Growth of inhibitory innervation in a lobster muscle

The fine structure of inhibitory innervation to a limb muscle was examined in larval, juvenile, and adult lobsters. The innervation is essentially similar in qualitative features among these different stages, although there are some marked quantitative changes associated with growth. From being localized to discrete regions in the larval muscle, the inhibitory innervation spreads to groups of muscle fibers in the early juvenile muscle and to single fibers in the late juvenile and adult muscles. Concurrently, its neuromuscular synapses enlarge in area, become perforated, and acquire more active sites of transmitter release. Inhibitory nerve terminals occur in close proximity to their excitatory counterparts in the muscles of larval and early juvenile stages, although in later stages this juxtaposition occurs preferentially in some muscle fibers but not others. The inhibitory innervation is, nevertheless, much more restricted in occurrence than is the excitatory innervation.