ON THE MECHANISM OF THE MEDIOLATERAL GRADIENT OF PARASTERNAL ACTIVATION

Recent studies have shown that in spontaneously breathing dogs the par asternal intercostals are activated according to a mediolateral gradie nt. To assess the mechanism of this regionalization of activity, we as sessed the pattern of activation of these muscles after section of the dorsal roots and examined the topographic distribution of the muscle fiber types from the sternum to the chondrocostal junctions. The patte rn of parasternal activity after dorsal rhizotomy was similar in all r espects to that previously observed in intact animals. Thus activity i n the medial parasternal bundles at the onset of inspiration frequentl y preceded activity in the middle bundles, and no activity was recorde d from the lateral bundles. The amount of medial activity, when expres sed as a percentage of the activity recorded during supramaximal tetan ic stimulation of the internal intercostal nerve (maximal activity), w as also consistently greater than the amount of middle activity (52.6 +/- 4.6 vs. 23.1 +/- 2.6% maximal activity; P < 0.001). Furthermore, t he medial, middle, and lateral parasternal bundles had a higher propor tion of slow-twitch oxidative fibers than of fast-twitch oxidative-gly colytic fibers; no topographic difference in fiber type distribution w as observed. We conclude, therefore, that the mediolateral gradient of parasternal activity is probably due to the unequal distribution of c entral inputs throughout the pool of alpha-motoneurons.