Nerves - Sacral Plexus

Question 1

What spinal nerves contribute to the sacral plexus?

Answer 1

S1, S2, S3, S4, L4, and L5.

At each vertebral level, paired spinal nerves leave the spinal cord via the intervertebral foramina of the vertebral column.

Each nerve then divides into anterior and posterior nerve fibres. The sacral plexus begins as the anterior fibres of the spinal nerves S1, S2, S3 and S4. They are joined by the 4th and 5th lumbar roots, which combine to form the lumbosacral trunk. This descends into the pelvis to meet the sacral roots as they emerge from the spinal cord.

Question 2

What are the branches of the sacral plexus?

Answer 2

The spinal roots S1-S4 divide into several cords. These cords then combine to form the five major peripheral nerves of the sacral plexus.

These nerves have either two destinations:

1) Leave the pelvis via the greater sciatic foramen. These nerves then enter the gluteal region of the lower limb, innervating structures there.
2) Remain in the pelvis - these nerves innervate the pelvic muscles organs and perineum.

The following major branches are called:

1) Superior gluteal nerve
2) Inferior gluteal nerve
3) Sciatic nerve
4) Posterior femoral cutaneous
5) Pudendal nerve
6) Other branches (nerve to piriformis, nerve to obturator internus and nerve to quadratus femoris)

Question 3

Superior gluteal nerve?

Answer 3

The superior gluteal nerve leaves the pelvis via the greater sciatic foramen, entering the gluteal region superiorly to the piriformis muscle. It is accompanied by the superior gluteal artery and vein for much of its course.

Roots: L4, L5, and S1

Motor function: innervates the gluteus minimus, medius and tensor fascia lata.

Sensory functions: none.

Question 4

Inferior gluteal nerve

Answer 4

The inferior gluteal nerve leaves the pelvis via the greater sciatic foramen, entering the gluteal region inferiorly to the piriformis muscles.

It is accompanied by the inferior gluteal artery and vein for much of its course.

Roots: L5, S1 and S2

Motor functions: innervates the gluteus maximus

Sensory functions: none

Question 5

Sciatic nerve?

Answer 5

Roots: L4, L5, S1, S2, S3

Motor functions:

• Tibiall portions: innervates the muscles in the posterior compartment of the thigh (apart from the short head of the biceps femoris), and the hamstring component of adductor magnus innervates all the muscles in the muscles in the posterior compartment of the leg and sole of the foot.
• Common fibular portion: short head of biceps femoris, all muscles in the anterior and lateral compartment of the leg and extensor digitorum brevis.

Sensory function:

• Tibial portion: supplies the skin of the posterolateral leg, lateral foot and sole of the foot.
• Common fibular portion: supplies the skin of the lateral leg and the dorsum of the foot.

Question 6

Posterior femoral cutaneous

Answer 6

The posterior cutaneous nerve of the thigh leaves the pelvis via the greater sciatic foramen, entering the gluteal region inferiorly to the piriformis muscle. It descends deep to the gluteus maximus and runs down the back of the knee.

Roots: S1, S2 and S3

Motor functions: none

Sensory functions:innervates the skin of the posterior surface of the thigh and leg. Also innervates the skin of the perineum.

Question 8

Pudendal nerve?

Answer 8

The nerve leaves the pelvis via the greater sciatic foramen, then re-enters the lesser sciatic foramen. It moves anterosuperiorly along the lateral wall of the ischiorectal fossa, and terminates by dividing into several branches.

Roots: S2, S3 and S4.

Motor functions: innervates the skeletal muscles in the perineum, the external urethral sphincter, the external anal sphincter, and levator ani.

Sensory functions: innervates the penis and the clitoris and most of the skin of the perineum.

Question 9

Other branches?

Answer 9

These are nerves that directly supply muscles (with the exception of the perforating cutaneous nerve, which supplies the skin over the inferior gluteal region and pelvic splanchnic nerves, which innervate the abdominal viscera):

1) Nerve to piriformis
2) Nerve to obturator internus
3) Nerve to quadratus femoris

Question 10

Clinical relevance - lumbosacral plexopathy

Answer 10

A lumbosacral plexopathy is a disorder affecting either the lumbar or sacral plexuses. They are rare disorders causing damage to the nerve bundles.

A plexopathy is suspected if the symptoms cannot be localised to a single nerve. Patients may complain of neuropathic pains, numbness, weakness or wasting or muscles.

One of the main causes of lumbosacral plexopathy is diabetic amyotrophy, also known as lumbosacral radioplexus neurophagy. In this condition the high blood sugar levels causes damage to the nerves. Idiotic plexopathy is another cause, being the lumbosacral equivalent of Parsonage-Turner syndrome (which affects the brachial plexus). Tumours and other local invasions cause plexopathy due to compression.

Treatment depends on what is causing the symptoms. Tumours should be removed. In idiopathic cases, corticosteroids may help.