious decision. They are economical in terms of the number of central neurons involved and do without the intervention of the brain, which can "focus" on more important matters.

2. Polysynaptic spinal reflexes. At least two synapses located in the central nervous system participate in such reflexes, since the third neuron is included in the arc - intercalary, or intermediate(interneuron). There are synapses here between sensory and interneurons and between intercalary and motor neurons (Fig. 16.13, B). This type of reflex act is an example of a simple reflex that closes in the spinal cord. On fig. 16.14 presents in a greatly simplified form the reflex that occurs when a finger is pricked with a pin.

Simple reflex arcs of type 1 and 2 allow the body to carry out automatic involuntary reactions necessary to adapt to changes in the external environment (for example, the pupillary reflex or maintaining balance when moving) and to changes in the body itself (regulation of respiratory rate, blood pressure, etc.). ), as well as to prevent damage to the body, such as injury or burns.

3. Polysynantic reflexes involving both the spinal cord and the brain. In this type of reflex arc, a sensory neuron forms a synapse in the spinal cord with a second neuron that sends impulses to the brain. Thus, these second sensory neurons form ascending nerve pathways (Fig. 16.15, A). The brain interprets this sensory information and stores it for later use. Along with this, it can initiate motor activity at any given moment, and then the impulses will be transmitted by motor neurons along the descending nerve path directly to the spinal motor neurons through synapses located in the same area as the output synapses of the intercalary neurons (Fig. 16.15).

4. Conditioned reflexes. Conditioned reflexes are a type of reflex activity in which the nature of the response depends on past experience. These reflexes are coordinated by the brain. The basis of all conditioned reflexes (such as the habit of toileting, salivation at the sight and smell of food, awareness of danger) is learning (Sec. 16.9).

There are many situations where one of two possible reflex responses occurs involving a particular group of muscles, which can either contract or relax, which would lead to opposite results. In this situation, the usual spinal reflex would be carried out by the reflex arc shown in Fig. 16.14, however, the "conditions" under which the stimulus operates may alter the response. In such cases, a more complex reflex arc operates, including both excitatory and inhibitory neurons. For example, if we grab an empty metal frying pan with our hand, which turns out to be too hot and will burn our fingers, we will probably let go of it immediately, but we will carefully and quickly put the equally hot food on the expensive dish that burns our fingers in its place. The difference in response indicates that we are dealing with a conditioned reflex, which involves memory and a conscious decision made by the brain. In this situation, the response is carried out along a more complex reflex path, shown in Fig. 16.16.

In both cases, the stimulus causes impulses to travel to the sensory region of the brain along the ascending neural pathway. When these impulses enter the brain, it analyzes them, taking into account information coming from other senses, such as the eyes, and sets reason stimulus. The information entering the brain is compared with what is already stored in it - with information about what is likely to happen if the spinal reflex occurs automatically. In the case of a metal frying pan, the brain will calculate that if it is thrown it will not cause any harm to the body or the frying pan, and will send impulses along exciting way. This path goes down the spinal cord to the level where the stimulus entered the spinal cord and forms connections with the bodies of the motor neurons that carry out this reflex. The speed of conducting impulses along this path is such that impulses from the excitatory motor neuron of the brain reach a special motor neuron simultaneously with impulses from the intercalary neuron of a simple reflex arc. The effects of both impulses are summed up, and excitatory impulses arrive to the muscle effector along the axon of the spinal motor neuron, forcing them to throw the frying pan.

But in the case of a hot dish, the brain will quickly figure out that if you throw it, you can scald your legs, and besides, the food will be spoiled and an expensive dish will be broken. If the dish is held and carefully put into place, this will not cause severe burns to the fingers. After the brain makes such a decision, impulses will arise in it, which will also be transmitted to the spinal motor neurons, but this time along the braking path. They will arrive simultaneously with excitatory impulses from the intercalary neuron and extinguish their action. As a result, no impulses will come through the motor neurons to the corresponding muscles and the dish will be held in the hands. At the same time, the brain can give the muscles a different program of action, and the dish will be quickly and carefully put into place.

The above description of reflex arcs is, of course, greatly simplified. After all, the process of coordination, integration and regulation of functions in the body is much more complex. So, for example, certain neurons link different levels of the spinal cord that control, say, the arms and legs, so that the activity of one level is coordinated with the activity of another, and another group of neurons exercises general control from the brain.

While the joint activity of the brain and the endocrine system plays an important role in coordinating many of the types of nervous activity described later in this chapter, the regulation of autonomic functions is carried out by another reflex system, which is based solely on nervous activity. This system is called the autonomic or autonomic nervous system.

REFLEX. REFLECTOR ARC.

Reflex- This is the body's response to irritation of receptors, carried out with the participation of the central nervous system. The path along which the nerve impulse passes from the irritated receptor to the organ that responds to this irritation is called reflex arc. Anatomically, the reflex arc is a chain of nerve cells that ensures the conduction of nerve impulses from the receptor of a sensitive neuron to the effector ending in the working organ.

The reflex arc (Fig. 44) begins receptor.

Rice. 44. Scheme of the structure of the reflex arc: 1 - intercalary neuron, 2 - afferent nerve fiber, 3 - efferent nerve fiber, 4 - anterior root, 5 - anterior horn of the spinal cord, 6 - posterior horn of the spinal cord, 7 - posterior root, 8 - spinal ganglion, 9 - sensory neuron, 10 - motor neuron; the vegetative arc is shown by a dotted line

Each receptor perceives certain stimuli (mechanical, light, sound, chemical, temperature, etc.) and converts them into nerve impulses. From the receptor, nerve impulses along the path that is formed by the dendrite, body and axon of the sensitive neuron are transmitted to intercalary neurons central nervous system. Here the information is processed and transmitted to motor neurons that conduct nerve impulses to the working organs. The axons of efferent (motor or secretory) neurons located in the central nervous system form a motor or secretory pathway through which nerve impulses travel to muscles or glands and cause movement or secretion.

Thus, the reflex arc consists of 5 links: 1) a receptor that perceives an external (or internal) influence and, in response to it, forms a nerve impulse; 2) a sensitive path formed by a sensitive neuron, along which the nerve impulse reaches

nerve centers in the central nervous system; 3) intercalary neurons, through which the nerve impulse is sent to efferent neurons (motor or secretory); 4) efferent neuron, through which the nerve impulse is conducted to the working organ; 5) nerve ending - an effector that transmits a nerve impulse to the cells (fibers) of the working organ (muscle, gland).

Reflex arcs in which two neurons contact each other - sensitive and motor, and excitation passes through one synapse, are called the simplest, monosynaptic. Reflex arcs that have two or more synaptic switches are polysynaptic.

However, the reflex act does not end with the body's response to irritation. During the response, the receptors of the working organ are excited and from them information about the achieved result is sent to the central nervous system. Each organ reports its state (muscle contraction, secretion) to the nerve centers, which correct the actions of the nervous system and working organs. Thus, the reflex is carried out not just along the reflex arc, but along the reflex ring (circle).

The reflex provides a subtle, precise and perfect balancing of the relationship of the body with the environment, as well as control and regulation of functions within the body. This is its biological significance.

All nervous activity consists of reflexes of varying degrees of complexity. Some reflexes are very simple. For example, pulling back a hand in response to a prick or a skin burn, sneezing when irritating substances enter the nasal cavity. Here the response is reduced to a simple motor act carried out without the participation of consciousness. Many other functions of the human body are performed under the action of complex reflex arcs, in the formation of which many neurons participate, including brain neurons.

For the implementation of any reflex, the integrity of all links of the reflex arc is necessary. Violation of at least one of them leads to the disappearance of the reflex.

The nerve impulse in different parts of the reflex arc passes at different speeds. It passes more slowly in the structures of the central nervous system, where impulses are transmitted from one neuron to another. The slow conduction of a nerve impulse across a synapse is called synoptic delay. It should also be recalled that the synapse transmits a nerve impulse in only one direction - from the presynaptic membrane to the postsynaptic, from the nerve to the working organ. This property of a synapse is called one-way conduction of a nerve impulse.

A delay or even a complete cessation of the conduction of a nerve impulse can occur due to fatigue of the nerve centers. At the same time, the nerve fibers almost do not get tired.

In the central nervous system, along with the processes of excitation, processes of inhibition of the reflex occur. The process of inhibition is associated with the work of inhibitory neurons and inhibitory mediators. Inhibition limits the excitation of neurons.

Coordinated reflex activity is due to the interaction in the central nervous system of the processes of excitation and inhibition. Excitation provides the reaction of the body in response to irritation. Inhibition limits or reduces the excitation of neurons. The interaction of the processes of excitation and inhibition explains the mechanisms of coordination of movements. Thus, when a group of flexor muscles is contracted, the extensor muscles are simultaneously relaxed. Consequently, when a group of neurons innervating the flexor muscles is excited, inhibition occurs in the nerve cells innervating other extensor muscles.