The response of single units in the caudate nucleus to peripheral stimulation.

1. Extra- and intracellular recordings have been made of single units in the caudate nucleus of the cat, anaesthetized with alpha chloralose.2. Pentobarbitone was shown to depress the spontaneous activity and responses of units in the caudate nucleus.3. Spontaneous activity of units usually took the form of single spikes or bursts of up to four spikes occurring every 100-300 msec.4. The response to electrical stimulation of limb nerves was a burst of 1-4 spikes after a latency of about 30 msec. Convergence from all four limbs on to single units was observed. It is shown that muscle afferents do not cause these responses.5. Post stimulus histograms of spontaneously active units show, following stimulation of the limb nerves, recurring cycles of excitation and inhibition at a rate of 3-4 c/s and lasting for several seconds. It is suggested that each stimulus synchronizes the normally rhythmic spontaneous activity.6. Similar responses have been obtained following stimulation by clicks and flashes of light.7. Stimulation of the ipsilateral cerebral cortex gave excitatory and inhibitory responses similar to those described above.8. Possible mechanisms for these responses and their significance are discussed.