Intrinsic organization of the rat cutaneus trunci motor nucleus.

1. We have investigated how the organization of the cutaneus trunci muscle (CTM) motor nucleus might correspond to the pattern of its physiological activation (see accompanying report, 61) by using HRP to label retrogradely the motoneurons supplying different anatomically and physiologically defined muscle compartments. The CTM motoneuron pool was found to be comprised of a tightly packed column of ventrolaterally located cells extending from caudal C6 to rostral T1 and to contain 1,183 cells per side; this unusually large number of motoneurons is consistent with the precisely localized reflex behavior exhibited by the muscle. 2. Each of three major motor nerves that functionally divide the CTM into three major longitudinal muscle fields (dorsal, lateral, ventral) was found to derive from a distinct subcolumn of motoneurons, which extends for the full rostrocaudal length of the parent CTM motor nucleus, such that the dorsal motor nerves derive from a medially located subcolumn, the lateral motor nerves from a centrally located subcolumn, and the ventral motor nerves from a subcolumn of motoneurons that sits most laterally in the CTM motor nucleus. 3. Localized injections of HRP into different rostrocaudal regions of the muscle revealed a further subdivision of the CTM motor nucleus: motoneurons located rostrally in the motoneuron pool supplied the rostral regions of the muscle, whereas more caudally located motoneurons in the pool supplied progressively more caudal sites in the muscle. 4. These results reveal the existence of both a mediolateral and a rostrocaudal subdivision of the motor nucleus; thus this intrinsic spatial organization of motoneurons relates the CTM motor nucleus topographically to the body coordinates of its target, the muscle. The possible relation of these findings to the expression of local sign in the CTM reflex, and how the underlying circuitry could develop appropriately, is discussed.