Inductive Functions of the Nervous System

In addition to their responsibility for the minute to minute control of behavior and homeostasis, nerve cells are important in maintaining the long term stability of structure and function of the nervous system and the cells it innervates. The trophic action of motor nerve cells on skeletal muscles is a well known example of such activity. Miledi (270) defined trophic actions as due to an influence of the nerve not mediated by impulses, but it has recently been suggested that only some of the classical trophic effects on skeletal muscle are exerted independently of nerve im­ pulses and/or muscle contraction. For example, direct electrical stimulation of a denervated muscle may reverse the development of denervation hypersensitivity (83, 241). The presence and the function of muscie innervation have a variety of in de pen­ dentiy exerted long term effects on the motoneurone itself, and on the physiological and anatomical state of the muscle. For example, botulinus toxin affects most of the trophic actions of the nerve except the maintenance of end plate cholinesterase (82), preganglionic cholinergic nerves form functional synapses on frog skeletal muscle but do not induce endpiate cholinesterase (223), and adrenergic nerves can prevent denervation fibrillation without forming functional synapses (265). Similarly com­ plex sets of interactions have been described elsewhere in the peripheral and auto­ nomic nervous systems, and within the brain as well.