In addition to the specific afferent system, there is a second or nonspecific afferent system. Information from peripheral sensory nerve endings, represented by many types of receptors (baroreceptors, mechanoreceptors, chemoreceptors, thermoreceptors) in this system is depersonalized, coming through collateral connections to the same neurons.
All nerve signals sent to the central nervous system along the sensory pathways of the nonspecific afferent system enter the reticular formation, which takes part in the activation of the cerebral cortex and in the control of the spinal cord. Signals from the periphery keep the reticular formation in a retarded state.
Signals entering the reticular formation from the cerebral cortex, for example, during stress or upon awakening, activate the reticular formation, thereby providing an integral response of the body to stress or to the initiation of wakefulness.
The nonspecific afferent nervous system of the ANS is an important part of the system of regulation of adaptation metabolism, which makes it possible to increase its rate tenfold.
The property of the most vulnerable link of autonomic innervation, afferent nerve fibers “owe” to several features.
1) The peripheral part of the afferent system of innervation is a “servant of two states” – the sympathetic and parasympathetic divisions of the ANS, working alternately, and the “master” of the central division of the ANS, participating in the regulation of its work through the second nonspecific afferent system. Afferent connections of the ANS function around the clock, which leads to their primary depletion and degeneration.
One of the most probable mechanisms for the depletion and death of afferent axons is the competition of two energy-consuming processes for a secondary energy source – a gradient of sodium cations on the plasma membrane. One of these processes is represented by the transmission of nerve impulses along an extended axon, and the other is the work of Na>+-dependent glucose transport into a large volume of the cytoplasm. This is a system with mutual exhaustion, in which each of the processes slows down the other.
2) The presence of unique pseudo-unipolar neurons, when the same neuron provides energy-consuming transport of nutrients and mitochondria over long distances in two different directions, connecting the innervated periphery with neurons of the brain stem or with neurons of the spinal cord.
3) A large network of collaterals that communicate with the neurons of the second afferent system (additional energy costs).
4) A dense network of extended afferent nerve fibers of the ANS, each of which departs from a small group of cells of internal organs or blood vessels (high energy consumption for the operation of the network).
5) A decrease, for one reason or another, in the number of cells in such a small group, connected by electrotonic contacts and innervated by a separate axon, leads to an increasingly rare use of the nerve fiber and, as a result, to its degeneration.
As for the efferent part of the arc of the unconditioned reflex, it is devoid of this “disadvantage”, since its functioning is based on a huge number of neurons and neural networks of the central division of the ANS.
The efferent part of the arc of the unconditioned reflex of the sympathetic division of the ANS also has vulnerable links that contribute to its age-related degeneration.
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