Convergent effects on alpha motoneurones from the vestibulospinal tract and a pathway descending in the medial longitudinal fasciculus.

Abstract

1. Convergence of effects from the vestibulospinal tract and the pathway descending in the MLF on the same motoneurones has been investigated with intracellular recording in the lumbosacral spinal cord. The following effects from the MLP pathway are described: a) Monosynaptic EPSPs are common in knee, ankle and toe flexors as well as in hip and toe extensors. b) Disynaptic IPSPs are common in knee and ankle extensors but are also found in hip extensor and knee flexor cells. c) Disynaptic EPSPs are dominant in ankle flexors and toe extensors, but are occasionally found also in knee extensors. 2. The properties of the monosynaptic EPSP from the MLF and the vestibulospinal tract are very similar in various respects such as the amplitude, the time course and the response to repetitive stimulation. 3. Monosynaptic effects from the vestibulospinal tract and the MLF are exerted on different motor nuclei. 4. Excitatory and inhibitory effects from the vestibulospinal tract and the MLF pathway are often reciprocally organized on antagonistic motoneurones at the knee and ankle joints but not regularly for the other kinds of motoneurones investigated. 5. The physiological role of the two pathways is discussed with regard to the pattern of motor effects expected from the synaptic linkages. The possibility that the vestibulospinal tract and the MLF pathway are functionally coupled is discussed.