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.
1) The ganglia of the sympathetic part of the ANS, in which the neurons of the efferent part of the arc of the unconditioned reflex are localized, are supplied with blood (oxygen and nutrients) on a common basis with other cells of the body. while the neurons of the afferent part of the arc, located in the nerve nodes of the spinal cord and the brain stem, are supplied with oxygen and nutrients in priority, like the neurons of the central nervous system.
2) Degeneration of the afferent part of the arc of the unconditioned reflex, which provides negative feedback between the periphery and the central structures of the ANS located in the parts of the brain stem, leads to irreversible activation of the work of the efferent part of the arc. Long-term non-stop work of neurons of the efferent part of the arc leads to their depletion and death.