The short-latency projection from the baboon's motor cortex to fusimotor neurones of the forearm and hand.

1. The corticospinal connexions that are responsible for the firing of fusimotor impulses at short latency in response to brief, high-frequency stimulation of the baboon's motor cortex have been investigated by micro-electrode recording from antidromically identified fusimotor neurones of the forearm and hand.2. Of nineteen fusimotor neurones investigated by intracellular recording, six showed EPSPs at monosynaptic latency.3. In extracellular records, the latency of firing of an early fusimotor impulse was always too long to be explained by monosynaptic excitation by the corticospinal D volley, but could be explained by monosynaptic excitation from an I volley.4. Four of the nineteen intracellularly recorded fusimotor neurones showed short-latency (?disynaptic) IPSPs in response to brief high frequency cortical bursts.