ASCENDING PROPRIOSPINAL MECHANISMS

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Abstract
Some aspects of mediation of ascending long spinal reflex activity from hindlimb to forelimb have been examined in spinal cats. Phasic activity along the ascending propriospinal system has little or no apparent impact upon local cervico-thoracic motor pool excitability, depending upon the number of interposed segments between the sites of afferent stimulation and efferent recording. Only after strychnine administration in sub-tetanic doses is it possible to activate a sufficient number of motoneurons within the cervical enlargement to obtain evidence of ascending propriospinal activity travelling from lumbosacral to cervical levels. An estimate of conduction velocities of the ascending and descending propriospinal systems, after strychnine has enhanced rostral and caudal spread of the intersegmental reflexes throughout the spinal cord, gives values of 13 m./sec. for ascending and 29 m./sec. for descending conduction. When evaluating the functional importance of the ascending and descending propriospinal systems it becomes evident that phasic control of spinal motor activity exerted by the descending intrinsic system is more potent. On the other hand, when comparing the tonic effects upon spinal motor activity by the intersegmental propriospinal connections, the ascending system plays a much more powerful role.