The structure of the reflex arc. Reflector ring. Physiology of the nervous system

Each of us at least once in his life uttered the phrase "I have a reflex," but few knew what it was talking about. Almost all of our life is based on reflexes. In infancy, they help us survive, in adulthood - to work effectively and maintain health. Obeying reflexes, we breathe, walk, eat and much more.

Reflex

Reflex - this is the response of the body to the stimulus, carried out by the nervous system. They manifest themselves at the beginning or termination of any activity: muscle movement, gland secretion, changes in vascular tone. This allows you to quickly adapt to changes in the external environment. The value of reflexes in a person's life is so great that even a partial exclusion (removal during surgery, trauma, stroke, epilepsy) leads to a permanent disability.

The study of the central and peripheral nervous system involved IP. Pavlov and I.M. Sechenov. They left a lot of information for future generations of doctors. Previously, psychiatry and neurology were not shared, but after their work, neuropathologists began to practice separately, accumulate experience and analyze it.

Types of reflexes

Globally the reflexes are divided into conditional and unconditioned. The first arise in a person in the process of life and are associated, for the most part, with what he does. Some of the acquired skills eventually disappear, and their place is occupied by new, more necessary in these conditions. These include cycling, dancing, playing musical instruments, crafting, driving a car and much more. Such reflexes are sometimes called "dynamic stereotype".

Unconscious reflexes are laid in all people equally, and we have them since birth. They persist throughout life, since they are the ones that support our existence. People do not think that they need to breathe, contract the heart muscle, keep their body in space in a certain position, blink, sneeze, etc. This happens automatically, because nature took care of us.

Classification of reflexes

There are several classifications of reflexes that reflect their functions or indicate a level of perception. We can cite some of them.

By biological significance, the reflexes are distinguished:

• Food;
• Protective;
• Sex;
• Indicative;
• Reflexes determining the position of the body (pozotonic);
• Reflexes for movement.

By the arrangement of the receptors that perceive the stimulus, we can distinguish:

• Exteroceptors located on the skin and mucous membranes;
• Interoceptors located in internal organs and vessels;
• Proprioceptors that perceive irritation of muscles, joints and tendons.

Knowing the three classifications presented, one can characterize any reflex: whether it is acquired or innate, what function it performs and how to evoke it.

Reflection arc levels

For neuropathologists it is important to know the level at which the reflex closes. This helps to better define the area of injury and predict damage to health. There are spinal reflexes, the motor neurons of which are located in the spinal cord. They are responsible for the mechanics of the body, the contraction of muscles, the work of the pelvic organs. Rising to the level above - in the medulla oblongata, bulbar centers are found that regulate the salivary glands, some facial muscles, the function of breathing and palpitation. Damage to this department almost always ends in death.

In the middle brain, mesencephalic reflexes are closed. Basically, these are reflex arcs of cranial nerves. There are also diencephalic reflexes, the final neuron of which is located in the intermediate brain. And cortical reflexes, which are controlled by the cortex of the brain. As a rule, these are acquired skills.

It should be borne in mind that the structure of the reflex arc with the participation of the higher coordinating centers of the nervous system always includes the lower levels. That is, the corticospinal pathway will pass through the intermediate, medial, oblong and spinal cord.

The physiology of the nervous system is arranged in such a way that each reflex is duplicated by several arcs. This allows you to preserve the functions of the body, even with injuries and illnesses.

Reflex arc

The reflex arc is the path of transmission of the nerve impulse from the receptor (receptor) to the performer. The reflex neural arc consists of neurons and their processes that form a chain. This concept was introduced into medicine by M. Hall in the middle of the nineteenth century, but over time, it was transformed into a "reflex ring". It was decided that this term more fully reflects the processes that occur in the nervous system.

Physiology distinguishes monosynaptic, as well as two- and three-neural arches, sometimes there are polysynaptic reflexes, that is, including more than three neurons. The simplest arc consists of two neurons: the receptor and the motor. The impulse passes through the long process of the neuron to the nerve node, which, in turn, transmits it to the muscle. Such reflexes, as a rule, are unconditional.

Departments of the reflex arc

The structure of the reflex arc includes five sections.

The first is a receptor that perceives information. It can be located both on the surface of the body (skin, mucous membranes), and in its depth (the retina, tendons, muscles). Morphologically, the receptor may look like a long process of a neuron or a cluster of cells.

The second department is a sensitive nerve fiber, which transfers excitation further along the arc. The bodies of these neurons are located outside the central nervous system (CNS), in the spinal nodes. Their function is similar to the arrow on the railway track. That is, these neurons distribute the information that they receive to different levels of the central nervous system.

The third department is the place of switching the sensitive fiber to the motor fiber. For most reflexes, it is in the spinal cord, but some complex arcs pass directly through the brain, for example, protective, orienting, food reflexes.

The fourth section is represented by a motor fiber that delivers a nerve impulse from the spinal cord to the effector or motoneuron.

The last, the fifth department is an organ that performs reflex activity. Typically, it is a muscle or gland, for example, the pupil, heart, sex or salivary glands.

Physiological properties of nerve centers

The physiology of the nervous system is variable at different levels. The later the department is formed, the more difficult its work and hormonal regulation. Allocate six properties that are inherent in all nerve centers, regardless of their topography:

1. Carrying out excitation only from the receptor to the effector neuron. Physiologically, this is due to the fact that synapses (junctions of neurons) act only in one direction and can not change it.

2. The delay in carrying out nervous excitation is also associated with the presence of a large number of neurons in the arc and, as a consequence, synapses. In order to synthesize a neurotransmitter (chemical stimulus), to release it into the synaptic cleft and thus lead to excitation, it takes more time than if the pulse propagated simply along the nerve fiber.

3. Summation of excitations. This happens if the stimulus is weak, but constantly and rhythmically repeated. In this case, the mediator accumulates in the synaptic membrane until it is a significant amount, and only then transmits the impulse. The simplest example of this phenomenon is the act of sneezing.

4. Transformation of the rhythm of excitations. The structure of the reflex arc, as well as the characteristics of the nervous system, are such that even at a slow rhythm of the stimulus it responds with frequent impulses, from fifty to two hundred times per second. Therefore, the muscles in the human body are reduced tetanically, that is, intermittently.

5. Reflexive aftereffect. Neurons of the reflex arc are in the excited state for some time after the termination of the action of the stimulus. On this account, there are two theories. The first states that nerve cells transmit excitation for fractions of a second longer than the stimulus acts, and thus prolong the reflex. The second is based on a reflex ring that closes between two intermediate neurons. They transmit excitement until one of them can generate a pulse, or until the braking signal arrives from the outside.

6. Drowning of nerve centers occurs with prolonged stimulation of the receptors. This is manifested first by a decrease, and then by a lack of sensitivity.

Autonomic Reflex Arc

By the type of the nervous system, which realizes excitation and conducts a nervous impulse, the somatic and vegetative nerve arches are distinguished. The peculiarity is that the reflex to the skeletal musculature is not interrupted, but the vegetative one necessarily switches through the ganglion. All nerve nodes can be divided into three groups:

• Vertebral (vertebral) ganglia have a relationship to the sympathetic nervous system. They are located on both sides of the spine, forming poles.
• The pre-invertebral nodes are located at some distance from both the vertebral column and the organs. These include the ciliary node, cervical sympathetic nodes, solar plexus and mesenteric nodes.
• Intra-organ nodes, as not difficult to guess, are located in the internal organs: the heart muscle, bronchi, intestinal tube, glands of internal secretion.

These differences between the somatic and vegetative system go deep into phylogeny, and are associated with the speed of the spread of reflexes and their vital necessity.

Implementation of the reflex

From the outside, the receptor of the reflex arc receives irritation, which causes excitation and the appearance of a nerve impulse. This process is based on a change in the concentration of calcium and sodium ions, which are located on both sides of the cell membrane. The change in the number of anions and cations causes a shift in the electrical potential and the appearance of a discharge.

From the receptor, excitation, moving centripetally, enters the afferent link of the reflex arc - the spinal node. The outgrowth of it enters the spinal cord to the sensitive nuclei, and then switches to motor neurons. This is the central link of the reflex. The processes of the motor nuclei come out of the spinal cord together with the other roots and are sent to the corresponding executive organ. In the muscle mass, the fibers terminate in the motor plaque.

The pulse transfer rate depends on the type of nerve fiber and can range from 0.5 to 100 meters per second. Excitation does not pass to neighboring nerves due to the presence of membranes, isolating the processes from each other.

The value of inhibition of the reflex

Since the nerve fiber can maintain excitation for a long time, the inhibition is an important adaptive mechanism of the organism. Thanks to him, the nerve cells do not experience constant overexcitation and fatigue. The reverse afferentation, thanks to which the inhibition is realized, participates in the formation of conditioned reflexes and removes the need for the central nervous system to analyze secondary problems. This ensures the coordination of reflexes, for example, movements.

Reverse afferentation also prevents the spread of nerve impulses to other structures of the nervous system, while preserving their efficiency.

Coordination of the nervous system

In a healthy person, all organs are coordinated and coordinated. They are subject to a single system of coordination. The structure of the reflex arc is a special case, which confirms a single rule. As in any other system, there are also a number of principles or regularities in which man acts:

• Convergence (impulses from different sites can flow to one section of the CNS);
• Irradiation (prolonged and severe irritation causes excitation of neighboring sites);
• Reciprocity (inhibition of some reflexes by others);
• A common final path (based on a discrepancy between the number of afferent neurons and efferent neurons);
• Feedback (self-regulation of the system based on the number of received and generated pulses);
• Dominant (the presence of the main focus of excitation, which overlaps the rest).