Respiratory-related discharge of genioglossus muscle motor units.
BACKGROUND
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.


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


METHODS
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).


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.
John
Jooby
J
Department of Physiology, Gittings Building, College of Medicine, University of Arizona, Tucson, Arizona 85721, USA.
Bailey
E Fiona
EF
Fregosi
Ralph F
RF
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D004576:Electromyography; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D007002:Hypoglossal Nerve / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014059:Tongue / Q000294:innervation*