The afferent innervation of the face of sheep and goats

The sensory innervation of the maxillary hairy skin and buccal mucous membrane was studied in anaesthetised sheep and goats. An electrophysiological technique isolated 47 single afferent units from the infraorbital nerve under chloralose or halothane anaesthesia. Mechanoreceptors of hairy skin were located in association with the following features: sinus hairs (n = 9); central primary hairs (n = 18); clear marginal hairs (n = 7) and skin-not-hair (n = 3). Units responded to hair tip displacement of 35 to 50 microm. Afferent units were also located in the mucous membrane of the cheek either associated with conical papillae (n = 8), or unassociated with papillae (n = 1). Receptor responses associated with hairs were classified as rapidly adapting (n = 18) and slowly adapting (n = 18) mechanoreceptor responses during sustained hair deflection. Seven mechanoreceptors of hairy skin and mucous membrane were excited by a fall in surface temperature. Two specific cold thermoreceptors were found: one in hairy skin and one in the mucous membrane. These units had phasic discharges during abrupt thermal depression and static discharges at constant surface temperatures. All afferent units had myelinated axons as indicated by their conduction velocities (range 20 to 57 m sec-1, mean 34 m sec-1). It is concluded that the mechanoreceptors identified had similarities with those of other mammalian species and some distinct differences. An interesting feature of mechanoreceptors in the buccal mucosa was their cold sensitivity. They therefore shared characteristics with mechanoreceptors in the penile mucosa of the ram and tongue of the sheep.