Each of us at least once in his life uttered the phrase “I have a reflex,” but few understood what it was talking about. Almost all of our lives are based on reflexes. In infancy, they help us survive, in adulthood - to work efficiently and maintain health. By obeying reflexes, we breathe, walk, eat and much more.
Reflex
Reflex is a response of the body to an irritant carried out by the nervous system. They are manifested by the beginning or termination of any activity: muscle movement, secretion of glands, a change in vascular tone. This allows you to quickly adapt to changes in the environment. The importance of reflexes in a person’s life is so great that even their partial exclusion (removal during surgery, trauma, stroke, epilepsy) leads to permanent disability.
The study of the central and peripheral nervous system involved 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 shared, but after their work, neuropathologists began to practice separately, accumulate experience and analyze it.
Types of Reflexes
Globally, reflexes are divided into conditional and unconditioned. The first occur 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 occupied by new, more necessary in the given conditions. These include cycling, dancing, playing musical instruments, crafts, driving a car and much more. Such reflexes are sometimes called a “dynamic stereotype”.
Unconscious reflexes are inherent in all people equally and have been with us from the moment of birth. They persist throughout life, since it is they who 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 took care of us.
Reflex classification
There are several classifications of reflexes that reflect their function or indicate the level of perception. Some of them can be cited.
According to the biological value, the following reflexes are distinguished:
- food;
- protective;
- genital
- indicative;
- reflexes that determine the position of the body (posotonic);
- reflexes for movement.
By the location of the receptors that perceive the stimulus, we can distinguish:
- exteroreceptors located on the skin and mucous membranes;
- interoreceptors located in the internal organs and vessels;
- proprioreceptors that perceive irritation of muscles, joints and tendons.
Knowing the three presented classifications, one can characterize any reflex: acquired or innate, what function performs and how to call it.
Reflex arc levels
It is important for neuropathologists to know the level at which the reflex closes. This helps to more accurately determine the area of damage and predict damage to health. Distinguish spinal reflexes, motor neurons which are located in the spinal cord. They are responsible for body mechanics, muscle contraction, and pelvic organs. Rising to a higher level - into 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. These are mainly the reflex arches of the cranial nerves. Diencephalic reflexes are also distinguished, 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 borne in mind that the structure of the reflex arc with the participation of the higher coordinating centers of the nervous system always includes lower levels. That is, the corticospinal path will pass through the intermediate, middle, 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 maintain body functions even with injuries and illnesses.
Reflex arc
The reflex arc is the pathway of transmission of a nerve impulse from the perceiving organ (receptor) to the performing one. The reflex nerve arc consists of neurons and their processes that form a chain. This concept was introduced into medicine by M. Hall in the mid-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, polysynaptic reflexes are sometimes found, that is, including more than three neurons. The simplest arc consists of two neurons: sensory and motor. The impulse passes along the long process of the neuron to the nerve node, which, in turn, transfers it to the muscle. Such reflexes are usually unconditioned.
Reflex arc departments
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 (retina, tendon, muscle). Morphologically, the receptor may look like a long process of a neuron or a cluster of cells.
The second section is the 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 an arrow on a railway track. That is, these neurons distribute the information that comes to them at different levels of the central nervous system.
The third section is the place of switching of sensitive fiber to motor fiber. For most reflexes, it is located in the spinal cord, but some complex arcs pass immediately through the brain, for example, protective, orientational, food reflexes.
The fourth section is represented by a motor fiber, which delivers a nerve impulse from the spinal cord to the effector or motor neuron.
The last, fifth department is the body that carries out reflex activity. Typically, it is a muscle or gland, such as the pupil, heart, genital 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. There are six properties that are inherent in all nerve centers, regardless of their topography:
Carrying out excitement only from the receptor to the effector neuron. Physiologically, this is due to the fact that synapses (junctions of neurons) act in only one direction and cannot change it.
The delay in conducting nervous 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 into the synaptic cleft and, thus, initiate excitation, require 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 repeating. In this case, the mediator accumulates in the synaptic membrane until there is a significant amount of it, and only then transmits an 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 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 contract tetanically, that is, intermittently.
Reflex aftereffect. The neurons of the reflex arc are in an excited state for some time after the termination of the stimulus. There are two theories to this. The first claims that nerve cells transmit excitation for fractions of a second longer than the stimulus, and thereby prolong the reflex. The second is based on the reflex ring, which closes between two intermediate neurons. They transmit excitation until one of them can generate a pulse, or until a braking signal arrives 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.
Vegetative reflex arc
By the type of nervous system that implements excitation and conducts a nerve impulse, somatic and autonomic nerve arches are distinguished. The peculiarity is that the reflex to skeletal muscles is not interrupted, and the vegetative one 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 columns.
- The prevertebral nodes are located at a certain distance both from the spinal column and from the organs. These include the ciliary node, cervical sympathetic nodes, solar plexus and mesenteric nodes.
- It is not hard to guess that intraorgan nodes are located in the internal organs: the heart muscle, bronchi, intestinal tube, and endocrine glands.
These differences between the somatic and autonomic systems go deep into phylogenesis, and are associated with the speed of distribution of reflexes and their vital necessity.
Realization of the reflex
From the outside, the irritation receptor receives irritation, which causes excitement and the emergence of a nerve impulse. The basis of this process is 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.
The excitation from the receptor, moving centripetally, enters the afferent link of the reflex arc - the spinal node. Its process 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 exit the spinal cord along with other roots and are directed to the corresponding executive organ. In the thickness of the muscles, the fibers end with a motor plaque.
The pulse transmission 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 that isolate the processes from each other.
The value of inhibition of reflex
Since the nerve fiber is able to maintain arousal 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, is involved in the formation of conditioned reflexes and eliminates the need for the central nervous system to analyze secondary tasks. This ensures coordination of reflexes, for example, movements.
Reverse afferentation also prevents the spread of nerve impulses to other structures of the nervous system, while maintaining their performance.
Nervous system coordination
In a healthy person, all organs act harmoniously and coherently. They are subject to a single coordination system. The structure of the reflex arc is a special case, which confirms a single rule. As in any other system, a person also has a number of principles or patterns by which it operates:
- convergence (impulses from different sections can come to one section of the central nervous system);
- irradiation (prolonged and severe irritation causes excitation of neighboring areas);
- reciprocity (inhibition of some reflexes by others);
- common final path (based on the mismatch of the number of afferent neurons to efferent ones);
- feedback (system self-regulation based on the number of received and generated pulses);
- dominant (the presence of the main focus of excitation, which overlaps the rest).