The spinal course and distribution of fore and hind limb muscle afferent projections to the superior colliculus of the cat.

1. Projections to the superior colliculus from fore and hind limb muscle nerves have been examined. 2. Hind leg nerve stimulation at strengths sufficient to excite Group II and III fibres elicited unit discharge in all layers of the superior colliculus and underlying tegmentum. 3. Forelimb nerve stimulation excited units in all layers of the superior colliculus and underlying tegmentum. Twenty per cent were activated by stimulation below Group II threshold, the remainder by stimulation above Group II threshold. 4. Most units activated by limb afferent nerve stimulation were also activated by visual stimuli. There was considerable convergence from both flexor and extensor nerves and most units were excited by stimulation of nerves in more than one limb. 5. Units fired either with a short or long latency or with a paired discharge. Short latency units were most common in deep collicular layers and the tegmentum, long latency units were most common in superficial layers. 6. The spinal pathway of the afferents is mainly in the dorsolateral quadrant contralateral to the recording site. The anatomical characteristics of the pathway are similar to the lateral funiculus climbing fibrespinocerebellar pathway.