Mechanoreceptive units in the human infra-orbital nerve

Eighty-four low-threshold mechanoreceptive afferents innervating facial hairy skin or the red zone of the lip were recorded with micro-electrodes from the human infra-orbital nerve. Based on their responses to skin indentations, the units were classified as slowly or fast-adapting, with small or large receptive fields. The responses to hair movement, skin stretching and contraction of facial muscles were also studied. Both hairy skin and the red zone were innervated by slowly and by fast-adapting units. The innervation density was found to be highest at the corner of the mouth and on the upper lip. Slowly adapting units with small fields in hairy skin were most common and included units responding to sustained hair displacement. These units are suggested to have two types of end-organs, either pilo-Ruffini endings or Merkel cell-neurite complexes. The slowly adapting units with large fields were spontaneously active stretch receptors and may have corresponded to Ruffini corpuscles, although the possibility of other, intramuscular, receptors could not be ruled out. Only one afferent possibly innervated a Pacinian corpuscle. Most mechanoreceptors were also activated by skin stretching or contraction of facial muscles. Many of the slowly adapting units with small fields responded to the onset and release of stretch, whereas their discharge in response to sustained stretching adapted more or less completely. Spontaneously active units had the most sustained stretch response. It is concluded that several types of cutaneous mechanoreceptors can operate as sensitive proprioceptors of importance for facial kinaesthesia and motor control.