The craniofacial muscles: Arguments for uniqueness

The craniofacial muscles are small skeletal muscles associated with head and neck structures and involved in a wide array of non-locomotor activities such as mastication, swallowing, breathing, vocalization, facial expression, and even vision and other special senses. These muscles are the new kids in the block, starting with their relatively recent appearance with the evolution of the head and neck in vertebrates and to our growing understanding of their distinctive development programs, functions, and pathologies. For convenience, we can group the craniofacial muscles according to their developmental origin: extraocular muscles, branchiomeric muscles (facial, masticatory, pharyngeal, and laryngeal muscles), and tongue muscles (Noden and Francis-West 2006). There is growing recognition of clinical relevance of the craniofacial muscles in terms of diseases that are specific to them (strabismus, laryngeal dystonias, facial paralysis, and many others), but also their characteristic divergent response to certain systemic neuromuscular disorders (sparing by some muscular dystrophies, targeting by myasthenia gravis, to name a few). These are the basic arguments for the uniqueness of the craniofacial muscles that serve as the central theme for the following chapters.