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

1. Intracellular injections of horseradish peroxidase were made in a functionally identified population of motoneurones in spinal cords of cats. These motoneurones were activated by tactile stimulation of the hind-limb central foot pad. 2. Cell bodies of twenty-two such motoneurones were located in the dorsolateral portion of the ventral horn in the first sacral segment. The mean diameter of the major axis of transverse sections through twelve of these cell bodies was 68 . 2 micrometer, the mean diameter of the minor axis was 48 . 7 micrometer. The major axis tended to be oriented dorsomedially-ventrolaterally. 3. 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 seventeen motoneurones with well stained dendrites, the mean medial spread of the dendrites was 960 micrometer. Though the mean lateral spread was only 508 micrometer, all of these motoneurones sent dendritic projections into the lateral white matter. The mean dorsal spread of the dendrites was 693 micrometer, the mean ventral spread, 748 micrometer. In the rostrocaudal direction, the mean spread rostrally was 911 micrometer, the mean spread caudally was 998 micrometer. The maximum dendritic spread for a single motoneurone was 2,940 micrometer, in the rostro caudal direction. The sum of dendritic lengths over an entire dendritic tree for the best-stained motoneurones exceeded 13,000 micrometer. 4. The mean diameter of the initial segment of axons of nineteen motoneurones was 4 . 3 micrometer. These axons were notable for the lack or paucity of axon collaterals. Only five of twenty-one axons possessed collaterals; of these, only one possessed more than a single collateral system. This sparseness of the collateral system was reflected in a low level of recurrent inhibition. 5. A possible relationship is discussed between the prominent dorsomedially oriented dendritic bundles of the motoneurones and the axon collaterals of dorsal horn cells mediating cutaneous stimulation which can activate these motoneurones.