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Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation

¹Department of Life Science, National Taiwan Normal University, Taipei, Taiwan, Republic of China; and ²Department of Physical Therapy, University of Florida, Gainesville, Florida

Submitted 11 August 2005 ; accepted in final form 30 August 2006

ABSTRACT

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIIMED) and retractor muscles (lateral branch; XIILAT); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIIMED, and XIILAT neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 µg/kg but augmented bursting at 5 µg/kg. All doses reduced inspiratory amplitude in XIIMED and XIILAT (P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate (n = 101/153) or become silent (n = 39/153). Capsaicin also altered temporal characteristics such that both XIIMED and XIILAT inspiratory burst onset occurred after the phrenic burst (P < 0.05). Increases in tonic discharge after capsaicin were greater in XIIMED vs. XIILAT (P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.

hypoglossal motoneurons; rat

This article has been cited by other articles:

				K.-Z. Lee, P. J. Reier, and D. D. Fuller Phrenic Motoneuron Discharge Patterns During Hypoxia-Induced Short-Term Potentiation in Rats J Neurophysiol, October 1, 2009; 102(4): 2184 - 2193. [Abstract] [Full Text] [PDF]

				K.-Z. Lee, D. D. Fuller, I-J. Lu, L.-C. Ku, and J.-C. Hwang Pulmonary C-fiber receptor activation abolishes uncoupled facial nerve activity from phrenic bursting during positive end-expired pressure in the rat J Appl Physiol, January 1, 2008; 104(1): 119 - 129. [Abstract] [Full Text] [PDF]