Flexor carpi ulnaris: the anterior forearm muscle with a wholly ulnar nerve supply
Flexor carpi ulnaris, the muscle on the little-finger side that bends the wrist, is the one whole muscle of the anterior forearm supplied by the ulnar nerve.
Remember
Flexor carpi ulnaris is the ulnar-supplied muscle of the superficial anterior forearm, so it keeps working when the median nerve is injured at the elbow and weakens when the ulnar nerve is damaged.
The anterior compartment is the front group of muscles in the forearm. It holds the flexors (muscles that bend the wrist and fingers) and the pronators, which turn the palm downwards, and it is arranged in three layers: superficial, intermediate (the middle one, between the superficial and deep layers) and deep. The median nerve supplies nearly all of these muscles. The ulnar nerve supplies two exceptions: flexor carpi ulnaris and the medial half of flexor digitorum profundus (a deep finger flexor), the half that moves the ring and little fingers.
Flexor carpi ulnaris is the most medial of the four muscles of the superficial layer, which all arise from the medial epicondyle (a bony bump at the lower end of the humerus, on the inner side). Beside it in that layer lie pronator teres, palmaris longus and flexor carpi radialis, and the median nerve supplies all three. It attaches to the pisiform, a small wrist bone, and it flexes and adducts the wrist, which means it bends the wrist and tilts the hand towards the little-finger side (ulnar deviation).
This exception helps with nerve injuries. If the median nerve is injured at the elbow, those three muscles are expected to lose function, but flexor carpi ulnaris is spared (it keeps working). Flexion at the wrist survives but is still weak, and the hand is pulled towards the little-finger side during attempted flexion. If the ulnar nerve is injured where it runs behind the medial epicondyle, flexor carpi ulnaris is denervated (it has lost its nerve supply), so wrist flexion and adduction weaken, and any flexion that remains tends to drift towards the thumb side. In that ulnar nerve injury, turning the palm downwards (pronation) is preserved, because the median nerve supplies pronator teres.
Key facts
- Nerve supply
- flexor carpi ulnaris is supplied by the ulnar nerve, while the median nerve supplies nearly all of the rest of the anterior forearm.
- Other ulnar part
- the medial half of flexor digitorum profundus, the part for the ring and little fingers, is also supplied by the ulnar nerve.
- Action
- it flexes and adducts the wrist, so its loss weakens wrist flexion and ulnar deviation.
- Attachment
- it attaches to the pisiform, a small bone of the wrist.
- Position
- it is the most medial of the four muscles in the superficial layer.
- Name clue
- it is the only muscle in the superficial layer with ulnar in its name, and the ulnar nerve supplies it.
Exam traps
- Flexor digitorum superficialis is not in the superficial layer despite its name, because it forms the intermediate layer, and the median nerve supplies it.
- Flexor carpi radialis does not explain weak ulnar deviation, because it flexes the wrist and abducts it towards the thumb side, and the median nerve supplies it.
- Palmaris longus is a median nerve muscle, not an ulnar one, and it is tempting because it lies beside flexor carpi ulnaris in the superficial layer.
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