New types of synaptic connections in crayfish stretch receptor organs: an electron microscopic study

The synaptic input to crayfish (Orconectes limosus) stretch receptor neurons, and the synaptic interactions between the inhibitory and excitatory efferents were analysed by electron microscopy of serial sections. Several novel types of synaptic connections have been observed: (i) inhibitory synaptic input on the axon hillock and initial axon segment; (ii) serial synaptic terminals on the sensory cell body; (iii) simultaneous synaptic contacts of the same inhibitory terminal with sensory dendrites and muscle fibres; (iv) reciprocal synapses between the two types of inhibitory efferents; and (v) inhibitory synapses on the primary inhibitory axon. The possible functional significance of these synapses is discussed in the light of earlier electrophysiological and pharmacological findings.