Propriospinal neurones as a relay system for transmission of cortico-spinal influences.

1. Synaptic organization and transmission have been studied in the lateral group of short propriospinal neurones of the lumbar and cervical regions of the cat spinal cord. Special attention was paid to their role in the transmission of cortico-spinal volleys. 2. The majority of these neurones are mono- or oligosynaptically excited after pyramidal tract stimulation. Convergence of excitatory actions from rubrospinal and lateral reticulospinal tracts was typical for these cells. Neurones with relatively low-level and delayed effects from segmental afferents are frequent in this population. 3. Temporal summation is important for the transmission of descending vlleys through these neurones. Mutual excitatory and recurrent inhibitory connections are supposed to play a substantial role in their function. 4. Possible participation of the short lateral propriospinal system in the transmission, transformation and re-distribution of corticofugal signals to the segmental spinal mechanisms is discussed.