Responses of single units in the inferior olive to stimulation of the limb nerves, peripheral skin receptors, cerebellum, caudate nucleus and motor cortex.

1. Extra- and intracellular responses of single units in the inferior olive following stimulation of the cerebellum, limb nerves, skin receptors, the caudate nucleus and cerebral cortex have been described.2. Responses following cerebellar stimulation were assumed to be antidromic spikes when they occurred after a latency of less than 4 msec and followed stimulus frequencies greater than 220/sec. Other responses with longer latencies were thought to be transynaptic. Evidence for recurrent inhibition of the Renshaw type is given.3. Following limb nerve stimulation two types of units have been observed; units responding after a short latency and to stimulation of one limb only and units responding with a long latency following stimulation of one or more of the limbs.4. Units responding to stimulation of the limb nerve after a short latency could often be excited by hair movement and/or light touch on the pads. Other units were excited by pinching and others could not be excited by a physiological stimulus. The receptive field of these units was small and there was no evidence of fringe inhibition.5. Most units observed represented the contralateral forelimb but there was a significant number of units representing the ipsilateral limbs.6. The caudato-olivary pathway has been shown to be excitatory. Short latency limb units do not receive afferents from the caudate nucleus.7. Afferents from the motor cortex excite units in the inferior olive. A remarkable correlation of latencies of responses from both the cerebral cortex and the limbs on to individual units has been described.8. The results suggest a complex organization of neurones within the inferior olive. A possible plan of the organization of neurones is given and discussed.