ACTIVITY IN THE SIMPLEST SPINAL REFLEX PATHWAYS

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Abstract
A shock which excites 1/4 to 1/2 of the a fibers of a dorsal root of the decerebrated cat typically produces in the ventral root a reflex discharge with a central delay varying between 0.65 and about 1.0 msec. Conditioning the cord by means of a previous dorsal root volley decreases the central reflex time slightly, but never below 0.5 msec. Ventral root discharges resulting from direct-electrical stimulation of the central gray matter demonstrate that the synaptic delays at the motoneurons vary over a similar range of values. Consequently central reflex times of 0.5 to about 1.0 msec. represent activity in arcs of 2 neurons (one synaptic relay). The direct excitation of the motoneurons by the primary sensory fibers offers a method of testing the synaptic excitability of the motoneurons. The motoneurons are facilitated for some time after the arrival at the cord of a volley over a contralateral dorsal root. During the period of facilitation, as can be detd. from cord leads, the motoneurons are receiving a barrage of impulses from interneurons. Facilitation, therefore, is accomplished by convergence of internuncial impulses with the impulses of the testing volley. A period of inhibition sometimes follows the facilitation. The inhibition must be attributable, at least in part, to subnormality in the few motoneurons which were fired by the crossed volley. An additional factor for inhibition may be subnormality in the neurons of internuncial systems which tonically barrage the motoneurons; for following the increased activity of the period of facilitation, the number of internuncial impulses impinging upon the motoneurons is reduced below the resting level.

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