Reflex (biology)

The concept of reflex first appeared in Descartes' physics. Descartes developed a general mechanical picture of the world and wanted to include in it the behavior of living beings. Descartes’s concept was created in an era when various scientists gave materialistic explanations for phenomena occurring in nature: for example, physician William Harvey discovered and described the circulatory system, in which this system acted as a simple mechanism with elements known to mankind at that time - the pump, “pipes ", Etc. Descartes' theory further strengthened the principle of materialistic determinism, since Harvey considered only the internal structure of the animal’s body as a mechanism, and Descartes transferred this principle in this way as well as the interaction of organisms with the outside world, that is, in fact, mental activity. Descartes believed that the interaction of organisms with surrounding bodies is mediated by the nervous machine, in which the brain acts as the center, and "neural tubes" diverge from it. According to his scheme, external factors act on the ends of nerve “threads” located in the body, which, stretching, open the valves of the holes leading from the brain to the nerves. Through these open channels, “animal spirits” (Descartes expression) rush into the corresponding muscles, which, as a result, “inflate”. Thus, Descartes argued that the cause of the motor act is not a product of the brain or soul, but lies outside, outside the body. Descartes’s work was the result of Harvey’s inspirational influence: Descartes wrote that according to the movements of the circulatory system "as the first and most common thing that is observed in animals, one can easily judge everything else." Although Descartes did not have the term “reflex”, he managed to outline the contours of this concept quite clearly: I. P. Pavlov considered Descartes to be the founder of the concept of “reflex” in physiology and wrote that “Considering the activity of animals, as opposed to human, machine-like, Descartes ... established the concept of reflex as the main act of the nervous system ” ^[2] .

The assumption of the completely reflex nature of the activity of the higher parts of the brain was first developed by physiologist I.M.Sechenov . Before him, physiologists and neurologists did not dare to raise the question of the possibility of a physiological analysis of the mental processes that psychology was left to solve.

Further, the ideas of I.M.Sechenov were developed in the works of I.P. Pavlov , who opened the way for objective experimental study of the functions of the cortex, developed a method for developing conditioned reflexes, and created the doctrine of higher nervous activity. Pavlov in his works introduced the division of reflexes into unconditioned, which are carried out by congenital, hereditarily fixed nerve paths, and conditioned, which, according to Pavlov’s views, are carried out through neural connections formed in the process of individual life of a person or animal.

Charles S. Sherrington ( Nobel Prize in Physiology or Medicine , 1932) made a major contribution to the formation of the theory of reflexes. He discovered coordination, mutual inhibition and facilitation of reflexes.

The doctrine of reflexes has given a lot to understand the very essence of nervous activity. However, the reflex principle itself could not explain many forms of purposeful behavior. At present, the concept of reflex mechanisms is supplemented by the idea of ​​the role of needs in the organization of behavior, it has become generally accepted that the behavior of animals, including humans, is active and is determined not only by certain stimuli, but also by plans and intentions arising under the influence of certain needs. These new ideas were expressed in the physiological concepts of the "functional system" of P. K. Anokhin or the "physiological activity" of N. A. Bernshtein . The essence of these concepts comes down to the fact that the brain can not only adequately respond to stimuli, but also anticipate the future, actively plan behavior and implement them in action. The notions of an “acceptor of action,” or “models of a needed future,” allow us to talk about “ahead of reality.”

Neurons and pathways of nerve impulses during a reflex act form the so-called reflex arc :

stimulus - receptor - neuron - effector - reaction.

In humans, most reflexes are carried out with the participation of at least two neurons - the sensory and motor (motor neuron, executive neuron). In the reflex arcs of most reflexes, intercalary neurons (interneurons) are also involved - one or more. Any of these neurons in humans can be located both inside the central nervous system (for example, reflexes with the participation of central chemo- and thermoreceptors), and beyond (for example, reflexes of the metasympathetic part of the ANS).

According to a number of signs, reflexes can be divided into groups ^[3] .

1. By type of education: conditioned and unconditioned reflexes.
2. By types of receptors: exteroceptive (skin, visual, auditory, olfactory), interoceptive (from receptors of internal organs) and proprioceptive (from receptors of muscles, tendons, joints)
3. By effectors: somatic, or motor (skeletal muscle reflexes), for example flexor, extensor, locomotor, statokinetic, etc .; autonomic - digestive, cardiovascular, sweating, pupillary, etc.
4. By biological significance: defensive, or protective, digestive, sexual, indicative.
5. According to the degree of complexity of the neural organization of reflex arcs, monosynaptic arcs are distinguished, whose arcs consist of afferent and efferent neurons (for example, knee), and polysynaptic, whose arcs also contain one or more insertion neurons and have two or more synaptic switches (for example, flexor pain).
6. By the nature of the effects on the activity of the effector: excitatory - causing and enhancing (facilitating) its activity, inhibitory - weakening and suppressing it (for example, reflex increase in heart rate by the sympathetic nerve and its reduction or cardiac arrest - vagus).
7. According to the anatomical location of the central part of the reflex arches , spinal reflexes and brain reflexes are distinguished. Neurons located in the spinal cord are involved in the implementation of spinal reflexes. An example of a simple spinal reflex is the withdrawal of a hand from a sharp pin. Brain reflexes are carried out with the participation of brain neurons. Among them there are bulbar, carried out with the participation of neurons of the medulla oblongata; mesencephalic - with the participation of neurons of the midbrain; cortical - involving neurons of the cerebral cortex. There are also peripheral reflexes carried out by the metasympathetic division of the ANS without the participation of the brain and spinal cord.

Unconditioned

Unconditioned reflexes are hereditarily transmitted (innate) reactions of the body inherent in the whole species. Perform a protective function, as well as the function of maintaining homeostasis (constancy of the internal environment of the body).

Unconditioned reflexes are inherited, unchanged reactions of the body to certain influences of the external or internal environment, regardless of the conditions of occurrence and course of reactions. Unconditioned reflexes ensure the body's adaptation to unchanging environmental conditions. The main types of unconditioned reflexes: food, protective, indicative, sexual.

An example of a protective reflex is the reflex withdrawal of a hand from a hot object. Homeostasis is supported, for example, by reflex breathing, with an excess of carbon dioxide in the blood . Almost every part of the body and every organ is involved in reflex reactions.

The simplest neural networks, or arcs (according to Sherrington's expression), participating in unconditioned reflexes, are closed in the segmental apparatus of the spinal cord , but can be closed even higher (for example, in the subcortical ganglia or in the cortex). Other parts of the nervous system are also involved in reflexes: brain stem, cerebellum , cerebral cortex.

Arcs of unconditioned reflexes are formed at the time of birth and persist throughout life. However, they can change under the influence of the disease. Many unconditioned reflexes manifest themselves only at a certain age; Thus, the grasping reflex characteristic of newborns fades away at the age of 3-4 months.

An example of an unconditioned reflex in cats is their ability to land on four legs , despite the position of the cat at the beginning of the fall and from what height it fell. The reflex of landing on the paws begins to appear in kittens by the age of 3-4 weeks and is fixed by 6-7 weeks ^[4] .

The neural organization of a simple reflex

The simplest vertebral reflex is monosynaptic. If the arc of the spinal reflex is formed by two neurons, then the first of them is represented by the cell of the spinal ganglion, and the second by the motor cell (motor neuron) of the anterior horn of the spinal cord. The long dendrite of the spinal ganglion goes to the periphery, forming a sensitive fiber of some nerve trunk, and ends with the receptor . The axon of the neuron of the spinal ganglion is part of the posterior root of the spinal cord, reaches the motor neuron of the anterior horn and, through the synapse, connects to the body of the neuron or one of its dendrites. The axon of the anterior horn motor neuron is part of the anterior spine, then of the corresponding motor nerve and ends with a motor plaque in the muscle.

Pure monosynaptic reflexes do not exist. Even the knee-jerk reflex , which is a classic example of a monosynaptic reflex, is polysynaptic, since the sensitive neuron not only switches to the extensor muscle motor neuron, but also gives off the axon collateral, which switches to the insertion inhibitory neuron of the antagonist muscle, flexor muscle.

Conditional

Conditioned reflexes arise in the course of individual development and the accumulation of new skills. The development of new temporal connections between neurons depends on environmental conditions. The conditioned reflexes are formed on the basis of the unconditioned with the participation of the higher parts of the brain.

The development of the doctrine of conditioned reflexes is primarily associated with the name of I.P. Pavlov. He showed that a new stimulus can initiate a reflex reaction if it is presented for some time along with an unconditional stimulus. For example, if the dog is allowed to smell the meat , then gastric juice is secreted (this is an unconditioned reflex). If at the same time the bell rings, the dog’s nervous system associates this sound with food, and gastric juice will be released in response to the bell, even if no meat is presented. Conditioned reflexes underlie acquired behavior . These are the simplest programs. The world around us is constantly changing, so only those who can quickly and efficiently respond to these changes can live in it. As life experience gains in the cerebral cortex, a system of conditioned reflex connections is formed. Such a system is called a dynamic stereotype . It underlies many habits and skills. For example, having learned to skate, cycle, we subsequently no longer think about how to move, so as not to fall.

The axon-reflex is carried out along the axon branches without the participation of the body of the neuron . The reflex arc of the axon reflex does not contain synapses and neuron bodies. Using axon reflexes, the regulation of the activity of internal organs and blood vessels can be carried out (relatively) independently of the central nervous system ^{[5] [6]} .

Pathological reflexes are a neurological term for reflex reactions unusual for a healthy adult. In some cases, they are characteristic of the earlier stages of phylogenesis or ontogenesis.

There is an opinion that mental dependence on something is caused by the formation of a conditioned reflex. For example, mental dependence on drugs is associated with the fact that the intake of a certain substance is associated with a pleasant state (a conditioned reflex is formed that persists for almost the entire life).

Candidate of Biological Sciences Harlampy Tiras believes that "the idea of ​​conditioned reflexes with which Pavlov worked is completely based on forced behavior, and this gives incorrect registration of [results in experiments]." “We insist: the object should be studied when it is ready for this. Then we act as observers without raping the animal, and, accordingly, we obtain more objective results ” ^[7] .

• Reflex arc
• Knee jerk
• Receptor
• Renshaw Cell
• Stereotype of behavior

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