6. Identify the nerves of the pelvis. Identify the muscles of the pelvic wall and floor.
Next we're going to look at the innervations of the pelvis. First, pull the peritoneum off the posterior wall of the pelvis, or sacrum. When you do so, you should see the hypogastric nerve, which really just provides continuity between the superior hypogastric plexus, that is located at the bifurcation to the aorta, and the inferior hypogastric plexus, which is located on either side of the rectum. The hypogastric nerve provides primarily sympathetic fibers from the superior hypogastric plexus and brings them down to the inferior hypogastric plexus. The inferior hypogastric plexus is made up of both sympathetic and parasympathetic fibers which then leave the plexus to innervate their target organs in the pelvis. Locate the sympathetic chain entering the pelvis along the medial border along the sacral foramina. You will see sacral splanchnic nerves carrying sympathetic fibers from the sacral portion of the chain to the inferior hypogastric plexus. Next, locate the lumbosacral trunk and the first four sacral nerves as they emerge from the anterior sacral foramina: L4, L5, S1, S2, S3, and S4. These are ventral primary rami that converge to create the sacral plexus. Locate the gray rami communicantes that connect the sympathetic chain to the sacral spinal nerves. Be sure to understand the difference between the rami communicantes and the sacral splanchnics, both which contain sympathetic fibers in the pelvis. Be sure to also identify pelvic splanchnic nerves which carry parasympathetic fibers from sacral segments S2, S3, and S4 to the inferior hypogastric plexus. These are nerves leaving the spinal nerves S2, 3, and 4 and bringing parasympathetic fibers to the inferior hypogastric plexus. Now let's look at the pelvic wall and floor. Move the viscera medially so you can better visualize the pelvic wall and floor. In ths image the sacrum is at the top. Looking at the piriformis muscle, the pelvic brim, the pubic symphysis, and this is the obturator internus muscle, with its fascia, the obturator canal with the nerve and artery coursing through. The fascia of the obturator internus muscle thickens into what is known as the arcus tendineus levator ani. The muscles of the pelvic diaphragm attach to this tendinous arch then extend downward and medially to fuse with each other in the midline. The pelvic diaphragm is composed of the levator ani and coccygeus muscles. Using your cadaver you should be familiar with the three different divisions of the levator ani muscles: the puborectalis, pubococcygeus, and iliococcygeus.