Early nerve-muscle synapses in vitro release transmitter over postsynaptic membrane having low acetylcholine sensitivity.

Functional nerve-muscle synapses form rapidly in cultures of embryonic chicken spinal cord and muscle cells. As early as 30 min after nerve processes first contact muscle fibers they are able to release stimulus-evoked neurotransmitter. This release was detected only after wave-form averaging because of the exceedingly low amplitude of the synaptic potentials. This small size was likely due to a postsynaptic effect, because the electrophysiologically assayed acetylcholine sensitivity of the synaptic muscle membrane was low and did not differ significantly from extrasynaptic levels. Transmitter release was elicited both from along the lengths of nerve processes and from active growth cones.