HLTH1008 Chap.8 Lumbosacral Plexus and Lower Limb Nerves
Lumbosacral Plexus and Lower Limb Nerves
This chapter brings the lower-limb nerves together as a plexus. It traces the lumbar plexus, the lumbosacral trunk and the sacral plexus with their root values, then sets out the femoral, obturator, sciatic, tibial, fibular, plantar and pudendal nerves with their muscles and skin areas.
It works through foot drop after a plaster cast and a runner with weak dorsiflexion, showing how spared skin and spared movements locate a lesion, and ends with dermatomes, peripheral nerve fields, myotomes and the saphenous and sural nerves. The method throughout is to list what is lost and what is spared before naming any nerve. The same approach is used again for the upper limb nerves in the elbow and hand chapter.
What this chapter covers
- 01
Lumbar and sacral plexus roots
- 02
Femoral and obturator nerves
- 03
Sciatic nerve and sciatica
- 04
Common, deep and superficial fibular nerves
- 05
Tibial and plantar nerves
- 06
Foot drop and lesion level
- 07
Dermatomes versus peripheral nerve fields
- 08
Lower limb myotomes
Worked example · free
Common fibular palsy after a cast
- +1The common fibular nerve winds around the neck of the fibula, where a tight cast can compress it.
- +1Its deep branch supplies the dorsiflexors, so their loss produces foot drop.
- +1Its superficial branch supplies fibularis longus and brevis, so eversion is lost.
- +1The superficial branch also supplies the skin of the dorsum of the foot, explaining the numbness.
Key terms
- Lumbosacral trunk
- Part of L4 with L5, which joins the sacral plexus in front of the sacral ala.
- Common fibular nerve
- The sciatic branch that winds around the fibular neck.
- Deep fibular nerve
- The nerve of the anterior compartment of the leg.
- Meralgia paresthetica
- Compression neuropathy of the lateral cutaneous nerve of the thigh.
- Peripheral nerve field
- The skin area supplied by one named cutaneous nerve.
- Myotome
- The group of muscles supplied by a single spinal nerve root.
- Sural nerve
- The cutaneous nerve formed by the medial and lateral sural branches that supplies the lateral foot.
- Medial plantar nerve
- The tibial branch that supplies abductor hallucis, flexor digitorum brevis and flexor hallucis brevis.
- Tibial nerve
- The sciatic branch that supplies the posterior leg and ends as the plantar nerves.
Lumbosacral Plexus and Lower Limb Nerves FAQ
How is a dermatome different from a peripheral nerve field?
A dermatome is the skin supplied by one spinal nerve, such as S1 along the lateral border of the foot. A peripheral nerve field is the skin supplied by one named cutaneous nerve, such as the superficial fibular nerve on the dorsum of the foot, and one field may contain several dermatomes.
Which nerves run with the saphenous veins?
The saphenous nerve, a branch of the femoral nerve, runs with the great saphenous vein along the medial leg, and the sural nerve runs with the small saphenous vein along the lateral leg and foot. The pairing appears among the final exam revision items.
What can cause sciatica down the leg?
Sciatica is pain along the distribution of the sciatic nerve. The nerves lecture lists disc prolapse, spinal canal stenosis, degeneration, spondylolisthesis and piriformis syndrome as causes, and the nerve's roots, L4 to S3, explain why the pain can involve both the thigh and the leg.
What is foot drop?
Foot drop is an inability to lift the foot at the ankle, so the toes catch and the foot slaps down in walking. It follows loss of the dorsiflexors in the anterior compartment, from injury to the deep fibular nerve or to its parent, the common fibular nerve at the fibular neck.
What are the roots of the sciatic and common fibular nerves?
The nerves lecture gives the sciatic nerve as L4 to S3 and the common fibular nerve as L4 to S2. The sciatic nerve leaves the pelvis below piriformis and divides into the tibial and common fibular nerves above the knee, sometimes much higher.
Exam move
Draw the lumbosacral plexus as root bars from L1 to S4 and place each nerve on them. Then build a table of nerve, roots, muscles and skin for the femoral, obturator, sciatic, tibial and fibular nerves. Practise lesion questions by writing what is lost and what is spared before naming any nerve, and finish by defining a dermatome and a nerve field with one lower-limb example each.
Practise two lesion cases a day, alternating common fibular, deep fibular, femoral and sciatic injuries, and always finish with the skin area that would confirm your answer.
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