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Respiratory-related Discharge of Genioglossus Muscle Motor Units

Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona

Correspondence and requests for reprints should be addressed to Ralph F. Fregosi, Ph.D., Department of Physiology, Gittings Building, University of Arizona, Tucson, AZ 85721. E-mail: fregosi{at}u.arizona.edu

Rationale: Little is known about the respiratory-related discharge properties of motor units driving any of the eight muscles that control the movement, shape, and stiffness of the mammalian tongue.

Objectives: To characterize the respiratory-related discharge of genioglossus motor units as synaptic drive to the hypoglossal motoneuron pool is increased with hypercapnia.

Measurements: We recorded airflow, genioglossus muscle EMG activity, and the respiratory-related discharge of 30 genioglossus muscle motor units in spontaneously breathing, urethane-anesthetized rats under control conditions and in hypercapnia (inspired CO₂: 3, 6, 9, and 12%, 3–5 min at each level).

Main Results: All motor units were active throughout all or most of inspiration. Nine of 30 units showed "preinspiratory" activity (discharge onset within the last 20% of expiration), with continued discharge into inspiration. Six inspiratory units transitioned to a preinspiratory pattern when inspired CO₂ exceeded 6%. For the majority of units (23/30), discharge rate increased with hypercapnia, with the maximum increase averaging about 50%. The average variability of interspike intervals within a spike train increased from 33% under baseline conditions to 50% with maximal hypercapnia.

Conclusions: (1) The discharge pattern of genioglossus muscle motor units can be altered by hypercapnia; (2) most, but not all, genioglossus motor units receive synaptic input from CO₂-sensitive chemoreceptors; (3) individual motor units have a wide range of CO₂ sensitivities; and (4) hypercapnia significantly increases the variability of motor unit discharge, which may enhance muscle force output.

Key Words: electrophysiology • respiratory muscles • tongue

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