Discharge characteristics of laryngeal single motor units during phonationin young and older adults and in persons with Parkinson disease

The rate and variability of the firing of single motor units in the larynge al muscles of young and older nondisordered humans and people with idiopath ic Parkinson disease (IPD) were determined during steady phonation and othe r laryngeal behaviors. Typical firing rates during phonation were similar t o 24 s/s. The highest rate observed, during a cough, was 50 s/s. Decreases in the rate and increases in the variability of motor unit firing were obse rved in the thyroarytenoid muscle of older and IPD male subjects but not fe male subjects. These gender-specific age-related changes may relate to diff erential effects of aging on the male and female voice characteristics. The range and typical firing rates of laryngeal motor units were similar to th ose reported for other human skeletal muscles, so we conclude that human la ryngeal muscles are probably no faster, in terms of their contraction speed , than other human skeletal muscles. Interspike interval OSI variability du ring steady phonation was quite low, however, with average CV of similar to 10%, with a range of 5 to 30%. These values appear to be lower than typica l values of the CV of firing reported in three studies of limb muscles of h umans. We suggest therefore that low ISI variability is a special although not unique property of laryngeal muscles compared with other muscles of the body. This conceivably could be the result of less synaptic "noise" in the laryngeal motoneurons, perhaps as a result of suppression of local reflex inputs to these motoneurons during phonation.