Distribution of primary afferent fibers projecting from hindlimb cutaneous nerves to the medulla oblongata in the cat and rat

The dorsal column nuclear complex, one of the most important relays for tactile perception, has well been known to be somatotopically organized. Topographical arrangements of terminal sites of individual cutaneous nerves within the dorsal column nuclei, however, have not been examined systematically, although many studies have been done upon primary afferents to the medulla oblongata, including the dorsal column nuclear complex. Thus, in the present study, distribution of primary afferent fibers projecting from the hindlimb cutaneous nerves to the medulla oblongata was examined in the cat and rat by means of the transganglionic transport method with horseradish peroxidase. Cutaneous primary afferent fibers projecting from the hindlimb to the medulla oblongata were distributed mainly in the ipsilateral gracile nucleus. Terminal labeling in the gracile nucleus was seen at all rostrocaudal levels of the nucleus, occasionally including the nuclear part straddling the midline (the median or accessory nucleus). The labeled axon terminals in the gracile nucleus were more densely distributed in the middle and caudal parts of the nucleus than in the rostral part. Although the fields of termination of the hindlimb cutaneous nerves overlapped in the gracile nucleus, the foci of the terminal labeling of the nerves innervating the distal parts of the hindlimb were located more medially or dorsomedially than those of the nerves innervating the proximal parts. Terminal labeling was further found in a small zone immediately medial to the rostromedial border of the external cuneate nucleus. This hitherto undescribed zone (U zone) contained a small cluster of medium‐sized neurons in the cat. Although no particular cell cluster was found in the U zone of the rat, convergence of the primary afferent fibers of the cutaneous nerve from the hindlimb appeared to occur as in the U zone of the cat.