During normal postnatal development, mammalian skeletal muscles undergo an orderly process of synapse elimination, whereby each muscle fiber loses all but one of the multiple synapses with which it starts at birth. Dr. Van Essen will study several aspects of synaptic specificity and plasticity in the soleus muscle of the rabbit, using a variety of anatomical and physiological techniques. One objective is to ascertain the way in which two distinct populations of motor neurons (fast or slow) innervate their respective populations of muscle fibers. He will determine the incidence of mismatched connections (fast motor axons onto slow muscle fibers and vice versa) during the stage of extensive polyinnervation, and he will test for differences in the number of synapses initially made onto fast and slow fibers. Another objective is to study the regional distribution of muscle fibers within individual motor units and to ascertain whether this distribution within the muscle changes during the period of synapse elimination. Finally, Dr. Van Essen will determine whether there are changes in neuromuscular connections during a period of late postnatal development, when many muscle fibers are known to convert from fast to slow contractile type. These studies may provide valuable insights concerning the cellular basis of synaptic plasticity and the rules governing orderly rearrangements of synaptic connections.
