REFLEXES AND THEIR CHANGES

Functioning of the nervous system is manifested in the form of continuously emerging responses to irritating factors of external and internal environment. The functional unit of the nervous system is reflex as a response to irritation of the nervous system. Reflexes are divided into unconditioned and conditioned.

Unconditioned reflexes are inherited, they are inherent to all of the biological species; their arcs are formed to the birth and normally persists throughout life. However they can be changed because of the disease.

Conditioned reflexes arise by individual development and acquiring of skills. Development of new temporary connections depends on changing of environmental conditions. Conditioned reflexes are formed on the basis of unconditioned reflexes with the help of the higher parts of the brain.

The doctrine of reflexes gave a lot to understanding of the nervous system, but according to this position it was impossible to explain many forms of goal-directed behaviour. In recent years the concept of reflex mechanisms of the nervous activity was supplemented by the presentation of biological activity. Currently it is generally accepted that behaviour of both animals and humans is active and is determined not only by external irritation but also by the influence of certain needs. These views are reflected in new physiological concepts "functional system" or "physiological activity" [N.A. Bernstein, 1949 P.K. Anokhin, 1971]. The essence of these concepts means that the brain can not only adequately respond to external irritation but also can actively plan their behaviour. The doctrine of the "acceptor of the action," or "the model of the future" changes our understanding of the nature of the nervous system; it gives us the reason to talk about the "anticipation of reality." Knowledge of these new concepts is necessary for studying many aspects of the nervous system especially in terms of behaviour.

The arcs of unconditioned reflex close in the segmental apparatus of the spinal cord and brain stem, but they can also close above, for example in the subcortical ganglia and in the cerebral cortex. Reflex arc of segmental reflexes usually consists of two or three neurons. If spinal reflex arc is formed by two neutrons then the first of them is represented by the cell spinal ganglion, and the second is represented by the motor cage (motoneuron) of the anterior horn.

The dendrite cells of the spinal ganglion has considerable length, it goes on the periphery of the sensory fibers in the nerve stem. Dendrite ends with a special apparatus for the perception of irritation - receptor. The axon of the spinal ganglion is a part of dorsal root; the fibre goes to the anterior horn of the motor neuron with the help of synapse and connects the cell body with one of its dendrites. The axon of a neuron is a part of anterior roots, of the appropriate motor nerve and ends with neuromuscular synapse.

Spinal ganglion cell with its appendices is called a receptor cell (an afferent cell) or centripetal (a part of the reflex arc), and the anterior horn motoneuron is called an effector cell or centrifugal part of it. If reflex arc has 3 neurons then the third neuron is intercalary between the receptor and effector neurons.

For example the knee-jerk reaction is caused by a reflex hammer on lig. patellae. In response quadriceps muscle of thigh reduces and the inferior limb of the knee joint straightens itself up. The arc of this unconditioned reflex consists of two neurons. It closes at level L_n-L_IV - (Pic. 9). The arcs of other deep reflexes close at different segments of the spinal cord (Table 1).

Table 1

Date: 2015-12-11; view: 1224