Recording single motor unit activity of human nasal muscles with surface electrodes: applications for respiration and speech.

A method is presented for recording nasal single motor unit (SMU) potentials from the skin surface using a 3-pole 'branched' bipolar electrode. Stable, high-quality recordings of single motor unit activity were obtained for up to 3 h. Branched electrode arrays were capable of locating an SMU's maximal voltage point within 5 mm. We examined nasal SMU discharge patterns in relation to respiration in 9 adult humans. The majority of SMUs which discharged during quiet breathing began firing late in expiration and ceased firing in mid-inspiration, other SMUs discharged only during expiration, and a few fired continually with frequency modulation during breath cycles. With increased ventilation, new SMUs were recruited, and previously active SMUs increased the frequency and duration of their discharge. We examined the discharge of 13 units (5 adults) which discharged during speech but were never active during quiet or moderately increased breathing. Some of these SMUs fired during production of nasal consonants, and others were active for articulations involving facial movements (bilabial stops, labio-dental fricatives, and vowels produced with lip movement). By providing information about motor neuron recruitment which cannot be obtained from gross EMG recordings, surface recording of unit potentials may be useful in studying the central nervous control of the nasal upper airway, face, and neck for respiration and speech.