Innervation

Sacral Plexus:

1. Rami are L4 - L5 (lumbosacral trunk) and S1 - S4 ventral primary rami of spinal nerves

2. Divisions (anterior and posterior) are formed by rami dividing into anterior and posterior divisions

3. Major terminal branches;

   • Superior gluteal nerve (L4-S1) innervates the gluteus medius, minimus and tensor fascia lata muscles

   • Inferior gluteal nerve (L5-S2_ innervates the gluteus maximus muscle

   • Nerve to piriformis (S1,S2) innervates the piriformis muscle

   • Common fibular nerve (L4, L5, S1, S2)

   • Tibial nerve (L4, L5, S1, S2, S3) this and the common fibular nerve comprise the sciatic nerve

   • Nerve to quadratus femoris and inferior gemellus (L5-S1) innervates the quadratus femoris and inferior gemellus muscles

   • Nerve to obturator internus and superior gemellus (L5-S2) innervates the obturator internus and superior gemellus muscles

   • Posterior femoral cutaneous nerve (S1-S3) innervates the skin of the buttock, thigh, and calf (sensory). It also gives rise to the inferior cluneal nerves and perineal branches

   • Perforating cutaneous nerve (S2, S3) innervates the skin in the perineal area.

   • Pudendal nerve (S2-S4) passes through the greater sciatic foramen, crosses the ischial spine, and enters the perineum with the internal pudendal artery through the pudendal canal. It gives rise to the inferior rectal nerve, perineal nerve, and dorsal nerve of the penis (or clitoris)

   • Nerve to the levator ani and coccygeus )S3,S4) innervates the levator anu muscles and coccygeus muscle

   • Perineal branch of spinal nerve S4 innervates the skin of the perineum (sensory)

Coccygeal Plexus

1. Rami are the S4 and S5 ventral primary rami of spinal nerves

2. Coccygeal nerve innervates the coccygeus muscle, part of the levator ani muscles and the sacrococcygeal joint

3. Branches:

   • Anococcygeal nerve which innervates the skin between the tip of the coccyx and the anus

Autonomic components

1. Superior hypogastric plexus is a continuation of the intermesenteric plexus from the inferior mesenteric ganglion below the aortic bifurcation and receies the L4 and L4 lumbar splanchnic nerves. This plexus contains ganglionic neuronal cell bodies upon which preganglionic sympathetic axons of the L3 and L4 lumbar splanchnic nerve synapse on. The superior hypogastric plexus descends anterior to the L5 vertebrae and ends by dividing into the right hypogastric nerve and left hypogastric nerve.

2. Right and left hypogastric nerves descend on either side lateral to the rectum and join the right or left inferior hypogastric plexus respectively.

3. Right and left inferior hypogastric plexuses are located against the posterolateral pelvic wall lateral to the rectum, vagina, and base of the bladder. The right and left inferior hypogastric plexuses are formed by the union of the right or left hypogastric nerves, sacral splanchnic nerves (L5 and S1-3), and pelvic splanchnic nerves (S2-4). This plexus contains ganglionic neuronal cell bodies upon which preganglionic sympathetic axons of the sacral splanchnic nerves (L5 and S1-3) synapse on.

4. Sacral sympathetic trunk is a continuation of the paravertebral sympathetic chain ganglia in the pelvis. The sacral trunks descend on the inner surface of the sacrum medial to the sacram foramina and converge to form the small median ganglion impar anterior to the coccyx.

Micturition

1. This is an involuntary reflex where stretch receptors in the walls of the bladder send sensory signals through the parasympathetic pelvic nerves. Signals are sent back to the bladder through the same parasympathetic pelvic nerves which trigger the detrusor muscle to contract and the internal urethral sphincter to relax.

2. The external urethral sphincter is under somatic motor control.

Spinal Cord Injuries

1. Spinal cord injuries above the spinal level of S2 causes hypertonic and hyper reflexive bladder issues. In such cases, the bladder fills and empties reflexively and micturition is not voluntarily controlled.

2. Spinal cord injuries at the sacral segment result in a flaccid bladder without reflex control. In such cases, the bladder fills to capacity and overflows but is only able to emptied slowly. The bladder can be emptied by manually compressing the lower anterior abdominal wall. This can lead to chronic urinary tract infections or eventual kidney failure.