Of particular importance is the participation of the ANS in maintaining homeostasis under different environmental changes and the internal state of the organism. Thus, the increase in air temperature is accompanied by sweating reflex, the reflex extension of the peripheral blood vessels and increased heat transfer. This helps to maintain the body temperature at a constant level and prevents overheating. Severe blood loss is accompanied by increased heart rhythm, narrowing of blood vessels and release into the General circulation is deposited in the spleen blood, the result is normal blood flow to the organs. All these and many other reactions of the body are formed the highest parts of the Central nervous system, the effects of which are implemented through the autonomic nervous system.
The ANS regulates by making small shifts in activity of two of its overall balanced divisions – the sympathetic and the parasympathetic nervous system, which leads to the predominant influence of one or another Department. Each of these departments has a touch element, perceiving various physical or chemical factors of the internal environment, and the effector link, producing the changes needed to the current activities of the body.
The life of organisms in their natural environment is in a continuous struggle for existence. In higher organisms have evolved a special structure – the sympathetic nervous system, can fully mobilize the motor and intellectual activity, launching into action all the resources and reserves of the body. This mobilization requires generalized inclusion in the reaction of many authorities and agencies. Apparently, therefore, the sympathetic ganglia are at a great distance from the innervated organs and tissues and have great potential of animation nerve impulses, which provides a generalized impact on a structure. Excitation of the sympathetic nervous system is the first stage of the circuit of hormonal responses characteristic of stress.
The sympathetic nervous system is often destabilizie physiological processes, ensuring maximum exertion of all those organs and systems that are necessary to great efforts, for the salvation of the body by fight or flight. Therefore, the influence of the sympathetic nervous system often disrupt homeostasis.
Restoring and maintaining homeostasis for any violations and the changes caused by the excitation of the sympathetic division, is a function of the parasympathetic nervous system. In this sense, the activities of the two divisions of the ANS can sometimes manifest as antagonism. However, the functions of organs and tissues are not exclusively antagonistic effects. Parasympathetic nerve fibers, in some cases, can both stimulate and inhibit the function of their regulatory bodies, providing all the process of current regulation necessary to preserve the constancy of the internal environment of the body.
Mechanisms of regulation of vegetative functions have a multi-level hierarchical structure. First the lower level of this hierarchy are the peripheral intraorgan reflexes, closes in the intramural ganglia of the ANS. The second level are reflex reactions, closing in prevertebral and paravertebral sympathetic ganglia of the solar plexus, sympathetic trunk, etc. Great contribution to the study of these reflexes have made scientists of the Institute of physiology of the Academy of Sciences of Belarus under the guidance of academician I. A. Bulygin. The third level of the specified hierarchy are lower autonomic centers in the spinal cord, the medulla and the midbrain. Higher levels of regulation of vegetative functions provided by the centres located in the reticular formation, the cerebellum, the hypothalamus, subcortical nuclei, limbic system and the cerebral cortex.