Nucleus z in the rat: spinal afferents from collaterals of dorsal spinocerebellar tract neurons.

Proprioceptive information from the hindlimb of the cat is now known to be relayed to the somatosensory thalamus and cortex via axons in the dorsolateral fasciculus and a medullary relay in nucleus z. The aim of this study was to identify nucleus z in the rat, to locate the cells of origin of spinal afferents to nucleus z, and to determine whether they are collaterals of the dorsal spinocerebellar tract. The location and extent of nucleus z were studied by filling the axon terminals of collaterals of the dorsal spinocerebellar tract (dsc) with horseradish peroxidase (HRP), which was injected into the inferior cerebellar peduncle. Nucleus z in the rat was found to be similar in location to nucleus z in other mammals. It was located just below the dorsal surface of the medulla, bounded laterally by the rostral pole of the cuneate nucleus and medially by the nucleus of the solitary tract. The cells of origin of the spinal afferents to nucleus z were studied by using the retrograde transport of HRP. They were located in Clarke's column (dorsal nucleus) and in lamina 10 of the dorsal horn. They were similar in location and morphology to neurons giving rise to the dorsal spinocerebellar tract, but were smaller in average diameter. A double retrograde labeling technique was used to determine whether the spinal afferents to nucleus z are collaterals of neurons giving rise to the dsc. It was estimated that up to 92% of the spinal afferents to nucleus z were collaterals of dsc neurons, while approximately 3% of all dsc neurons gave rise to collaterals terminating in nucleus z.