Motor responses to positive-pressure breathing in the developing opossum

The suckling opossum exhibits an expiration-phased discharge in abdominal muscles during positive-pressure breathing (PPB); the response becomes apparent, however, only after the 3rd-5th wk of postnatal life. The purpose of this study was to determine whether the early lack of activation represented a deficiency of segmental outflow to abdominal muscles or whether comparable effects were observed in cranial outflows to muscles of the upper airways due to immaturity of afferent and/or supraspinal pathways. Anesthetized suckling opossums between 15 and 50 days of age were exposed to PPB; electromyogram (EMG) responses in diaphragm and abdominal muscles were measured, along with EMG of larynx dilator muscles and/or upper airway resistance. In animals older than approximately 30 days of age, the onset of PPB was associated with a prolonged expiration-phased EMG activation of larynx dilator muscles and/or decreased upper airway resistance, along with expiratory recruitment of the abdominal muscle EMG. These effects persisted as long as the load was maintained. Younger animals showed only those responses related to the upper airway; in fact, activation of upper airway muscles during PPB could be associated with suppression of the abdominal motor outflow. After unilateral vagotomy, abdominal and upper airway motor responses to PPB were reduced. The balance between PPB-induced excitatory and inhibitory or disfacilitory influences from the supraspinal level on abdominal motoneurons and/or spinal processing of information from higher centers may shift toward net excitation as the opossum matures.