Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Sensory endings and synaptic structures

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Abstract
The sensory endings, neuromuscular junctions and interneuronal synapses in the crayfish muscle receptor organ have been studied by electron microscopy. The dendrites of the receptor neuron terminate as endings which are either free in the connective tissue matrix of the central region of the receptor strands, or abut on the muscle membrane forming a specialized junction with a narrow cleft of about 18 nm. Efferent nerve endings are classified into three types on the basis of their fine structural features. Type 1 endings contain mainly spherical vesicles with a diameter of about 55 nm and a few large granular vesicles with a diameter of about 100 nm, and synapse exclusively on muscle fibres. Type 2 endings have a high proportion of elongated vesicles measuring about 30 × 80 nm and a few large granular vesicles, and synapse on both sensory neurons and muscle. Type 3 endings are characterized by the high electron density of the axoplasm and numerous large granular vesicles with a diameter of about 100 nm; they synapse only on the sensory neuron of the slow receptor unit. It is suggested that Type 1 endings are excitatory, and Type 2 and 3 endings are inhibitory. Several differences in postsynaptic structure were observed between the putative excitatory and inhibitory neuromuscular junctions. Axo-axonal synapses between endings of Type 1 and Type 2, the latter being presynaptic to the former, are also found. Functional implications and possible roles of these structures are discussed.