PHYSIOLOGY OF MOTOR UNITS IN AN UPPER AIRWAY MUSCLE

The upper airway muscles of the nose, mouth, tongue and larynx control
respiratory airflow resistance, and contract rhythmically in phase with
breathing. Despite their significance, information about the physiological
properties of upper airway motor units and their role in the gradation of
muscle force is unavailable. - The proposed studies are designed to
provide fundamental information about the neurophysiology of the nasal
dilator muscle motoneurOn pool in healthy human subjects. The nasal
dilator muscle was chosen as the experimental model because it is involved
in controlling the breath-by-breath diameter of the nasal airway, it is
readily accessible in humans, and it responds to reflex-mediated
ventilatory stimuli in a manner analogous to the upper airway muscles of
the larynx, tongue and face. The Specific Aims include: l) to estimate the
number of motor units in the nasal dilator muscles; 2) to determine if the
nasal dilator motoneuron pool behaves in accordance with the Size
Principle, as occurs in limb muscle; 3) to determine the contributions of
rate modulation and motor unit recruitment to the gradation of muscle
force; 4) to determine the relationship between the neural drive to the
nasal dilator muscles, and the force produced by these muscles during
rhythmic, involuntary contractions; and to estimate the relative
contributions of motor unit recruitment and rate modulation to the
rhythmic force variations; and 5) to determine the contractile and
endurance properties of these muscles. The measurements to be made
include muscle force, single motor unit electrophysiology, muscle twitch
properties and evoked potentials, and spike-triggered average force of
single motor units in conscious human subjects. There is great clinical
interest in upper airway muscle physiology because malfunction of these
muscles often initiates or exacerbates the obstructive sleep apnea
syndrome. This syndrome is associated with hypoxia and respiratory
acidosis, acute and chronic hypertension, and often results in death in
both infants and the elderly. These studies will contribute to our
understanding of upper airway muscle physiology, and will provide the
foundation for the development of treatment strategies.