The Origin of a Descending Pathway with Monosynaptic Action on Flexor Motoneurones

The postsynaptic effects evoked in lumbar motoneurones were studied following electrical stimulation of the brain stem in the cat. The spinal cord was transected at the lower thoracic level leaving only the ipsilateral ventral quadrant intact. With a stereotactical method a low threshold focus was found in the medial brain stem from which monosynaptic EPSPs could be evoked in flexor motoneurones. It is concluded that this effect is mediated by fibres descending in the ipsilateral medial longitudinal fascicle and it is tentatively suggested that these fibres originate from the ipsilateral upper medullary or lower pontine reticular formation. Monosynaptic EPSPs were also evoked in some extensor motoneurones from this medial brain stem region and at such a strength of stimulation that stimulus escape to the lateral vestibulospinal tract is excluded.