DESCENDING PROPRIOSPINAL AXONS IN THE HINDLIMB ENLARGEMENT OF THE RED-FARED TURTLE - CELLS OF ORIGIN AND FUNICULAR COURSES

Spinal neurons with descending axons are important components of spina l sensorimotor networks. We used an anatomical tracing technique to st udy the distribution of descending propriospinal axons and cell bodies in red-eared turtles. We injected horseradish peroxidase into a porti on of one funiculus in the middle of the hindlimb enlargement and exam ined six spinal segments rostral to the injection site (dorsal 3 throu gh dorsal 8) for labeled neuronal cell bodies. Injections into each re gion of the white matter labeled substantial numbers of descending pro priospinal neurons. Each injection labeled cell bodies over most of th e six spinal segments examined. Each injection also labeled cell bodie s in the ipsilateral dorsal horn, intermediate zone, and ventral horn as well as the contralateral intermediate zone and ventral horn. Injec tions into each of four regions of the white matter, the dorsal funicu lus, the medial part of the lateral funiculus, the lateral part of the lateral funiculus, and the ventral funiculus reliably gave rise to a distinct distribution of labeled cell bodies. These experiments establ ish that descending propriospinal axons in red-eared turtles are found in all regions of the spinal white matter. This finding contrasts wit h a popular contemporary view of the organization of descending propri ospinal axons in mammals. These experiments also demonstrate that neur ons in each region of the gray matter give rise to a different distrib ution of descending, funicular axons, although these distributions are widely overlapping. Different funicular axon distributions could be a ssociated with different sets of synaptic contacts with the white-matt er dendrites of spinal neurons. (C) 1994 Wiley-Liss, Inc.