Musculotopic organization of muscle afferents in the external cuneate nucleus of the gerbil as revealed by transganglionic transport of horseradish peroxidase.

The projections of muscle afferents from six regions (hand, forearm, arm, thorax, shoulder and neck) to the external cuneate nucleus (ECN) of gerbils were investigated using transganglionic transport of horseradish peroxidase (HRP). These were chiefly ipsilateral projections which terminated in six discrete longitudinal columns in the ECN. A medial cell column confined to the caudal ECN received projections from the hand muscles, while the ventromedial and ventrolateral cell columns in the posterior half of the ECN received projections from the forearm and arm muscles respectively. In the caudal ECN, the thoracic and shoulder muscles were represented by the dorsomedial and dorsolateral cell columns respectively. The last mentioned two columns extended anteriorly into the rostral ECN occupying a dorsal and intermediate portion leaving the ventral portion which represented the neck muscles. The latter also extended posteriorly into the middle 1/3 of ECN occupying a lateral portion. Serial sections and reconstruction studies showed that the ECN was located in the dorsal region of the upper medulla, approximately 0.3-1.8 mm rostral to the obex. The rostrocaudal extent of the six cell columns from hand proximally to neck were measured to be 0.5-0.8, 0.3-1.1, 0.3-1.3, 0.4-1.8, 0.4-1.8 and 0.8-1.8 mm rostral to the obex.