Now that we’ve looked at the blood vessels, we’ll look at the principal nerves of the pelvic region. We’ll look at the sacral plexus, then at the pudendal nerve, then at the autonomic nerves of the region.
Here’s the sacral plexus. Its lower part lies on the front of the piriformis muscle. The sacral plexus is formed mainly by the anterior primary rami of the spinal nerves S1 through S4. In addition, the plexus receives a contribution from L4 and 5, through this big nerve bundle, the lumbo-sacral trunk. The major branches of the sacral plexus leave the pelvis by passing through the greater sciatic foramen, either above piriformis, or below it.
Almost all the nerves that arise from the sacral plexus go to the lower extremity. They’re shown in Volume 2 of this Atlas. The branches of the sacral plexus that do concern us here are the pudendal nerve, which is the principal nerve of the perineum, and also the small motor nerves to the pelvic diaphragm.
A small branch or branches from S3 or 4 supply most of the levator ani muscle, and the coccygeus muscle, on their pelvic surfaces.
Here’s the pudendal nerve. It’s derived from S2, 3 and 4. It arises from the plexus just above the sacro-spinous ligament, which is here, and passes immediately through the greater sciatic foramen. To see where it goes, we’ll go round to the back.
Here’s the pudendal nerve again. Here next to it is the internal pudendal artery, which we’ve already met. We’ll go to an underneath view to follow the pudendal nerve.
It passes forwards on the side of obturator internus along with the internal pudendal artery. Its branches supply the anal sphincter, the muscles of the urogenital diaphragm, and the external genitals.
Lastly we’ll look at the autonomic nerves of the pelvic region. The autonomic nerves in the pelvis that belong to the sympathetic nervous system are the tail end of the sympathetic trunk, and the so-called hypogastric nerve. The parasympathetic nerves in the pelvis are the pelvic splanchnic nerves.
All these nerves, sympathetic and parasympathetic, are connected to a diffuse and extensive plexus of autonomic nerves called the pelvic plexus. The pelvic plexus lies within the fascia that covers this part of the pelvic wall and floor.
A small part of the pelvic plexus has been partially dissected out here. The pelvic plexus distributes the sympathetic and parasympathetic supply to the distal colon, the pelvic organs, and the external genital organs.
Feeding into the pelvic plexus from above is the hypogastric nerve, single here, but often taking the form of several small nerves. It’s the distal continuation of the aortic plexus.
Here’s the distal end of the sympathetic trunk. It enters the pelvis deep to the common iliac vessels, and descends just medial to the sacral foramina. It gives rami communicantes to the anterior rami of the sacral nerves.
Lastly, here are the pelvic splanchnic nerves, sometimes called the nervi erigentes. These are the source of all parasympathetic innervation in this region. They arise in this case from S3, also often from S2 and S4. They break up into branches which enter the pelvic plexus. From the plexus their fibers are distributed to the pelvic organs and external genitals.