Musculocutaneous Nerve: Anatomy, Course, Function, Clinical Imaging

Topic

Musculocutaneous nerve

The musculocutaneous nerve is a terminal branch of the lateral cord of the brachial plexus (roots C5–C7). It is primarily a motor nerve to the flexor muscles of the anterior arm—namely the biceps brachii, brachialis, and coracobrachialis—and provides sensory innervation to the lateral aspect of the forearm through its terminal continuation, the lateral cutaneous nerve of the forearm.

It is one of the key nerves of the arm, contributing to elbow flexion and forearm supination, and can be injured in shoulder dislocations, brachial plexus trauma, or iatrogenic surgical procedures involving the anterior arm.

Synonyms

Musculospiral nerve of the arm (archaic)
External cutaneous nerve of the forearm (for terminal branch)

Origin, Course, and Branches

Origin:

Arises from the lateral cord of the brachial plexus in the axilla, receiving fibers mainly from C5–C7 spinal roots.

Course:

Emerges lateral to the axillary artery.
Pierces the coracobrachialis muscle, entering the anterior compartment of the arm.
Descends obliquely between biceps brachii and brachialis muscles, supplying both.
Continues distally to the elbow as the lateral cutaneous nerve of the forearm, emerging lateral to the biceps tendon and lying superficial to the brachioradialis.

Branches:

Muscular branches:
- To coracobrachialis (near origin)
- To biceps brachii (mid-arm)
- To brachialis (lower arm)
Cutaneous branch:
- Lateral cutaneous nerve of forearm—provides sensation to skin over the lateral forearm.
Articular branches:
- To the elbow joint (via branch to brachialis).

Relations

Proximally: Lateral to the axillary artery
Medially: Median nerve (before musculocutaneous separates)
Laterally: Coracobrachialis and short head of biceps brachii
Anteriorly: Biceps brachii
Posteriorly: Brachialis muscle
Distally: Emerges lateral to biceps tendon in cubital fossa, continues as lateral cutaneous nerve of forearm

Nerve Supply

Roots: C5, C6, and C7 (from lateral cord of brachial plexus)
Motor supply: Coracobrachialis, biceps brachii, and brachialis
Sensory supply: Skin on lateral forearm via lateral cutaneous nerve of forearm

Function

Motor:
- Flexes the elbow (biceps brachii, brachialis)
- Assists in forearm supination (biceps brachii)
Sensory:
- Provides cutaneous sensation to the lateral forearm (radial border from wrist to elbow)
Reflex involvement:
- Biceps reflex (C5–C6) tests musculocutaneous nerve integrity

Clinical Significance

Nerve injury: Rare in isolation but may occur in upper brachial plexus injuries or during shoulder or humeral surgery.
Deficit: Weak elbow flexion and supination; sensory loss over lateral forearm.
Entrapment: Possible at coracobrachialis or between biceps and brachialis; presents with pain, weakness, or paresthesia.
Surgical relevance: Must be preserved in deltopectoral and coracoid approach surgeries.
Iatrogenic causes: Common in anterior shoulder arthroscopy and coracoid transfer (Latarjet) procedures.

MRI Appearance

T1-weighted images:
- Nerve appears as a thin, low-to-intermediate signal tubular structure within the anterior arm compartment.
- Surrounded by bright perineural fat, providing contrast for visualization.
- Muscles supplied by the nerve (biceps, brachialis, coracobrachialis) show intermediate signal in normal state.
- Denervated muscles may appear slightly hyperintense in subacute stages due to fatty infiltration.
T2-weighted images:
- Normal nerve: intermediate signal
- Pathologic nerve (neuritis, entrapment, or trauma): hyperintense due to edema or inflammation.
- Denervated muscle shows bright T2 signal due to edema in acute injury.
STIR:
- Normal nerve: intermediate-to-dark linear structure.
- Pathology: hyperintense nerve and muscle signal in neuritis, traction injury, or entrapment.
Proton Density Fat-Saturated (PD FS):
- Normal: Nerve appears dark-to-intermediate signal with surrounding fat suppression.
- Pathologic: Bright hyperintensity along nerve or muscle belly in acute denervation.
- Excellent for detecting perineural edema or scar tissue post-surgery.
T1 Fat-Sat Post-Contrast:
- Normal nerve: minimal or no enhancement.
- Neuritis or perineural inflammation: uniform or focal enhancement.
- Postoperative scarring: mild linear enhancement around nerve.
- Neuroma: nodular or fusiform enhancement with disrupted fascicular pattern.

CT Appearance

Non-Contrast CT:

Nerve itself not well visualized due to similar attenuation as surrounding muscle.
Indirectly identified by its position relative to biceps and brachialis.
May show secondary changes—atrophy or fatty infiltration of innervated muscles in chronic injury.

MRI image

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