Enzyme-histochemical and morphological characteristics of muscle fibre types in the human buccinator and orbicularis oris.

Human masticatory muscles, originating from the first branchial arch and innervated by the trigeminal nerve, have a fibre composition distinct from that of limb and trunk muscles. The zygomatic muscles, originating from the second branchial arch and innervated by the facial nerve, differ in fibre composition from either the masticatory or the limb and trunk muscles. To elucidate further the structural basis for function, and the influence of embryological origin and innervation on oro-facial muscles, the buccinator and orbicularis oris muscles, which originate from the second branchial arch and are innervated by the facial nerve, were investigated. Like the masticatory and zygomatic muscles, they have a large representation in the cerebral cortex. Both muscles were composed of type I, type IIA and a few type IIC fibres of about equal diameter. However, the type I fibres had a different myofibrillar ATPase reaction from those in masticatory, zygomatic, limb and trunk muscles; this was a moderate to strong staining at pH 9.4, indicating a special isomyosin composition. Whereas the buccinator was composed of 53% type I fibres, the orbicularis oris had a 71% predominance of type II fibres. In both muscles, the mean fibre diameter and its marked intramuscular variability were similar to earlier findings in the zygomatic muscles. No muscle spindles were found. The large number of type I fibres in the buccinator implies a capacity for endurance during continuous work at relatively low levels of force. The predominance of type II fibres in the orbicularis oris indicates that it is built up of fast-twitch motor units, related to properties such as rapid acceleration and high speed during intermittent oro-facial movements. The similarities and differences in fibre-type composition between the facial, masticatory and limb muscles imply that specific functional demands are of greater importance for muscle differentiation than embryological origin and nerve supply.