Respiratory-related discharge of genioglossus muscle motor units

Am J Respir Crit Care Med. 2005 Nov 15;172(10):1331-7. doi: 10.1164/rccm.200505-790OC. Epub 2005 Sep 1.

Abstract

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.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Action Potentials / physiology
  • Animals
  • Electromyography
  • Humans
  • Hypercapnia / physiopathology
  • Hypoglossal Nerve / physiology*
  • Male
  • Motor Neurons / physiology*
  • Muscle, Skeletal / innervation*
  • Rats
  • Rats, Sprague-Dawley
  • Tongue / innervation*