Inhibitory reflexes in human perioral facial muscles: a single-motor unit study

OBJECTIVE: To describe the reflex responses evoked by trigeminal stimulation in perioral facial motor units (MUs) in humans. METHODS: We recorded single motor units (MUs) from perioral muscles performing three movements: elevation of the upper lip (levator labii superioris muscle--LLS), protrusion of the lips (orbicularis oris muscle--OOr) and depression of the lower lip (depressor anguli oris and depressor labii inferioris muscles--DAO/DLI) with concentric needle electrodes. MUs were tested during constant voluntary activation with non-painful cutaneous electrical stimuli applied to the mental or supraorbital nerves and intraorally. Analysis was performed with peristimulus histograms and cumulative sum. RESULTS: Eighty MUs were sampled from 17 subjects. Cutaneous stimulation induced inhibition of discharge in 100% of the lip-depressor MUs, inhibition in 65-70% of LLS MUs and in 25% of OOr MUs. Mean latency of inhibition was of 35+/-12ms. Intraoral stimulation produced an equivalent percentage of inhibitory or facilitatory effects with no difference among the three muscles. CONCLUSIONS: Reflex responses to cutaneous stimulation identify a completely inhibitory (DAO/DLI), a mainly inhibitory (LLS) and a mixed (OOr) pattern in perioral muscles. SIGNIFICANCE: A purely inhibitory trigemino-facial reflex is present in lip-lowering muscles with potential use in clinical practice.