Descending and segmental inhibition of transmission through the spinocervical tract.

1. Micro-electrode recordings were made from axons of the spinocervical tract in unanaesthetized decerebrate-spinal cats.2. The effects of stimulation of (1) descending systems at the level of the upper cervical spinal cord and (2) hind limb cutaneous nerves, on discharges of spinocervical tract neurones were examined.3. Effects were obtained from bilateral spinal cord regions in the dorsolateral funiculi and the most medial and ventral parts of the ventral funiculi and also from the dorsal columns in the upper cervical region even though the columns had been transected at low thoracic and upper lumbar levels.4. Stimulation of either descending or segmental systems inhibited spontaneous and evoked responses. Facilitation was not seen. The inhibition had a time course of up to 250 msec, with maximal action at 20-40 msec and was greatest for polysynaptic responses or those evoked from the smaller myelinated cutaneous axons.5. It is suggested that the descending and segmental systems converge on to common inhibitory interneurones.