Morphology of spinal motoneurones mediating a cutaneous spinal reflex in the cat.

Abstract

Intracellular injections of horseradish peroxidase were made in a functionally identified population of motoneurons in spinal cords of cats. These motoneurons were activated by tactile stimulation of the hind-limb central foot pad. Cell bodies of 22 such motoneurons were located in the dorsolateral portion of the ventral horn in the 1st sacral segment. The mean diameter of the major axis of transverse sections through 12 of these cell bodies was 68.2 .mu.m, the mean diameter of the minor axis was 48.7 .mu.m. The major axis tended to be oriented dorsomedially-ventrolaterally. In the transverse plane, the dendrites had a characteristic configuration, with a prominent group of dendrites travelling from the cell body dorsomedially into the dorsal horn, entering Rexed''s lamina VI. For 17 motoneurons with well stained dendrites, the mean medial spread of the dendrites was 960 .mu.m. Though the mean lateral spread was only 508 .mu.m, all of these motoneurons sent dendritic projections into the lateral white matter. The mean dorsal spread of the dendrites was 693 .mu.m, the mean ventral spread, 748 .mu.m. In the rostrocaudal direction, the mean spread rostrally was 911 .mu.m and the mean spread caudally was 998 .mu.m. The maximum dendritic spread for a single motoneurons was 2940 .mu.m in the rostrocaudal direction. The sum of dendritic lengths over an entire dendritic tree for the best-stained motoneurons exceeded 13,000 .mu.m. The mean diameter of the initial segment of axons of 19 motoneurons was 4.3 .mu.m. These axons were notable for the lack or paucity of axon collaterals. Only 5 of 21 axons possessed collaterals; of these, only 1 possessed more than a single collateral system. This sparseness of the collateral system was reflected in a low level of recurrent inhibition. A possible relationship is discussed between the prominent dorsomedially oriented dendritic bundles of the motoneurons and the axon collaterals of dorsal horn cells mediating cutaneous stimulation which can activate these motoneurons.