The Autonomic Nervous System

The Autonomic Nervous System

 

The autonomic nervous system (Figs. 1 and 2) affecting the pelvic viscera can by divided into a sympathetic (thoracolumbar) system and a parasympathetic (craniosacral) system. The neurotransmitter for the sympathetic nervous system is adrenergic, whereas the parasympathetic is cholinergic. In general, adrenergic stimulation favors storage and cholinergic stimulation favors evacuation.

 

Sympathetic nerves at different vertebral levels

(Fig.1) Sympathetic nerves at different vertebral levels. A, At vertebral levels T1 to L2, preganglionic sympathetic neuron cell btxiies are located in the intermediate horn, and their axons travel in the ventral root to reach the sympathetic chain. The axons then traverse the white ramus communicans to enter the sympathetic ganglion and, as shown here, synapse in a neuron in that ganglion (other options would be to enter the splanchnic nerve or to ascend or descend in the sympathetic chain). When they synapse in the sympathetic ganglion, the postganglionic axon then traverse the gray ramus communicans to reenter the spinal nerve. (continued)

 

The postganglionic axons traverse the gray ramus to enter the spinal nerve for that level

(Fig. 2) cont’d B, At a level where no lateral horn exists, the sympathetic ganglion receives preganglionic input through the sympathetic chain. The postganglionic axons traverse the gray ramus to enter the spinal nerve for that level.

 

Autonomic innervation of the bladder

(Fig. 3) Autonomic innervation of the bladder. The sympathetic innervation begins with preganglionic axons arising in the lower thoracic spinal cord (A) in the intermediolateral cell column. These axons must synapse in a peripheral ganglion, and it is from the neurons of those ganglia that the axons actually innervating the intended target tissues arise. For pelvic viscera, preganglionic axons travel as part of the group of splanchnic nerves (greater, lesser, and least) that separate from the segmental thoracic nerves and descend along the posterior thoracic wall to enter the abdomen. Here they synapse in one of the large prevertebral ganglia (B) along the aorta (e.g., the celiac, superior mesenteric, renal, etc.). Postganglionic axons arising in these ganglia distribute themselves along the arterial branches of the abdominal aorta, and, in addition, a portion of them continues inferiorly to descend from the bifurcation of the aorta into the pelvis and the hypogastric nerve (or plexus) (C). Parasympathetic innervation begins with preganglionic axons arising in spinal cord segments S2 to S4 (D). These axons travel in the pelvic nerve (E) to small groups of neurons embedded in the walls of target organs. Here the preganglionic axons synapse, and the small neurons give rise to postganglionic axons (F), which actually innervate the muscle or glands in that organ.

 

SYMPATHETIC PATHWAY

The sympathetic pathway from the spinal cord to the viscera always involves two neurons. The synapse occurs either in the paravertebral ganglia of the sympathetic trunk or in prevertebral ganglia (i.e., the infer­ior mesenteric ganglion).

Preganglionic sympathetic fibers supplying the pelvic organs originate from the white rami of the upper two lumbar nerves and from the lowest thoracic nerve. These fibers pass through the corresponding ganglia of the sympathetic trunk (paravertebral gang­lia) terminating in the inferior mesenteric ganglia (prevertebral). Postganglionic fibers or axons are then sent by way of the superior hypo­gastric and inferior hypogastric plexi to the pelvic viscera.

Each spinal nerve receives gray rami communicantes from the sympathetic trunk. The gray ramus consists of postganglionic sympathetic fibers, which innervate blood vessels, hair, and glands of the body wall. The four pairs of ganglia of the sympathetic trunk anterior to the sacrum contribute branches from the first two ganglia to the inferior hypogastric plexus, whereas the last two supply the rectum directly.

 

 

PARASYMPATHETIC SYSTEM

 

The parasympathetic system consists of preganglionic fibers that originate from the second, third, and fourth sacral nerves forming the pelvic nerve (also termed nervi erigentes) and terminate in the ganglia of the pelvic plexi. Postganglionic fibers from these plexi supply the effectors of the pelvic organs, including the urinary bladder, the descending colon, the rectum, and the reproductive organs.

 

Anatomically (Fig. 4), the two sympathetic trunks descend into the pelvis along the medial side of the anterior sacral foramina and end as a single ganglion impar anterior to the coccyx. Each trunk usually has four ganglia, which send gray rami to the sacral and the coccygeal nerves.

 

Sympathetic chains and splanchnic nerves.

(Fig. 4) Sympathetic chains and splanchnic nerves. The sympathetic chains emit medial branches from the T6 to T12 levels (the splanchnic nerves). These synapse in ganglia positioned at the origin of branches of the abdominal aorta, especially the celiac ganglion. The sympathetic chains continue inferiorly in the abdomen and eventually meet in the midline, anterior to the coccyx (the ganglion impar).

 

The superior hypogastric plexus lies in front of the fifth lumbar vertebra, the sacral promontory, and the ventral surface of the lower aorta. This plexus also receives important pain fibers from the pelvic viscera. From the superior hypogastric plexus descend two hypogastric nerves along the course of the internal iliac vessels. These nerves terminate as the inferior hypogastric plexus.

The inferior hypogastric plexus consists of the uterovaginal plexus along the medial surface of the uterine vessels lateral to the uterosacral ligament. In addition, extensions of the inferior hypogastric plexi are located on the walls of the pelvic viscera. This includes the rectal plexus and the vesical plexus.

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