Transmitter release from frog motor nerve terminals depends on motor unit size.

It has been postulated that the success with which a motor nerve terminal competes for synaptic connections or the ability of an axon to maintain sprouts may depend on the support each terminal receives from its coma, presumably in the form of some substance(s) synthesized there. The support received by each terminal may in turn depend on the total number of terminals maintained by that soma, namely, motor unit size. We show here that when motor unit size is experimentally decreased, transmitter release from the terminals is markedly enhanced. This is consistent with the view that the extent of support from the soma may also influence the effectiveness of synaptic transmission.