Root Origin
- Spinal nerve roots: C8, T1 (minor contribution from C7 in some individuals)
- Plexus: Brachial plexus
- Cord: Medial cord
- Type: Mixed (motor and sensory)
Course
- Medial cord. The ulnar nerve arises from the medial cord of the brachial plexus, carrying primarily C8 and T1 fibers. It begins in the axilla, medial to the axillary artery.
- Medial arm. The nerve descends the medial arm alongside the brachial artery and median nerve. It gives off no branches in the arm — it is purely a conduit at this level. About halfway down the arm, it pierces the medial intermuscular septum to enter the posterior compartment, moving from anterior to posterior. It then travels on the medial head of the triceps toward the elbow.
- Medial epicondyle (cubital tunnel). The nerve passes behind the medial epicondyle in the cubital tunnel — a fibro-osseous channel formed by the medial epicondyle (floor), the olecranon (lateral wall), and the Osborne ligament or arcuate ligament (roof, connecting the two heads of FCU). This is the most superficial point of any major nerve in the upper extremity — you can feel it rolling under your fingertip. This is the primary site of conditions/cubital-tunnel-syndrome, the second most common peripheral nerve entrapment after CTS.
- Proximal forearm. The nerve enters the forearm between the two heads of the flexor carpi ulnaris (the humeral head attaching to the medial epicondyle and the olecranon head attaching to the olecranon). It descends the forearm deep to FCU, lying on the flexor digitorum profundus. It supplies FCU and the medial half of FDP (ring and little fingers) along this course.
- Distal forearm. The nerve becomes superficial at the wrist, traveling lateral to the FCU tendon and lateral to the pisiform bone. About 5 cm proximal to the wrist, it gives off the dorsal cutaneous branch, which wraps around the ulnar side of the wrist to supply dorsal hand sensation.
- Guyon's canal (wrist). The nerve enters Guyon's canal (ulnar tunnel) — a fibro-osseous tunnel between the pisiform and the hook of the hamate, roofed by the pisohamate ligament and the palmar carpal ligament. The ulnar artery travels with it. Within the canal, the nerve divides into a superficial branch (sensory — ring and little finger) and a deep branch (motor — intrinsic hand muscles). This is the site of Guyon's canal syndrome.
- Hand (deep motor branch). The deep branch curves around the hook of the hamate and dives deep across the palm, supplying the hypothenar muscles, all interossei, the medial two lumbricals, and the adductor pollicis. It ends at the adductor pollicis, which is the last muscle it innervates.
Motor Distribution
Forearm
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/flexor-carpi-ulnaris | Wrist flexion, ulnar deviation | The FCU tendon at the wrist is the landmark for locating the ulnar nerve and pisiform |
| anatomy/muscles/flexor-digitorum-profundus (medial half — ring + little) | Flexes DIP joints of digits 4-5 | The lateral half (index + middle) is median nerve — this split is a classic exam question |
Hand — Hypothenar Muscles
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/abductor-digiti-minimi | Abducts little finger | The hypothenar equivalent of the abductor pollicis brevis |
| anatomy/muscles/flexor-digiti-minimi-brevis | Flexes MCP of little finger | Small muscle, rarely tested in isolation |
| anatomy/muscles/opponens-digiti-minimi | Opposes little finger to thumb | Cups the hand — contributes to power grip |
Hand — Deep Intrinsic Muscles
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/adductor-pollicis | Adducts thumb | The key muscle for strong pinch; tested with Froment sign. Loss = unable to pinch paper firmly between thumb and index finger |
| anatomy/muscles/palmar-interossei (3 muscles) | Adduct digits toward the middle finger, flex MCP, extend IP | "PAD" — Palmar ADduct |
| anatomy/muscles/dorsal-interossei (4 muscles) | Abduct digits away from the middle finger, flex MCP, extend IP | "DAB" — Dorsal ABduct |
| anatomy/muscles/lumbrical-3 | Flexes MCP, extends IP of ring finger | Lumbricals 1-2 are median; 3-4 are ulnar |
| anatomy/muscles/lumbrical-4 | Flexes MCP, extends IP of little finger | The ulnar lumbricals |
Sensory Distribution
- Dorsal cutaneous branch. Exits the main trunk approximately 5 cm proximal to the wrist and wraps around the ulnar border of the forearm to supply the dorsal surface of the little finger, the ulnar half of the ring finger, and the ulnar side of the dorsal hand.
- Clinical significance: This branch exits BEFORE Guyon's canal. If dorsal hand sensation is intact but the palmar little finger is numb, the lesion is in Guyon's canal. If dorsal sensation is also lost, the compression is proximal to the canal — at the cubital tunnel or higher.
- Superficial terminal branch. Supplies the palmar surface of the little finger and the ulnar half of the ring finger.
- The ulnar-median boundary. The ring finger is split between the two nerves — ulnar takes the ulnar half, median takes the radial half. When a patient says "my ring finger is numb," ask which side. This single question separates ulnar from median compression.
Entrapment Sites
1. Cubital Tunnel (Medial Epicondyle)
- Location: Behind the medial epicondyle, in the cubital tunnel
- Structure: The nerve lies in a shallow groove between the medial epicondyle and the olecranon, roofed by the Osborne ligament (connecting the FCU heads). The tunnel narrows with elbow flexion — the Osborne ligament tightens and the medial collateral ligament bulges into the tunnel, reducing volume by up to 55%.
- Condition: conditions/cubital-tunnel-syndrome
- Presentation: Numbness and tingling in the little finger and ulnar half of the ring finger. Aching along the medial elbow. In advanced cases: hand weakness, loss of grip strength, difficulty with fine motor tasks (buttons, keys), visible wasting of the first dorsal interosseous and hypothenar muscles. Symptoms worse with prolonged elbow flexion (sleeping, phone use, driving).
- Key differentiator from C8-T1 radiculopathy: Cubital tunnel symptoms are limited to ulnar territory in the hand with no neck pain. C8-T1 radiculopathy produces broader medial arm and forearm numbness plus potential weakness of median-innervated muscles (FPL, APB).
- MT relevance: The most common peripheral nerve compression at the elbow. Elbow flexion sustained beyond 90 degrees compresses the nerve — counsel patients on sleeping posture (towel roll around the elbow to limit flexion), phone habits, and desk ergonomics. Myofascial release of FCU and the surrounding fascia can reduce compression. Avoid sustained pressure directly over the cubital tunnel during treatment.
2. Guyon's Canal (Ulnar Tunnel at the Wrist)
- Location: Ulnar wrist, between the pisiform and the hook of the hamate
- Structure: Fibro-osseous tunnel. The nerve divides into superficial (sensory) and deep (motor) branches within the canal. Compression can be selective — affecting motor only, sensory only, or both, depending on the location within the canal.
- Condition: Guyon's canal syndrome (ulnar tunnel syndrome)
- Presentation: Three zones of compression produce different presentations. Zone 1 (proximal, before division): combined motor and sensory — hand weakness plus little finger numbness. Zone 2 (deep branch only): pure motor — hand weakness without numbness, mimicking ALS or motor neuron disease. Zone 3 (superficial branch only): pure sensory — little finger numbness without weakness.
- Causes: Ganglion cysts (most common), hook of hamate fractures (in cyclists and golfers), repetitive palmar pressure (cycling — "handlebar palsy"), ulnar artery thrombosis.
- Key differentiator from cubital tunnel: Dorsal hand sensation is intact in Guyon's canal syndrome (the dorsal branch exits before the canal). FCU strength is preserved (innervated in the forearm, not the hand).
- MT relevance: Cyclists, golfers, and anyone using vibrating tools (jackhammers, drills) are at risk. Padded cycling gloves and modifying hand position on handlebars reduce compression. Differentiate from cubital tunnel by testing dorsal hand sensation.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| ULTT3 (ulnar bias) | Shoulder abduction, elbow flexion, wrist and finger extension, forearm supination or pronation. Applied as a sequential tensioning of the ulnar nerve. | Reproduction of ulnar symptoms (little finger tingling, medial elbow pain). Symptoms increase with cervical lateral flexion away. | The ulnar nerve is mechanosensitive somewhere along its path. Does not localize the compression site. |
| Tinel's at the elbow | Tap over the ulnar nerve in the cubital tunnel groove behind the medial epicondyle. | Tingling or electric sensation radiating into the little finger and ulnar ring finger. | Nerve irritability at the cubital tunnel. Moderate sensitivity — a negative Tinel's does not rule out cubital tunnel syndrome. |
| Elbow flexion test | Patient holds both elbows in full flexion with wrists extended for 60 seconds. | Reproduction of ulnar symptoms. | Equivalent to Phalen's test for the ulnar nerve. Full elbow flexion tightens the Osborne ligament and narrows the cubital tunnel by up to 55%. Sensitivity ~75%. |
| Froment sign | Ask the patient to pinch a piece of paper firmly between the thumb and the side of the index finger. Attempt to pull the paper away. | The patient flexes the thumb IP joint (using FPL — median nerve) to compensate for weak adductor pollicis (ulnar nerve). The thumb cannot maintain adduction with a straight IP joint. | Adductor pollicis weakness — the most specific motor sign of ulnar nerve dysfunction. Named the "sign of the newspaper." |
| Wartenberg sign | Ask the patient to adduct all fingers together tightly. | The little finger remains abducted (sticks out) due to unopposed action of the extensor digiti minimi without the palmar interosseous to adduct it. | Intrinsic muscle weakness in ulnar distribution. The abducted little finger catches on pockets and draws the patient's attention. |
| Tinel's at Guyon's canal | Tap over the ulnar nerve between the pisiform and the hook of the hamate. | Tingling into the little finger. | Nerve irritability at Guyon's canal. Compare with cubital tunnel Tinel's to localize the compression. |
Clinical Notes
- Cubital tunnel syndrome is the second most common peripheral nerve entrapment. Only carpal tunnel syndrome is more common. The ulnar nerve's superficial position behind the medial epicondyle makes it vulnerable to direct pressure (leaning on elbows, armrests) and to stretching during elbow flexion. Cubital tunnel symptoms are often bilateral in desk workers.
- Elbow flexion is the primary aggravating factor. The cubital tunnel narrows by up to 55% in full elbow flexion. Patients who sleep with elbows flexed (most people), hold phones to the ear, or work with sustained elbow flexion develop cumulative nerve irritation. The single most effective conservative intervention is a night splint or towel roll to limit elbow flexion during sleep.
- "Claw hand" develops late. In chronic ulnar nerve palsy, the ring and little fingers develop a clawed posture — hyperextended at the MCP joints and flexed at the IP joints. This occurs because the interossei and lumbricals (ulnar nerve) that normally flex the MCPs and extend the IPs are paralyzed, while the extrinsic extensors (radial nerve) and FDP (median/ulnar) are unopposed. The ulnar paradox: a high lesion (elbow) produces less clawing than a low lesion (wrist) because the high lesion denervates FDP to the ring and little fingers, reducing the IP flexion component of the claw.
- First dorsal interosseous wasting is the first visible sign. Before claw hand develops, the first dorsal interosseous (the muscle between the thumb and index metacarpals on the dorsal hand) begins to waste. A visible "gutter" between the thumb and index metacarpals indicates chronic ulnar nerve compression. Check this in every suspected cubital tunnel case.
- Handlebar palsy is preventable. Cyclists who ride in the drops or on the hoods for extended periods compress the ulnar nerve at Guyon's canal. Padded gloves, bar tape, frequent hand position changes, and proper bike fit prevent this. The deep motor branch is often affected alone — producing hand weakness without numbness, which the cyclist may notice only when they struggle to turn a key.
- The dorsal cutaneous branch is your localizer. This sensory branch exits before Guyon's canal. If dorsal hand sensation on the ulnar side is intact, the lesion is at or distal to the wrist. If dorsal sensation is also reduced, the compression is at the cubital tunnel or higher. This is the equivalent of the median nerve's palmar cutaneous branch for localization.
Related Nerves
- anatomy/nerves/median-nerve — Lateral and medial cords. The ulnar nerve's counterpart in the hand — together they innervate every intrinsic hand muscle. The ring finger boundary (median takes the radial half, ulnar takes the ulnar half) is the key sensory differentiator. Combined median and ulnar nerve lesions produce a devastating "simian hand" with total loss of intrinsic function.
- anatomy/nerves/radial-nerve — Posterior cord. Shares dorsal hand sensory territory — the radial nerve covers the radial side, the ulnar nerve covers the ulnar side, with overlap at the ring finger dorsum.
- anatomy/nerves/medial-cutaneous-nerve-of-forearm — Medial cord, C8-T1. Provides sensation to the medial forearm — the territory just proximal to the ulnar nerve's hand territory. Can be confused with ulnar nerve symptoms when the medial forearm is involved.
Key Takeaways
- Second most common peripheral nerve entrapment (cubital tunnel at the elbow) — elbow flexion reduces tunnel volume by up to 55%, making night splinting the most effective conservative intervention.
- Supplies all intrinsic hand muscles except the LOAF group (median) — the Froment sign (compensatory thumb IP flexion during pinch) is the most specific motor test.
- The dorsal cutaneous branch exits before Guyon's canal — intact dorsal hand sensation localizes the lesion to the wrist, not the elbow.
- First dorsal interosseous wasting (visible gutter between thumb and index metacarpals) is the earliest visible sign of chronic ulnar nerve compression.