The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2131-2139
Copyright ©1999 by the American Physiological Society

Discharge Characteristics of Laryngeal Single Motor Units During Phonation in Young and Older Adults and in Persons With Parkinson Disease

 ¹Department of Speech Pathology and Audiology, University of Iowa, Iowa City, Iowa 52242;  ²Department of Speech, Language, Hearing Science, University of Colorado, Boulder, Colorado 80309;  ³Wilbur James Gould Voice Research Center, Denver Center for Performing Arts, Denver, Colorado 80204; and  ⁴Division of Otolaryngology, University of Utah School of Medicine, Salt Lake City, Utah 84132

Luschei, Erich S., Lorraine O. Ramig, Kristin L. Baker, and Marshall E. Smith. Discharge Characteristics of Laryngeal Single Motor Units During Phonation in Young and Older Adults and in Persons With Parkinson Disease. J. Neurophysiol. 81: 2131-2139, 1999.Discharge characteristics of laryngeal single motor units during phonation in young and older adults, and in persons with Parkinson disease. The rate and variability of the firing of single motor units in the laryngeal muscles of young and older nondisordered humans and people with idiopathic Parkinson disease (IPD) were determined during steady phonation and other laryngeal behaviors. Typical firing rates during phonation were ~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 observed in the thyroarytenoid muscle of older and IPD male subjects but not female subjects. These gender-specific age-related changes may relate to differential effects of aging on the male and female voice characteristics. The range and typical firing rates of laryngeal motor units were similar to those reported for other human skeletal muscles, so we conclude that human laryngeal muscles are probably no faster, in terms of their contraction speed, than other human skeletal muscles. Interspike interval (ISI) variability during steady phonation was quite low, however, with average CV of ~10%, with a range of 5 to 30%. These values appear to be lower than typical values of the CV of firing reported in three studies of limb muscles of humans. 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.