Proprioception is a quality of sensibility that originates in specialized sensory organs (proprioceptors) that inform the central nervous system about static and dynamic conditions of muscles and joints. The facial muscles are innervated by efferent motor nerve fibers and typically lack proprioceptors. However, facial proprioception plays a key role in the regulation and coordination of the facial musculature and diverse reflexes. Thus, facial muscles must be necessarily supplied also for afferent sensory nerve fibers provided by other cranial nerves, especially the trigeminal nerve. Importantly, neuroanatomical studies have demonstrated that facial proprioceptive impulses are conveyed through branches of the trigeminal nerve to the central nervous system. The multiple communications between the facial and the trigeminal nerves are at the basis of these functional characteristics. Here we review the literature regarding the facial (superficial) communications between the facial and the trigeminal nerves, update the current knowledge about proprioception in the facial muscles, and hypothesize future research in facial proprioception.
Hemimasticatory spasm is characterized by involuntary spasm of jaw closure due to contraction of unilaterally hypertrophied temporalis and masseter muscles (Thompson and Carroll, 1983a) and occasionally jaw deviation due to additional involvement of medial pterygoid muscle (Kumar et al., 2008). The spasms are triggered by chewing or talking and often result in biting the tongue and cheek. The combination of masticatory spasm, facial hemiatrophy, and linear scleroderma or en coup de sabre is referred to as the Parry–Romberg syndrome (Kaufman, 1980; Thompson et al., 1986; Ebersbach et al., 1995; Kim et al., 2000). Neurophysiological studies reveal high-frequency discharges, afterdischarges, and absence of the ipsilateral masseter stretch reflex, suggesting a peripheral nerve hyperexcitability syndrome due to a trigeminal sensorimotor neuropathy (Thompson and Carroll, 1983a, b; Thompson et al., 1986; Auger et al., 1992; Cruccu et al., 1994; Esteban et al., 2002; Mir et al., 2006).
Masticatory spasm has also been observed in brainstem lesions due to multiple sclerosis (Thompson et al., 1986) or brainstem infarction (Krasnianski et al., 2003; Gunduz et al., 2007) and Foix–Chavany–Marie syndrome (acquired biopercular syndrome) (Jiménez-Jiménez et al., 2008), presumably related to disordered central control of masticatory motor neurones.
Unilateral jaw dystonia with lateral jaw deviation may occur as an isolated manifestation of primary cranial dystonia (Thompson et al., 1986) and unilateral jaw closure has been observed as a manifestation of tardive dystonia following metoclopramide use (Friedman, 1986).
Dystonic jaw tremor characterized by vertical or complex jaw motion at frequencies of 4–6 Hz may occur in cranial and segmental primary dystonia (Schneider and Bhatia, 2007). Jaw tremor is also seen in Parkinson's disease and severe essential tremor (Bain et al., 1994; Louis et al., 2006).
Focal jaw tremor (frequency 5 Hz) can be position-specific and apparent only during partial mouth opening (Tarsy and Ro, 2006) or task-specific when drinking from a glass (Miles et al., 1997). A rapid vertical jaw tremor with frequency 14–16 Hz causing paroxysms of involuntary teeth chattering on jaw closure was reported in two patients (Schrag et al., 1999; Gonzalez-Alegre et al., 2006). The tremor was suppressed briefly by jaw opening or clenching the teeth. Coherence analysis of a comparable rapid postural jaw tremor revealed 12 and 15 Hz tremor frequencies, depending on jaw posture (Sowman et al., 2008). Similar high-frequency jaw tremor was described in association with orthostatic limb and trunk tremor (Köster et al., 1999). Postural hand tremor at slower frequencies and voice tremor were observed even in cases of apparently isolated 14–16 Hz jaw tremor (Schrag et al., 1999; Gonzalez-Alegre et al., 2006).
Involuntary jaw movements are frequently seen as part of edentulous and tardive dyskinesias, task-specific embouchure dystonia (Frucht et al., 2001; Ragothaman et al., 2007; Díaz-Sánchez et al., 2008), and secondary dystonic syndromes (Tarlaci, 2004; Fabbrini et al., 2009).