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 ± 12 ms. 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.