Median Nerve: Anatomy, Course, Branches, Function, and Imaging

Topic

Median nerve

The median nerve is a major mixed (motor and sensory) peripheral nerve of the upper limb, originating from both the lateral and medial cords of the brachial plexus (roots C5–T1). It supplies most of the flexor muscles of the forearm, the thenar muscles, and provides sensory innervation to the palmar aspect of the lateral three and a half digits.

It is the principal nerve involved in fine motor control of the hand, particularly thumb opposition, and is clinically significant in carpal tunnel syndrome, pronator teres syndrome, and traumatic nerve injuries.

Synonyms

Nervus medianus
Median trunk of brachial plexus

Origin and Course

Origin: Formed by the union of the lateral root (from lateral cord, C5–C7) and the medial root (from medial cord, C8–T1) of the brachial plexus in the axilla, anterior to the third part of the axillary artery.
Arm: Descends lateral to the brachial artery, then crosses anteriorly to lie medial to it in the distal arm. It gives no branches in the arm.
Cubital fossa: Lies medial to the brachial artery and biceps tendon, entering the forearm between the two heads of the pronator teres muscle.
Forearm: Courses between flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), giving off the anterior interosseous nerve.
Wrist: Passes deep to the flexor retinaculum through the carpal tunnel into the hand.
Hand: Divides into recurrent (thenar) and digital branches to supply thenar muscles and lateral digits.

Relations

In the arm:
- Medial to brachial artery (distally)
- Anterior to brachialis and triceps fascia
In the cubital fossa:
- Medial to brachial artery and biceps tendon
- Covered by bicipital aponeurosis and skin
In the forearm:
- Between FDS and FDP muscles
- Deep to pronator teres and flexor retinaculum
At wrist:
- Deep to palmaris longus tendon (if present)
- Lateral to FDS tendons
In the hand:
- Lies under flexor retinaculum, giving recurrent and digital branches

Branches

In the arm: None
In the forearm:
- Muscular branches to superficial flexors (except flexor carpi ulnaris)
- Anterior interosseous nerve (AIN): Supplies flexor pollicis longus, pronator quadratus, and lateral FDP
- Palmar cutaneous branch: Arises proximal to flexor retinaculum, supplies palm skin
In the hand:
- Recurrent (thenar) branch: Thenar muscles (abductor pollicis brevis, opponens pollicis, superficial head of flexor pollicis brevis)
- Digital branches: Palmar skin of lateral 3½ digits and nail beds of same fingers

Nerve Supply (Motor and Sensory)

Motor:
- All forearm flexors except flexor carpi ulnaris and medial half of FDP
- Thenar muscles (except adductor pollicis)
- First and second lumbricals
Sensory:
- Lateral palm and palmar surface of the thumb, index, middle, and radial half of the ring finger
- Dorsal distal phalanges of same digits

Function

Motor functions:
- Forearm flexion (except ulnar flexors)
- Wrist flexion and pronation
- Thumb opposition and abduction
- Fine motor control of lateral digits
Sensory functions:
- Sensation to lateral palm and fingers
Reflexes:
- Contributes to flexor reflexes at wrist and fingers

Clinical Significance

Carpal tunnel syndrome: Compression beneath flexor retinaculum; causes nocturnal pain, paresthesia, thenar atrophy.
Pronator teres syndrome: Proximal compression between pronator teres heads; mimics carpal tunnel but spares palm sensation.
Anterior interosseous nerve syndrome: Weakness of thumb flexion and index finger flexion (“pinch sign”).
Laceration or trauma: Produces loss of thumb opposition and sensory deficit over lateral palm (“ape hand”).
Entrapment or neuropathy: Evaluated via MRI for nerve swelling, compression, or signal change.

MRI Appearance

T1-weighted images:
- Normal nerve: Intermediate signal intensity, slightly darker than muscle
- Perineural fat: Bright signal outlining the nerve path
- Pathology: Nerve enlargement or discontinuity with heterogeneous signal in trauma or neuritis
T2-weighted images:
- Normal nerve: Intermediate signal, slightly brighter than muscle but darker than fat
- Pathology: Hyperintense (bright) in neuritis, entrapment, or trauma
- Denervated muscle: Increased T2 signal due to edema
STIR:
- Normal nerve: Intermediate-to-dark signal
- Abnormal: Bright hyperintense nerve signal with or without surrounding fat stranding
- Sensitive for neuritis, entrapment, or post-traumatic neuropathy
Proton Density Fat-Saturated (PD FS):
- Normal nerve: Intermediate-to-dark signal
- Pathologic nerve: Hyperintense with possible perineural edema
- Denervated muscle: Bright signal in acute phase; chronic denervation leads to fatty replacement (bright on T1)
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal enhancement
- Inflamed or compressed nerve: Patchy or diffuse enhancement
- Tumors (schwannoma/neurofibroma): Homogeneous or heterogeneous enhancement with fusiform morphology

CT Appearance

Non-Contrast CT:

Nerve: Soft-tissue density structure adjacent to arteries, difficult to distinguish from surrounding fat
Course: Seen as fine tubular density medial to brachial artery (arm), deep to FDS (forearm), and beneath flexor retinaculum (wrist)
Pathology: Indirect findings such as muscle atrophy, space-occupying lesion, or displaced fat planes

Post-Contrast CT (standard):

Normal nerve: No significant enhancement
Pathologic nerve: May show mild enhancement in inflammation or neoplastic infiltration
CT usefulness: Evaluates bony entrapment (fractures, osteophytes), post-traumatic changes, and space-occupying lesions compressing the nerve

MRI images