Monosynaptic transmission from afferents of one segment to motoneurons of other segments in the spinal cord

Monosynaptic connections between collaterals of one dorsal root fiber and motoneurons of several adjacent segments were described in 1909. In mammals, however, monosynaptic reflex activity is strictly confined to the motoneurons which supply the muscle from where the stimulated afferents originate. Activation of motoneurons subserving a synergistic muscle occurs in the cat under special conditions only. In this paper successful attempts to establish effective monosynaptic transmission from afferents of one segment to motoneurons of other segments (polysegmental monosynaptic reflex connections) in the spinal cord of frogs are reported. Experiments were designed to demonstrate the monosynaptic character of reflexes recorded from several spinal nerves on stimulation of the tenth dorsal root. Total reflex latency, afferent and efferent conduction time, central reflex time and synaptic delay were measured by classical methods. Reflex latency and synaptic delay were found to be highly dependent on temperature, but were nevertheless comparable with the values for frogs reported in the literature. Effective monosynaptic transmission always occurred from dorsal root X to ipsilateral motoneurons of segment IX, and in some cases to motoneurons of segment VIII. Polysynaptic activity was sometimes observed in ipsilateral spinal nerve VIII (when monosynaptic response was absent) and III, as well as in the contralateral spinal nerve X.