Projections of primary afferent fibres from muscle to neurones of the spinocervical tract of the cat.

1. Micro-electrode recordings were made from axons in the spinocervical tract of decerebrate-spinal cats. 2. The effect of electrical stimulation of afferent fibres in muscle nerves was examined. 3. Stimulation of group III fibres in muscle nerves excited hair only and hair and pressure units in the spinocervical tract. Hair and pressure units also responded to electrical stimulation of group IV fibres. 4. Units of the spinocervical tract excited by stimulation of muscle nerves had receptive fields in the skin overlying the relevant muscles. 5. Stimulation of group III fibres in muscle nerves produced long lasting inhibition of spinocervical tract units. In two such units the inhibitory action of afferent myelinated fibres was enhanced by increasing the stimulus strength so that group IV fibres were recruited.