Estimation of the number and size of motor units in intrinsic laryngeal muscles using morphometric methods

The number and size of motor units in the intrinsic laryngeal muscles were estimated by morphometric methods. Laryngeal muscles with their respective nerve branches were obtained from 64 fresh cadavers (32 older than 60 years, mean age 74 +/- 9 years and 32 younger than 60 years, mean age 51 +/- 8 years). Myelinated nerve fibers and the total number of muscle fibers were counted. Motor unit size was estimated by dividing the total number of muscle fibers by the total number of motor units in each case. The mean number of motor units ranged from 268 +/- 1.3 (interarytenoid muscle) to 431 +/- 1.6 (cricothyroid muscle). Thyroarytenoid and cricothyroid muscle presented the smallest (9.8 +/- 0.2) and largest (20.5 +/- 0.9) motor unit size, respectively, suggesting that thyroarytenoid muscle has a greater capacity to fine-tune its total force compared with the other intrinsic laryngeal muscles. No differences in motor unit number or size were observed between the right and left sides or between younger and older subjects. It is suggested that synaptic rearrangements may occur at the level of the neuromuscular junction in the human larynx that may explain the age-related changes in motor units reported by clinical methods.