Control of abdominal muscles.

Abdominal muscles serve many roles; in addition to breathing, especially at higher levels of chemical drive or at increased end-expiratory lung volumes, they are responsible for, or contribute to, such protective reflexes as cough, sneeze, and vomiting, generate the high intra-abdominal pressures necessary for defecation and parturition, are active during postural adjustments, and play an essential role in vocalization in many species. Despite this widespread involvement, however, their control has, with rare exceptions, received little attention for two major reasons. First, in most anesthetized or decerebrate preparations, they are relatively inactive at rest, in part because the position of the preparation (supine or prone with abdomen supported), reduces lung volume and, therefore, their activity. Second, unlike phrenic motoneurons innervating the diaphragm, identification of motoneurons to a particular abdominal muscle is difficult. At the lumbar level, a given motoneuron may innervate any one of the four abdominal muscles; at the thoracic level, they are also intermixed with those innervating the intercostals. The two internal muscles, the internal oblique and the transverse abdominis, respond more to increases in chemical or volume-related drive than the two external muscles, the rectus abdominis and external oblique; the basis for this differential sensitivity is unknown. Segmental reflexes at the thoracic and lumbar levels are sufficient to activate abdominal motoneurons in the absence of descending drive but the basis for these reflex effects is also unknown. Neuroanatomical experiments demonstrate many more inputs to, and outputs from, the nucleus retroambigualis, the brainstem region in which the premotor neurons are located, than can be accounted for by their respiratory role alone. These other connections likely subserve activities other than respiration. Studies of the multifunctional roles of the abdominal muscles, on the basis of recent work, hold considerable promise for improving our understanding of their control.