Phenotype and contractile properties of mammalian tongue muscles innervated by the hypoglossal nerve.

The XIIth cranial nerve plays a role in chewing, respiration, suckling, swallowing, and speech [Lowe, A.A., 1981. The neural regulation of tongue movements. Prog. Neurobiol. 15, 295-344.]. The muscles innervated by this nerve are functionally subdivided into three categories: those causing protrusion, retrusion, and changing the shape of the tongue. Myosin heavy chain (MHC) II isoform makes up the majority of the MHC phenotype with some variability among mammalian species and some evidence suggests between genders. In addition, there are regional differences in fiber type within some of these muscles that suggest functional compartmentalization. The transition from developmental MHC isoforms to their adult phenotype appears to vary not only from muscle to muscle but also from species to species. Motor units within this hypoglossal motor system can be categorized as predominantly fast fatigue resistant. Based on twitch contraction time and fatigue index, it appears that hypoglossal innervated muscles are more similar to fast-twitch muscles innervated by spinal nerves than, for example, extraocular muscles.