MARSHALL, C.D.*; HERRING, S.W.: Motor Control of Perioral Muscles: Somatotopic Organization in the Pig Facial Nucleus.
The lips and cheeks of mammals form a complex structure of constant volume that appears to function as a muscular hydrostat. The muscles involved, primarily orbicularis oris and buccinator, are part of the facial musculature innervated by CN VII. Their fine motor control is likely accompanied by specialized innervation. The degree of perioral muscle elaboration is related to food acquisition and processing. For example, the lips of herbivores (e.g., guinea pigs) are well developed and complex compared to carnivores (e.g., felids). The size and organization of the facial nucleus may reflect the behavioral use of perioral muscles. The lips and cheeks of pigs provide an intermediate model of a mammalian muscular hydrostat in which the innervation can be investigated. We conducted a retrograde nerve tracing study of CN VII in pigs to determine (1) the location and morphology of the facial motor nucleus, (2) the location of motor neuron pools that innervate perioral muscles, and (3) the organization of these motor pools. Lips and cheeks were injected with fluorescent neurotracers. Subjects survived for 7 days to allow retrograde transport of tracers. The pig facial nucleus is located in the ventral pontine tegmentum and is comprised of 7 subnuclei. Perioral muscle motor neuron pools are found in 3 of the 7 subnuclei and, in general, are segregated from motor neurons innervating other facial muscles. These motor pools are organized somatotopically; neurons within the lateral, ventral lateral, and ventral intermediate subnuclei innervate the superior orbicularis oris, inferior orbicularis oris, and buccinator muscles respectively. The size and organization of the pig facial nucleus lie between those of guinea pigs and cats.