Functional organization in the terminal segments of the spinal cord with a consideration of central excitatory and inhibitory latencies in monosynaptic reflex systems.

Prominent monosynaptic and disynaptic reflex discharges characterize ipsilateral reflex transmission in the third sacral segment. Convergence upon the motoneurons from the two sides of the body is inhibitory, that through disynaptic paths excitatory. The relative latencies of excitation and inhibition of reflex responses, of excitatory and inhibitory synaptic potentials, and of various aspects of impulse discharge in motoneurons are considered. It is concluded: (1) that a direct (i.e. monosynaptic) action of primary afferent collaterals upon motoneurons is responsible for inhibition of monosynaptic reflex discharge of antagonist motoneurons within a myotatic unit; (2) that the inhibitory postsynaptic potential as described is not the primary agency for monosynaptic reflex inhibition of monosynaptic reflex discharge; (3) that, however, a common causal agent may be responsible for inhibition of reflex discharge and for generation of an inhibitory postsynaptic potential; and (4) that the inhibitory post-synaptic potential may be linked with, or be the agent for, inhibition of soma response.