Somatotopic Organization of Perioral Musculature Innervation within the Pig Facial Motor Nucleus

Abstract

The orbicularis oris and buccinator muscles of mammals form an important subset of the facial musculature, the perioral muscles. In many taxa, these muscles form a robust muscular hydrostat capable of highly manipulative fine motor movements, likely accompanied by a specialized pattern of innervation. We conducted a retrograde nerve-tracing study of cranial nerve (CN) VII in pigs (Sus scrofa) to: (1) map the motor neuron pool distributions of the superior and inferior orbicularis oris, and the buccinator, to test the hypothesis that perioral muscle motor neuron pools exhibit a somatotopic organization within the facial motor nucleus; and (2) test the hypothesis that portions of the superior orbicularis oris (SOO) motor neuron pool also exhibit a somatotopic organization, reflecting a potential compartmentalization of function of the rostral, middle, and caudal segments of this muscle. Cresyl violet histological staining showed that the pig facial motor nucleus was comprised of 7 well-defined subnuclei. Neuroanatomical tracers injected into these perioral muscles transported to the motor neuron pools of the lateral 4 of the 7 subnuclei of the facial motor nucleus. The motor neuron pools of the perioral muscles were generally segregated from motoneurons innervating other facial muscles of the rostrum. However, motor neuron pools were not confined to single nuclei but instead spanned across 3–4 subnuclei. Perioral muscle motor neuron pools overlapped but were organized somatotopically. Motor neuron pools of portions of the SOO overlapped greatly with each other but exhibited a crude somatotopy within the SOO motor neuron pool. The large and somatotopically organized SOO motor neuron pool in pigs suggests that the upper lip might be more richly innervated than the other perioral muscles and functionally divided.