Journal of Applied Physiology, Vol 71, Issue 5 1723-1728, Copyright © 1991 by American Physiological Society
Inhomogeneous response of expiratory muscle activity to cold block of the ventral medullary surface
T. Chonan, S. Okabe, W. Hida, T. Izumiyama, Y. Kikuchi and T. Takishima
First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan.
We assessed the effects of cooling the ventral medullary surface (VMS) on
the activity of chest wall and abdominal expiratory muscles in eight
anesthetized artificially ventilated dogs after vagotomy and denervation of
the carotid sinus nerves. Electromyograms (EMGs) of the triangularis
sterni, internal intercostal, abdominal external oblique, abdominal
internal oblique, and transversus abdominis muscles were measured with EMG
of the diaphragm as an index of inspiratory activity. Bilateral localized
cooling (2 x 2 mm) in the thermosensitive intermediate part of the VMS
produced temperature-dependent reduction in the EMG of diaphragm and
abdominal muscles. The rib cage expiratory EMGs were little affected at 25
degrees C; their amplitudes decreased at lower VMS temperatures (less than
20 degrees C) but by significantly fewer degrees than the diaphragmatic and
abdominal expiratory EMGs at a constant VMS temperature. With moderate to
severe cooling (less than 20 degrees C) diaphragmatic EMG disappeared, but
rib cage expiratory EMGs became tonic and resumed a phasic pattern shortly
before the recovery of diaphragmatic EMG during rewarming of the VMS. These
results indicate that the effects of cooling the VMS differ between the
activity of rib cage and abdominal expiratory muscles. This variability may
be due to inhomogeneous inputs from the VMS to expiratory motoneurons or to
a different responsiveness of various expiratory motoneurons to the same
input either from the VMS or the inspiratory neurons.
