Ultrastructural morphology of axon terminals of an inhibitory spinal interneurone in the cat.

Axon collateral terminals of an interneurone mediating Ia reciprocal inhibition of spinal a-motoneurones have been ultrastructurally investigated after intracellular staining with horseradish peroxidase. The size of bouton profiles, the size and shape of synaptic vesicles and the characteristics of synaptic contacts were analyzed in 38 terminals. The morphology of the terminals was compared with the morphology of previously investigated terminals of excitatory spinocervical tract cells. The flatness of synaptic vesicles was only marginally greater in the terminals of the inhibitory interneurone.