Tissue Growth, Maintenance, and Regeneration

Abstract

Although most of the functions of the nervous system described in previous chapters are contingent on impulse conduction and changes in the membrane potential at synaptic sites, it has been recognized since the time of Claude Bernard that nerves also transmit chemical substances toward the periphery that are necessary for growth and maintenance or regeneration of the tissues and organs innervated. This relatively slow chemical modulation of target tissues and organs is thought to be mediated by substances synthesized in the cell bodies of either sensory or motorneurons that are transported peripherally by means of the microtubular system (fast axoplasmic transport) or by a “mitochondrial shuttle” (slow axoplasmic transport) (see Ochs, 1974). A central axoplasmic transport toward the cell body has also been clearly demonstrated and is thought to be the mechanism for reciprocal effects between the nerve cells and the target cells “so as to influence the structure or function of either member of the pair” (Drachman, 1974). Neural viruses such as herpes simplex and poliomyelitis and toxins such as tetanus toxin that spread into the central nervous system along peripheral nerves are probably moved by this retrograde axoplasmic transport (Bak et al.,1977). Although most investigators working in the area of neurotrophism (e.g., see Guth, 1971; Gutman, 1976) define neurotrophism in this more limited sense of chemical control by axoplasmic transport, others (e.g., Drachman, 1974) include impulse conduction and usage in neurotrophic modulation. In this chapter, the term neurotrophism will be used in its more restricted sense, i.e., regulation not mediated by nerve impulses or by functional demand.