CONTROL OF ABDOMINAL MUSCLES

Abdominal muscles serve many roles; in addition to breathing, especial ly at higher levels of chemical drive or at increased end-expiratory l ung volumes, they are responsible for, or contribute to, such protecti ve reflexes as cough, sneeze, and vomiting, generate the high intra-ab dominal pressures necessary for defecation and parturition, are active during postural adjustments, and play an essential role in vocalizati on in many species. Despite this widespread involvement, however, thei r control has, with rare exceptions, received little attention for two major reasons. First, in most anesthetized or decerebrate preparation s, they are relatively inactive at rest, in part because the position of the preparation (supine or prone with abdomen supported), reduces l ung volume and, therefore, their activity. Second, unlike phrenic moto neurons 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 th e thoracic level, they are also intermixed with those innervating the intercostals. The two internal muscles, the internal oblique and the t ransverse abdominis, respond more to increases in chemical or volume-r elated drive than the two external muscles, the rectus abdominis and e xternal oblique; the basis for this differential sensitivity is unknow n. 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. Neuroanatomica l experiments demonstrate many more inputs to, and outputs from, the n ucleus retroambigualis, the brainstem region in which the premotor neu rons are located, than can be accounted for by their respiratory role alone. These other connections likely subserve activities other than r espiration. Studies of the multifunctional roles of the abdominal musc les, on the basis of recent work, hold considerable promise for improv ing our understanding of their control. (C) 1998 Elsevier Science Ltd. All rights reserved.