Competition in the Synaptic Marketplace: Activity Is Important

Abstract

The developing nervous system is replete with excessive synaptic connections, many of which will be lost during development. This process of pruning leads to a more precise pattern of synaptic connectivity of the mature nervous system. To sculpt away inappro priate synaptic connections is considered a crucial determinant in establishing the pre cision of synaptic interactions in the nervous system. Much evidence suggests that the pattern of nerve impulse activity in the afferent fiber component of the synapse plays a primary role in loss of superfluous synaptic connections during development. Perhaps the best studied example of this developmental phenomenon is at the neuromuscular junction of the neonatal mouse where multiple motor neurons innervate a given muscle fiber; the multiple synapses are usually located on the same motor endplate. However, over a few weeks all but one motor neuron are eliminated so that a given muscle fiber receives its innervation from only one motor neuron (Fig. 1). The mechanisms for this retraction of inappropriate or redundant synaptic connections have broad implications for the under standing of the development of the synaptic organization of the nervous system as well as for other forms of neuronal plasticity, including those that may underlie learning and memory processes in the adult nervous system. A novel approach by Balice-Gordon and Lichtman (Nature 1994;519-524) to understanding the mechanisms of the loss and pres ervation of synapses in the developing and regenerating mammalian nervous system is reviewed in this Update. The Neuroscientist 1:185-187, 1995