Autonomic Nerves of the Abdomen
The autonomic innervation to the abdominal viscera is provided by perivascular plexuses of nerves accompanying the arterial supply to each organ. The plexuses comprise sympathetic and parasympathetic fibres of both motor and sensory type. The autonomic nerves control glandular secretion, smooth muscle activity and vasomotor tone; they are also sensory, mediating the distension of hollow organs and the tension on mesenteries.
Most of the parasympathetic supply is provided by the vagus (X cranial) nerves, but there are small contributions to the distal part of the colon from branches of the pelvic splanchnic nerves that arise from the sacral spinal nerves. The vagi and the pelvic splanchnic nerves carry preganglionic parasympathetic fibres, which synapse with postganglionic fibres in the walls of the relevant organs.
From the oesophageal plexus (p. 62) two or more vagal trunks (gastric nerves) emerge and accompany the oesophagus through the diaphragm. The anterior trunk, derived mostly from the left vagus nerve (Fig. 4.96), enters the abdomen in front of the oesophagus and gives branches to the anterior surface of the stomach and to the liver. The posterior trunk, derived mostly from the right vagus, descends behind the oesophagus and supplies the posterior surface of the stomach and the coeliac plexus. From this plexus, some vagal fibres pass inferiorly to the root of the superior mesenteric artery. The perivascular plexuses that accompany branches of the coeliac and superior mesenteric arteries convey these vagal fibres to all parts of the digestive system as far distally as the splenic flexure of the colon.
The remainder of the large intestine receives its parasympathetic supply from branches of the pelvic splanchnic nerves (nervi erigentes). These ascend through plexuses in the pelvis (p. 236) and cross the left common iliac vessels in the root of the sigmoid mesocolon to reach the root of the inferior mesenteric artery. By accompanying branches of this artery, parasympathetic fibres from the sacral segments of the spinal cord supply the descending and sigmoid parts of the colon and the rectum.
The sympathetic system in the abdomen comprises two ganglionated trunks and a network of nerves (the aortic or prevertebral plexuses) covering the surface of the aorta.
Each sympathetic trunk enters the abdomen behind the medial arcuate ligament of the diaphragm (Fig. 4.97) and descends along the medial border of psoas major, the trunk on the right lying posterior to the inferior vena cava. Each trunk passes behind the common iliac vessels and crosses the pelvic inlet at the ala of the sacrum.
Within the abdomen, each trunk bears five lumbar ganglia. Only the upper two ganglia receive fibres from the central nervous system. These preganglionic sympathetic fibres are conveyed in white rami communicantes arising from the first and second lumbar spinal nerves. Some preganglionic fibres synapse in the sympathetic trunk, while others pass into branches of the trunk and synapse nearer the target organs.
Each ganglion of the trunk gives a branch, a grey ramus communicans, to the corresponding spinal nerve. The postganglionic sympathetic fibres in the grey rami are distributed to the body wall and the lower limb. In addition, the ganglia supply branches (containing both pre- and postganglionic fibres) to the abdominal and pelvic organs. Branches from the upper ganglia reinforce the aortic plexuses, while the lumbar splanchnic nerves descend from the lower ganglia and cross anterior to the common iliac vessels (Fig. 4.98). The lumbar splanchnic nerves from the right and left trunks unite below the bifurcation of the aorta to form the hypogastric plexus from which branches descend to reach the pelvic autonomic plexuses (p. 236).
The dense network of autonomic nerves (both parasympathetic and sympathetic) that invests the abdominal aorta (Fig. 4.98) has several component plexuses, which are named according to the larger branches of the aorta. These are the coeliac, aorticorenal, renal, superior mesenteric, intermesenteric and inferior mesenteric plexuses. From these perivascular plexuses, nerves are distributed to the abdominal organs.
The parasympathetic fibres in the aortic plexuses are derived mainly from the posterior vagal trunk, which enters the abdomen on the wall of the oesophagus, while the sympathetic fibres are provided principally by the thoracic splanchnic nerves (p. 64). These branches of the thoracic portions of the sympathetic trunks pierce the crura of the diaphragm to reach the coeliac plexus. Additional sympathetic fibres are provided by the upper ganglia of the lumbar sympathetic chain. Within the aortic plexuses are numerous small ganglia in which the pre- and postganglionic sympathetic fibres synapse.