Differential recruitment of expiratory muscles during opossum development.

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Relationships between time-averaged electromyogram (EMG) discharge in abdominal (ABD) and lateralintercostal (IC) muscles (interspaces 1-10) were evaluated when Inactin-anesthetized supine opossums between 20 and 100 days of age were challenged by positive-pressure breathing (PPB) (3, 6, and 8 cmH2O). Expiratory activity upon initiation of PPB was observed in ABD muscles after the 30th postnatal day; recruitment of IC muscles requires further maturation. For example, at a PPB level of 6 cmH2O, animals up to 50-55 days of age recruited IC muscles from the lowest three interspaces on 44% of the occasions where ABD muscles were activated; in older animals the incidence of IC recruitment increased to 87%. Additionally, the occurrence of IC relative to ABD recruitment increased with increasing levels of PPB; IC muscles of highest interspaces (1-3) were activated less often at all pressures. At the onset of PPB, ABD muscles were usually recruited before IC muscles; this effect was more prominent in the younger animals. During the 5th min of PPB, ABD muscles were recruited earlier in expiration than IC muscles in the majority of animals at all ages. Since IC and ABD motor groups are activated from similar levels of the spinal cord, the delayed maturation of IC muscle responses to PPB may reflect developmental processes involving reflexes from the chest or abdomen, and/or may be a function of nonrespiratory utilization (i.e., postural) of ABD and IC muscles.