Sensory receptors in the sheep's foot.

1. Median and ulnar nerves of anaesthetized Soay sheep have been investigated in vivo.2. The compound action potentials of these nerves indicate that the most rapidly conducting fibres had mean velocities of 87 m/sec in the median nerve and 57 m/sec in the ulnar nerve. The mean conduction velocities of the axons responsible for the Group A gammadelta wave in the median nerve were 23-30 m/sec and those responsible for the Group A delta wave in the ulnar nerve conducted at 23-26 m/sec.3. Single myelinated afferents were isolated by microdissection of median and ulnar nerves in the metacarpal region, and action potentials were recorded from hair follicle units, cutaneous mechanoreceptors, deeply situated proprioceptors, and from units of unknown function which were spontaneously active.4. The hair follicle units fell into two categories: ;diffuse' units which innervated many hairs in an area of about 1-3 cm(2) anywhere on the hairy skin above the hoof, and ;localized' units which innervated a few hairs within small areas of the skin adjacent to the hoof. The ;diffuse' units generally resembled the ordinary hair follicle units of other animals except that in the sheep the afferents had higher conduction velocities. The ;localized' units also had large myelinated axons, but their physiological properties differed in several aspects from the guard hair, tylotrich and carpal hair units which have been studied in other animals.