🎓 Prepared by students from Boğaziçi University

What are Spinal Nerve Plexuses?

Spinal nerve plexuses are networks where adjacent spinal nerves intertwine to reorganise and distribute fibres to body regions. Four major plexuses (cervical, brachial, lumbar, sacral) form branches that innervate the limbs and trunk.

Short answer

Spinal nerve plexuses are tangles of nerve fibres from adjacent spinal nerves that unite and rearrange to form peripheral nerves. This allows fibres from multiple spinal segments to supply a single limb region.

The Four Spinal Nerve Plexuses

1
Cervical Plexus (C1–C4)
Supplies neck, shoulder, and diaphragm (phrenic nerve). Branch: long thoracic nerve (serratus anterior)
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2
Brachial Plexus (C5–T1)
Supplies upper limb. Three trunks → six divisions → three cords. Major branches: radial, median, ulnar, musculocutaneous nerves
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3
Lumbar Plexus (L1–L4)
Supplies lower abdomen and medial/anterior thigh. Major branches: femoral and obturator nerves
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4
Sacral Plexus (L5–S4) + Coccygeal (S4–Co1)
Supplies gluteal region, posterior thigh, leg and foot. Major branch: sciatic nerve (largest peripheral nerve; splits into tibial and common peroneal)

Step-by-step worked examples

A patient with a Erb's palsy (brachial plexus birth injury) cannot abduct the shoulder. Which nerve root is affected?

Erb's palsy = injury to C5–C6 nerve roots (upper trunk of brachial plexus)
Abduction of shoulder = supraspinatus and deltoid muscles
Deltoid innervated by axillary nerve (from posterior cord, derived from C5–C6)
→ C5–C6 lesion produces Erb's palsy with arm adduction and internal rotation

A patient presents with foot drop and loss of ankle dorsiflexion. Which nerve is damaged?

Foot drop and inability to dorsiflex = common peroneal nerve (branch of sciatic)
Common peroneal = L4–S2 nerve roots via sacral plexus
Common cause: compression at fibular neck (trauma, prolonged crossing of legs)
Result: foot hangs in plantarflexion (foot drop)

A patient cannot straighten the leg at the knee after a femoral nerve injury. Why?

Knee extension = quadriceps muscle
Quadriceps innervated by femoral nerve (L2–L4)
Femoral nerve from lumbar plexus (L1–L4)
Femoral nerve injury → quadriceps paralysis → inability to extend knee

Flashcards

Quick quiz

Q1.The phrenic nerve, which innervates the diaphragm, arises from which plexus?

Correct answer: B. The phrenic nerve arises from the cervical plexus (C3–C5), making phrenic nerve injury a serious concern in neck trauma.

Q2.A patient has weakness in knee flexion after a sciatic nerve injury. Which muscle is paralysed?

Correct answer: C. Knee flexion is performed by the hamstrings (biceps femoris, semitendinosus, semimembranosus), which are innervated by the sciatic nerve.

Q3.Which spinal levels contribute to the lumbar plexus?

Correct answer: A. The lumbar plexus forms from spinal nerve roots L1 through L4. L5 contributes to the sacral plexus.

Q4.A patient has weakness in ankle plantarflexion. Which nerve is affected?

Correct answer: B. Plantarflexion is performed by the soleus and gastrocnemius muscles, innervated by the tibial nerve (sacral plexus, L5–S2).

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Common mistakes

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Thinking each spinal nerve supplies only its own segment's muscles. — Correct: Plexuses allow multiple spinal nerves to intertwine, so a single peripheral nerve carries fibres from several spinal levels.

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Confusing Erb's palsy with Klumpke's palsy. — Correct: Erb's = upper trunk (C5–C6) injury; Klumpke's = lower trunk (C8–T1) injury. Different clinical presentations and outcomes.

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Assuming the sciatic nerve comes from the lumbar plexus. — Correct: The sciatic nerve is the major branch of the sacral plexus (L5–S3), not the lumbar plexus.

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Forgetting that plexuses allow collateral innervation. — Correct: Because multiple spinal levels supply a region via a plexus, partial plexus injury may cause incomplete paralysis, and recovery is possible.

FAQ

Why do plexuses exist? Why don't spinal nerves just go straight to their targets?

Plexuses allow efficient redistribution of nerve fibres. A single limb receives innervation from multiple spinal segments (e.g., brachial plexus C5–T1), and plexuses reorganise these fibres into functionally organized peripheral nerves.

What is a nerve 'trunk', 'division', and 'cord' in the brachial plexus?

Brachial plexus anatomy: nerve roots (C5–T1) → three trunks (upper, middle, lower) → six divisions → three cords (lateral, medial, posterior) → major peripheral branches (radial, median, ulnar, musculocutaneous).

Why is the fibular head a common site of peroneal nerve compression?

The common peroneal nerve wraps tightly around the fibular neck just below the knee, making it vulnerable to pressure injury (e.g., prolonged leg crossing, tight casts, trauma).

How is plexus anatomy used in regional anaesthesia?

Anaesthetists target specific plexuses (e.g., brachial plexus block, epidural at lumbar plexus level) to numb entire limb regions with minimal systemic drug use.