Hypoxia initiates two independent primary structural events.
1) Death of free living cells by apoptosis or necrosis (see above). 2) Degeneration and slowing down of regeneration of afferent nerve fibers of the autonomic nervous system (ANS).
The sympathetic division of the ANS is responsible for stimulating the metabolism of activity (catabolism) associated with the fight-or-flight response. The parasympathetic division of the ANS is responsible for stimulating resting metabolism (anabolism): “rest and digestion” and “feeding and reproduction”.
Afferent fibers are not divided into sympathetic and parasympathetic.
Metabolism at rest is less intense than in a state of physical activity, since the main consumer of free energy is skeletal muscles that are inactive at rest.
The state of activity is characterized by sharp ups and downs of metabolic activity, in contrast to the slow monotonic changes in metabolism in a state of rest. It is in connection with these differences that the degeneration of afferent fibers in the first place negatively affects the efficiency of the functioning of the sympathetic rather than the parasympathetic division of the ANS.
The weak link in autonomic regulation is the afferent, sensitive nerve fibers, each of which departs from a small group of cells or from single specialized receptors (bodies). A decrease, for one reason or another, in the number of cells in such a group or in a specialized receptor innervated by a separate axon leads to an increasingly rare use of the nerve fiber and, as a result, to its degeneration.
The most common cause of degeneration of afferent nerve fibers, apparently, is the death of nerve endings under conditions of deep and/or prolonged hypoxia. Nerve endings are the most distant from the neuron body and oxygen deficiency, leading to a deficiency of free energy, should primarily affect the delivery of nutrients and “building materials” necessary to maintain the integrity of nerve endings and for their regeneration, namely in the nerve endings.
Such degeneration of nerve endings belongs to the physiological category, in contrast to pathological degeneration caused by a violation of the integrity of nerve fibers as a result of injury or inflammation.
Afferent nerve fibers carry out negative feedback in the arc of an unconditioned reflex, turning off the activating effect of efferent fibers on innervated cells.