Ulnar Nerve Anatomy

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

ORIGIN AND COURSE

• The ulnar nerve originates from the C8 and T1 nerve roots.

• These roots form the medial cord of the brachial plexus.

• The nerve descends along the medial aspect of the arm.

• At the elbow, it passes posterior to the medial epicondyle of the humerus.

• This superficial position makes the nerve vulnerable to trauma and compression.

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COURSE AT THE ELBOW (CUBITAL TUNNEL)

• Behind the medial epicondyle, the ulnar nerve runs through a fibro-osseous tunnel known as the cubital tunnel.

• The cubital tunnel is formed by:

  • The medial epicondyle

  • The olecranon

  • The cubital tunnel retinaculum

• Compression or irritation at this site results in cubital tunnel syndrome.

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ENTRY INTO THE FOREARM

• After passing behind the medial epicondyle, the ulnar nerve enters the forearm.

• It passes between the two heads of the flexor carpi ulnaris muscle.

• In the forearm, it travels:

  • Medial to the flexor digitorum profundus

  • Deep to the flexor carpi ulnaris

• The nerve begins giving motor and sensory branches in the upper forearm.

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MOTOR INNERVATION IN THE FOREARM

• Flexor carpi ulnaris

• Medial half of the flexor digitorum profundus (to the ring and little fingers)

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COURSE AT THE WRIST

• The ulnar nerve continues along the medial border of the forearm, adjacent to the ulna.

• At the wrist, it lies just lateral to the pisiform bone.

• It enters the hand through Guyon canal.

• Within Guyon canal:

  • The ulnar nerve and ulnar artery pass superficial to the flexor retinaculum.

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BRANCHES IN THE HAND

Deep Branch (Motor)

The deep branch of the ulnar nerve supplies:

1. Abductor digiti minimi

2. Flexor digiti minimi

3. Opponens digiti minimi

4. Third and fourth lumbricals

5. Palmar interossei

6. Dorsal interossei

7. Adductor pollicis

8. Deep head of flexor pollicis brevis

• The ulnar nerve supplies all intrinsic hand muscles medial to the flexor pollicis longus, except the lateral two lumbricals.

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Superficial Branch (Sensory)

• Provides palmar sensory innervation to:

  • Little finger

  • Medial half of the ring finger

• Also provides dorsal sensory innervation to the same digits.

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CLINICAL TEST

Tinel Sign at the Elbow

• Performed by tapping over the ulnar nerve at the medial epicondyle.

• A positive test produces:

  • Tingling

  • Electric shock-like sensation

  • Numbness radiating to the ring and little fingers

• This sensation is similar to striking the “funny bone”.

• Indicates ulnar nerve entrapment at the cubital tunnel.

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CLINICAL FEATURES OF ULNAR NERVE COMPRESSION

• Pain at the elbow

• Numbness or tingling in the hand

• Paresthesia of the fourth and fifth digits

• Weak grip and pinch strength

• Intrinsic muscle weakness

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CLAW HAND DEFORMITY

• Injury or entrapment of the ulnar nerve below the elbow may result in ulnar claw hand.

• Caused by:

  • Loss of intrinsic muscle function

  • Unopposed action of the flexor digitorum profundus

• Characterized by:

  • Hyperextension at the metacarpophalangeal joints

  • Flexion at the interphalangeal joints of the ring and little fingers

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KEY POINTS

• The ulnar nerve is most vulnerable at the medial epicondyle and Guyon canal.

• It plays a critical role in fine motor control of the hand.

• Early recognition of compression is essential to prevent permanent intrinsic muscle wasting.