The Ulnar Nerve
The ulnar nerve is a major peripheral nerve of the upper limb. In this article, we shall look at the applied anatomy of the nerve – its anatomical course, motor functions and cutaneous innervation. We shall also consider the clinical correlations of damage to the ulnar nerve.
Spinal roots: C8-T1.
Motor functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus.
Sensory functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and the associated palm area.
The ulnar nerve is derived from the brachial plexus. It is a continuation of the medial cord, containing fibres from spinal roots C8 and T1.
After arising from the brachial plexus, the ulnar nerve descends down the medial side of the upper arm. At the elbow, it passes posterior to the medial epicondyle of the humerus, entering the forearm. At the medial epicondyle, the nerve is easily palpable and vulnerable to injury.
In the forearm, the ulnar nerve pierces the two heads of the flexor carpi ulnaris, and travels alongside the ulna. Three branches arise in the forearm:
- Muscular branch: innervates some muscles in the anterior compartment of the forearm.
- Palmar cutaneous branch: innervates the skin of the medial half of the palm.
- Dorsal cutaneous branch: innervates the skin of the medial 1 and 1/2 fingers, and the associated palm area.
(The functions of these nerves are explored in more detail later in the article).
At the wrist, the ulnar nerve travels superficially to the flexor retinaculum. It enters the hand via the ulnar canal (or Guyon’s canal). In the hand the nerve terminates by giving rise to superficial and deep branches.
The Anterior Forearm
In the anterior forearm, the muscular branch of the ulnar nerve supplies two muscles:
- Flexor carpi ulnaris – Flexes and adducts the hand at the wrist.
- Flexor digitorum profundus (medial half) – Flexes the fingers.
The remaining muscles in the anterior forearm are innervated by the median nerve.
The majority of the intrinsic hand muscles are innervated by the deep branch of the ulnar nerve.
The hypothenar muscles (a group of muscles associated with the little finger) are innervated by the ulnar nerve. It also innervates some other muscles of the hand:
- Medial two lumbricals
- Adductor pollicis
- Interossei of the hand
- Palmaris brevis
The other muscles in the hand (such as the thenar eminence) are innervated by the median nerve.
There are three branches of the ulnar nerve that are responsible for its cutaneous innervation.
Two of these branches arise in the forearm, and travel into the hand:
- Palmar cutaneous branch: Innervates the skin of the medial half of the palm.
- Dorsal cutaneous branch: Innervates the skin of the medial one and a half fingers, and the associated dorsal hand area.
The last branch arises in the hand itself:
- Superficial branch – Innervates the palmar surface of the medial one and a half fingers.
Clinical Relevance: Lesions of the Ulnar Nerve
The ulnar nerve is most susceptible to injury at the elbow and the wrist. In this section, we shall look at how damage may occur, and what the clinical sequelae of such damage may be.
Damaged at the Elbow
How it commonly occurs: The nerve is most vulnerable to injury at the medial epicondyle, but it can also be compressed in the cubital tunnel. This is known as cubital tunnel syndrome.
Motor functions: Flexor carpi ulnaris and medial half of flexor digitorum profundus paralysed. Flexion of the wrist can still occur, but is accompanied by abduction. The interossei are paralysed, so abduction and adduction of the fingers cannot occur. Movement of the little and ring fingers is greatly reduced, due to paralysis of the medial two lumbricals.
Sensory functions: All sensory branches are affected, so there will be a loss of sensation over the areas that the ulnar nerve innervates.
Characteristic signs: Patient cannot grip paper placed between fingers.
Damaged at the Wrist
How it commonly occurs: Lacerations to the wrist
Motor functions: The interossei are paralysed, so abduction and adduction of the fingers cannot occur. Movement of the little and ring fingers is greatly reduced, due to paralysis of the medial two lumbricals. The two muscles in the forearm are unaffected
Sensory functions: The palmar branch and superficial branch are usually severed, but the dorsal branch is unaffected. Sensory loss over palmar side of medial one and a half fingers only.
Characteristic signs: Patient cannot grip paper placed between fingers. For long-term cases, a hand deformity called ‘Ulnar Claw’ develops.
Ulnar claw consists of:
- Hyper-extension of the metacarpophalangeal joints of the little and ring fingers – this is because of the paralysis of the medial two lumbricals, and the now unopposed action of the extensor muscles
- Flexion at the interphalangeal joints (if the lesion has occurred close to the elbow, this might not be evident, as the flexor digitorum profundus will be paralysed)