Nucleus Z, the medullary relay in the projection path to the cerebral cortex of group I muscle afferents from the cat's hind limb.

1. The medullary relay of hind limb muscle afferents was studied in cats anaesthetized with chloralose. Evoked potentials were recorded from the dorsal surface of the medulla oblongata and from the depth of the medulla with penetrating micro-electrodes. Graded electrical stimulation of the muscle nerves was used. The afferent volley was monitored by recordings from the lumbar dorsal roots or from the peripheral nerves.2. Group I muscle afferents from the ipsilateral quadriceps, posterior biceps-semitendinosus and gastrocnemius-soleus nerves evoked potentials in the nucleus Z of Brodal & Pompeiano (1957a). Focal and unitary potentials were recorded within 2.7-3.8 mm rostral to the obex and 2.8-4.1 mm lateral to the mid line. The recording positions were checked histologically.3. Focal and unitary responses appeared at threshold strengths of the afferent volleys. The latencies of the focal potentials varied between 4.5 and 7.0 msec, those of the unitary ones between 5.1 and 11.0 msec. Group II muscle afferents and low threshold cutaneous afferents also projected to nucleus Z.4. An antidromic discharge was evoked in the relay cells of nucleus Z by electrical stimulation of the contralateral thalamus.5. A lesion of nucleus Z abolished the response evoked in the contralateral post-sigmoid gyrus of the cerebral cortex by the Group I muscle afferents of the hind limb.6. A superficial transection of the ipsilateral dorsolateral fascicle at the 1st cervical segment abolished the potentials evoked in nucleus Z.7. Results of lumbar cord transections showed that most of the primary afferents of the Group I hind limb path supplying nucleus Z enter the dorsolateral fascicle at L3 level.8. The relation of the Group I hind limb path to the dorsal spinocerebellar tract is discussed.