Muscle activity and motor neuron death in the spinal cord of the chick embryo.

During embryonic development in vertebrates about half the spinal motor neurons degenerate naturally after an initial period of normal differentiation. Motor neuron survival during this period is regulated by influences associated with both afferent and target contacts. Target-associated influences are regulated, at least in part, by activity (i.e. neuromuscular transmission or muscle contraction). Pharmacological blockade of neuromuscular activity reduces or prevents normal cell death whereas induced hyperactivity of targets enhances the death of motor neurons. Information supporting these assertions is reviewed and evidence is presented from studies which attempt to elucidate the major site at which neuromuscular activity affects motor neuron survival and degeneration in the chick embryo. Finally, a model and some supporting evidence are described in which activity is thought to regulate the production or availability of a target-derived trophic factor required by motor neurons for their survival during certain critical phases of early development.