An ultrastruct study of the innervation of the musculature of the pond snail Lymnaea stagnalis (L.) with reference to peripheral neurosecretion.

The ultrastructure of nerve endings in the musculature of Lymnaea stagnalis was studied. Seventeen different types of axon ending were distinguished according to the size and morphology of the granules or vesicles they contain. Nine types of axon endings form neuromuscular synapses. Some of these types also form axo-somatic synapses on peripheral neurones. One axon may innervate several muscle cells. Furthermore more than one type of axon may innervate one muscle cell. One type of axon ending forms axo-axonic synapses on the presynaptic part of neuromuscular junctions. In the connective tissue near muscle cells seven types of free axon endings were found. These seem to be peripheral neurosecretory endings. Some of these are probably derived from known types of neurosecretory cells in the central nervous system, whereas others appear to originate from peripheral neuronal perikarya. Body wall muscles appear to be innervated by neuromusculatur synapses, whereas in the visceral musculature both neuromuscular junctions and free axon endings are found.