Each of us at least once in his life said the phrase "I have a reflex", but few understood what he was talking about. Almost all of our life is based on reflexes. In infancy, they help us survive, in adulthood - to work efficiently and stay he althy. Our reflexes allow us to breathe, walk, eat and more.
Reflex
Reflex is the body's response to a stimulus, carried out by the nervous system. They are manifested by the beginning or cessation of any activity: muscle movement, secretion of glands, changes in vascular tone. This allows you to quickly adapt to changes in the external environment. The importance of reflexes in human life is so great that even their partial exclusion (removal during surgery, trauma, stroke, epilepsy) leads to permanent disability.
I. P. Pavlov and I. M. Sechenov. They left behind a lot of information for future generations of doctors. Previously, psychiatry and neurology were not separated, but after their work, neuropathologists began to practice separately,accumulate experience and analyze it.
Types of reflexes
Globally, reflexes are divided into conditional and unconditional. The first ones 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 disappear over time, and their place is taken by new, more necessary in these conditions. These include cycling, dancing, playing musical instruments, crafts, driving and more. Such reflexes are sometimes referred to as the “dynamic stereotype.”
Unconscious reflexes are embedded in all people the same way and we have from the moment of birth. They persist throughout life, as they support our existence. People do not think about the fact 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 has taken care of us.
Classification of reflexes
There are several classifications of reflexes that reflect their functions or indicate the level of perception. You can cite some of them.
Reflexes are distinguished by biological significance:
- food;
- protective;
- sexual;
- indicative;
- reflexes that determine the position of the body (posotonic);
- reflexes for movement.
According to the location of the receptors that perceive the stimulus, we can distinguish:
- exteroreceptors located on the skin and mucous membranes;
- interoreceptors located ininternal organs and vessels;
- Proprioreceptors that perceive irritation of muscles, joints and tendons.
Knowing the three classifications presented, any reflex can be characterized: acquired or congenital, what function it performs and how to call it.
Reflex arc levels
For neurologists, it is important to know the level at which the reflex closes. This helps to more accurately determine the area of damage and predict damage to he alth. There are spinal reflexes, the motor neurons of which are located in the spinal cord. They are responsible for the mechanics of the body, muscle contraction, the work of the pelvic organs. Rising to a higher level - in the medulla oblongata, bulbar centers are found that regulate the salivary glands, some muscles of the face, the function of breathing and heartbeat. Damage to this department is almost always fatal.
Mesencephalic reflexes close in the midbrain. Basically, these are reflex arcs of the cranial nerves. There are also diencephalic reflexes, the final neuron of which is located in the diencephalon. And cortical reflexes, which are controlled by the cerebral cortex. As a rule, these are acquired skills.
It should be taken into account 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 tract will pass through the intermediate, middle, medulla oblongata and spinal cord.
The physiology of the nervous system is arranged in such a way that eachthe reflex is duplicated by several arcs. This allows you to maintain the functions of the body even with injuries and illnesses.
Reflex arc
A reflex arc is a way of transmitting a nerve impulse from a perceiving organ (receptor) to an executing one. The reflex neural arch consists of neurons and their processes, which 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.
In physiology, monosynaptic, as well as two- and three-neuron arcs are distinguished, sometimes there are polysynaptic reflexes, that is, including more than three neurons. The simplest arc consists of two neurons: perceiving and motor. The impulse passes along the long process of the neuron to the ganglion, which, in turn, transmits it to the muscle. Such reflexes are usually unconditioned.
Departments of the reflex arc
The structure of the reflex arc includes five departments.
The first is the receptor that receives information. It can be located both on the surface of the body (skin, mucous membranes) and in its depth (retina, tendons, muscles). Morphologically, the receptor may look like a long process of a neuron or a cluster of cells.
The second section is a sensitive nerve fiber that transmits excitation further along the arc. The bodies of these neurons are located behindoutside the central nervous system (CNS), in the spinal nodes. Their function is similar to a switch on a railway track. That is, these neurons distribute the information that comes to them to different levels of the central nervous system.
The third section is the place where the sensory fiber switches to the motor one. For most reflexes, it is located in the spinal cord, but some complex arcs pass directly through the brain, such as protective, orienting, food reflexes.
The fourth section is represented by a motor fiber that delivers a nerve impulse from the spinal cord to an effector or motor neuron.
The last, fifth department is an organ that carries out reflex activity. Typically, this is a muscle or gland, such as the pupil, heart, gonads, or salivary glands.
Physiological properties of nerve centers
The physiology of the nervous system is changeable at its different levels. The later the department is formed, the more difficult its work and hormonal regulation. There are six properties that are inherent in all nerve centers, regardless of their topography:
- Conducting 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 cannot change it.
- The delay in the conduction of nerve excitation is also associated with the presence of a large number of neurons in the arc and, as a result, synapses. In order to synthesize a neurotransmitter (chemical stimulus), release it intothe synaptic cleft and conduct, thus, excitation, it takes more time than if the impulse propagated simply along the nerve fiber.
- 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 there is a significant amount of it, and only then transmits the impulse. The simplest example of this phenomenon is the act of sneezing.
- Transformation of the rhythm of excitations. The structure of the reflex arc, as well as the features of the nervous system, are such that it responds even to a slow rhythm of the stimulus with frequent impulses - from fifty to two hundred times per second. Therefore, the muscles in the human body contract tetanically, that is, intermittently.
- Reflex aftereffect. The neurons of the reflex arc are in an excited state for some time after the cessation of the stimulus. There are two theories on this. The first states that nerve cells transmit excitation for a fraction of a second longer than the stimulus acts, and thereby prolong the reflex. The second is based on a reflex ring, which closes between two intermediate neurons. They transmit excitation until one of them can generate an impulse, or until a braking signal is received from the outside.
- Drowning of the nerve centers occurs with prolonged irritation of the receptors. This is manifested first by a decrease, and then by a complete lack of sensitivity.
Vegetativereflex arc
According to the type of nervous system that realizes excitation and conducts a nerve impulse, somatic and autonomic nerve arcs are distinguished. The peculiarity is that the reflex to the skeletal muscles is not interrupted, and the vegetative necessarily switches through the ganglion. All nerve nodes can be divided into three groups:
- Vertebral (vertebral) ganglia are related to the sympathetic nervous system. They are located on both sides of the spine, forming pillars.
- Prevertebral nodes are located at some distance from the spinal column, and from the organs. These include the ciliary ganglion, cervical sympathetic ganglions, solar plexus, and mesenteric ganglia.
- Intraorgan nodes, as you might guess, are located in the internal organs: the muscle of the heart, bronchi, intestinal tube, endocrine glands.
These differences between the somatic and vegetative systems go deep into phylogenesis, and are associated with the speed of propagation 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 occurrence 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. A change in the number of anions and cations causes a shift in the electric potential and the appearance of a discharge.
From the receptor, excitation, moving centripetally, enters the afferentthe link of the reflex arc is the spinal node. Its process enters the spinal cord to sensitive nuclei, and then switches to motor neurons. This is the central link of the reflex. The processes of the motor nuclei leave the spinal cord along with other roots and go to the corresponding executive organ. In the thickness of the muscles, the fibers end with a motor plaque.
The speed of impulse transmission 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 sheaths that isolate the processes from each other.
The value of reflex inhibition
Since the nerve fiber is able to maintain excitation for a long time, inhibition is an important adaptive mechanism of the body. Thanks to him, nerve cells do not experience constant overexcitation and fatigue. Reverse afferentation, due to which inhibition is realized, participates in the formation of conditioned reflexes and relieves the CNS of the need to analyze secondary tasks. This ensures the coordination of reflexes, such as movements.
Reverse afferentation also prevents the spread of nerve impulses to other structures of the nervous system, keeping them working.
Coordination of the nervous system
In a he althy person, all organs work harmoniously and coordinated. They are subject to a single system of coordination. The structure of the reflex arc is a special case that confirms a single rule. As in any other system,a person also has a number of principles or patterns according to which it operates:
- convergence (impulses from different areas can come to one area of the CNS);
- irradiation (prolonged and severe irritation causes excitement of neighboring areas);
- reciprocity (inhibition of some reflexes by others);
- general final path (based on the discrepancy between the number of afferent and efferent neurons);
- feedback (system self-regulation based on the number of received and generated impulses);
- dominant (the presence of the main focus of excitation, which overlaps the rest).
