Monosynaptic activation of long descending propriospinal neurons from the lateral vestibular nucleus and the medial longitudinal fasciculus.

In decerebrate cats long descending propriospinal (LDP) neurons were recorded extracellularly in the cervical enlargement. They were identified antidromically by spinal cord stimulation at the L1-L2 level. Inputs to these cells were tested by stimulating the medial longitudinal fasciculus (MLF) 5 to 6 mm rostral to the obex, the lateral vestibular nucleus (LVN), the upper MLF 1 mm caudal to the trochlear nucleus, and the medial vestibular nucleus (MVN), all on the ipsilateral side. Action potentials were elicited in 44% (64/144) of LDP neurons in the ventral horn (laminae VII, VIII) at a segmental latency of 1 ms or less following brain stem stimulation. This was considered to be a monosynaptic latency. The most effective stimulation sites were the MLF and the LVN. MLF stimulation accounted for about two-thirds of the monosynaptically elicited action potentials and LVN for about one-third. Another 22% of LDP neurons responded at longer latencies, but some of those responses may also have been monosynaptic. Stimulation of the upper MLF and the MVN were much less effective, indicating that the MLF input was predominantly from fibers originating in the medullary and/or pontine reticular formation.