Expiratory effects of cerebellar stimulation in developing opossums.

Positive-pressure breathing (PPB) causes expiratory activation of abdominal (ABD) and intercostal (IC) muscles in anesthetized opossums. Developmentally these responses begin to appear at about the 30th postnatal day and become prevalent over the next 3-4 wk. The purpose of the present study was to examine effects of constant-current cerebellar stimulation, applied over a single breath, on respiratory timing and expiratory electromyogram (EMG) activity in ABD and IC muscles during PPB in anesthetized opossums from 30-35 days of age to weaning. Electrode placements in the region of the deep cerebellar nuclei showed strong expiration-phased effects. For animals showing expiratory activation of both ABD and IC muscles during PPB, the response at low-current density consisted of suppression of the IC and ABD EMGs; relative inhibition often differed, with reduction of the ABD EMG being greater than for IC muscles. In animals of all ages tested, cerebellar stimulation prolonged the expiratory phase of the stimulated breath. Frequency of cerebellar stimulation needed to achieve maximum prolongation of the breath was significantly lower for younger vs. older animals. Pathways from deep cerebellar nuclei may differentially influence the activity of motor groups with comparable respiratory functions; this may be related to a cerebellar function of coordinating respiratory and nonrespiratory (i.e., movement and posture) functions of specific motor groups.