The Nerves of the Pelvis
The pelvis is easy to access during physical examination, so it can tell you a lot if you understand its anatomy. The pelvic girdle is innervated by nerves that come from the sacral plexus, coccygeal plexus, and pelvic autonomic nerves.
The sacral plexus
The 4th and 5th lumbar spinal nerves form the lumbosacral trunk. The lumbosacral trunk goes on to join the 1st through 4th sacral nerves as they exit the sacrum to form the sacral plexus. The sacral plexus runs down on the posterior pelvic wall anterior to the piriformis muscle.
The nerves that stem from the sacral plexus include the following:
Sciatic nerve: This nerve is formed by the 4th lumbar through 3rd sacral spinal nerves. It’s the largest nerve in the body. It leaves the pelvis through the greater sciatic foramen to enter the gluteal area.
Pudendal nerve: This nerve is formed from the 2nd through 4th spinal sacral nerves. It exits the pelvis through the greater sciatic foramen and enters the perineum through the lesser sciatic foramen to innervate the muscles and skin of the perineum.
Superior gluteal nerve: Formed by the 4th lumbar through the 1st sacral spinal nerves, this nerve leaves the greater sciatic foramen to innervate gluteal muscles.
Inferior gluteal nerve: This nerve’s formed by the 5th lumbar through 2nd sacral spinal nerves. Like the superior gluteal nerve, it runs through the greater sciatic foramen to innervate gluteal muscles.
Nerve to the quadratus femoris muscle: This nerve is formed from the 4th lumbar through the 1st sacral spinal nerves. It leaves the greater sciatic foramen to innervate hip muscles.
Nerve to the obturator internus muscle: This nerve is formed by fibers from the 5th lumbar through the 2nd sacral spinal nerves. It also leaves the greater sciatic foramen to innervate hip muscles.
Nerve to the piriformis muscle: Stemming from the 1st and 2nd sacral spinal nerves, this nerve innervates the piriformis muscle.
Perforating cutaneous nerve: This nerve is formed from the 2nd and 3rd sacral spinal nerves and innervates the skin over the lower and medial portion of the buttock.
Posterior femoral cutaneous nerve: This nerve’s formed from the 2nd and 3rd sacral spinal nerves and innervates the skin of the perineum and the back surface of the thigh and leg.
Pelvic splanchnic nerves: Stemming from the 2nd through 4th sacral spinal nerves, these nerves provide the parasympathtetic innervation to the pelvic organs.
The coccygeal plexus
The coccygeal plexus of nerve fibers is formed by the 4th and 5th sacral spinal nerves and the coccygeal nerves. It supplies the coccygeus and levator ani muscles and the sacrococcygeal joint. Anococcygeal nerves innervate the skin between the coccyx and anus.
The obturator nerve arises from the from the lumbar plexus and doesn’t innervate anything in the pelvis, but it runs through the pelvis to the medial thigh.
Pelvic autonomic nerves
Pelvic autonomic nerves innervate the pelvic cavity; in general, autonomic nerves control things like blood flow, hormone levels, and body functions that you don’t consciously think about. Pelvic autonomic nerves include the following:
Sacral sympathetic trunks: These nerves are a continuation of the lumbar sympathetic trunks. They run in front of the sacrum and behind the rectum. The sacral sympathetic trunks each have four ganglia. The right and left sacral sympathetic trunks unite at the ganglion impar anterior to the coccyx. These trunks provide postganglionic sympathetic fibers to the sacral plexus that innervate the lower extremities. In addition, they provide fibers to the hypogastric plexus discussed later on.
Superior hypogastric plexus: This nerve sits in front of the sacral promontory. It contains sympathetic fibers from the aortic plexus. It descends into the pelvis and divides into the left and right hypogastric nerves.
Inferior hypogastric plexuses: These plexuses are formed when the right and left hypogastric nerves are joined by preganglionic parasympathetic fibers from the pelvic splanchnic nerves. The plexuses are located on each side of the rectum and the base of the bladder. They contain both sympathetic and parasympathetic fibers.
Pelvic splanchnic nerves (S2–S4): These nerves are preganglionic parasympathetic fibers that originate from the 2nd, 3rd, and 4th sacral spinal segments. They join the hypogastric nerves to form the inferior hypogastric plexuses.
Parasympathetic stimulation increases peristalsis and contraction of the bladder and rectum for urination and defecation. Parasympathetic stimulation of the genital erectile tissue stimulates erection. Sympathetic stimulation inhibits peristalsis and stimulates muscle contraction of genital organs during orgasm.