Monosynaptic excitatory and inhibitory pathways from medial midbrain nuclei to trochlear motoneurons.

Stimulation of both an ipsi and contralateral medial midbrain area in pentobarbital anesthetized cats evoked an EPSP-IPSP sequence in trochlear motoneurons (TMn). The latencies of both the EPSP and IPSP were in the monosynaptic range. Experiments on chronic medial longitudinal fasciculus sectioned cats showed that the TMn effects were not produced by collaterals of ascending vestibular axons antidromically excited from the medial midbrain. Stimulation of the same area had no effect in abducens motoneurons, but produced EPSPs and spikes in reticular neurons just ventral to the VI nucleus. The effective area for producing TMn PSPs, as delimited by multiple stimulation electrode arrays, extended only over the interstitial nucleus of Cajal-nucleus of Darkschewitsch area (IN). The effective bilateral IN areas appeared homogeneous in their ability to produce both the EPSP and IPSP components in TMns, even at minimum stimulus strength. It was concluded that the IN area is the origin of monosynaptic excitatory and inhibitory pathways to TN, and it was discussed how the IN has the synaptology required of an integration center for reciprocal control of conjugate vertical and rotational eye movements. [ABSTRACT FROM AUTHOR]