ELECTROPHYSIOLOGICAL STUDIES OF CEREBELLAR INFLOW: I. ORIGIN, CONDUCTION AND TERMINATION OF VENTRAL SPINO-CEREBELLAR TRACT IN MONKEY AND CAT

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Abstract
Electrical activity evoked by stimulation of peripheral nerves was used to provide information regarding the mode of activation, the site of origin and the course of the V.S.C.T. up to its termination in the cerebellum. The tract is activated by skin afferents as well as those from muscles, and from ipsilateral and contralateral nerves. The tract from the lumbar cord derives from the nucleus anteromarginalis which is innervated by crossed as well as uncrossed primary collaterals. The response of the cells arises after a synaptic delay of about 0.9 msec. Since the axons of the cells then cross to ascend in the anterolateral column of the opposite side, the V.S.C.T. from the lumbar spinal cord is primarily once-crossed and contralateral to the peripheral initiating activation, but also contains a twice crossed, ipsilateral component. The latter is small in the lumbar derivation of the V.S.C.T. but is predominant in its cervical derivation. The cells forming the final common path of the tract are activated by different peripheral pathways, giving rise to interactive effects of these pathways. The nucleus also appears to be endowed with complex interneuronal circuits. The fibers of the tract have conduction velocities of 30-80 m.p.s., and the tract probably comprises most though not all of the larger fibers (5 to 14u) in the antero-lateral column. At the superior cerebellar peduncle the lumbar component of the tract occupies a layer about 0.6 mm. thick under a similarly thin superficial layer which is the cervical component. The lumbar component of the V.S.C.T. terminates in the lobulus centralis and first two folia of the culmen, supplying the whole vermian portion and the medial half of the hemispheral portion ipsilateral to the peripheral nerve stimulated. The cervical component supplies the entire ipsilateral vermian and hemispheral parts of the rest of the culmen. The V.S.C.T. therefore supplies essentially the same cerebellar area as does the dorsal spino-cerebellar tract, and to do this its fibers recross in the cerebellum. The activity brought to the cerebellum in the V.S.C.T. arrives about 1-1.5 msec. later than activity supplied by the D.S.C.T. Together, these responses form Potential I, the first of 3 action sequences at the cerebellum. Potential II is composed of later afferent activity as well as that arising in granule cells. The most prominent cerebellar response, the large, late Potential HI, is an intracerebellar response caused by the complex of preceding activity, initiated by the various cerebellar inflows in conjunction with activity in intracerebellar neuronal circuits. This potential is more widely distributed than are the incoming activating spinal afferents and cannot be used as an index of the somatotopic organization of inflow to the cerebellum.