Anatomy · Brain & nerves

Ulnar nerve

The 'funny bone' nerve

Peripheral nerves · upper limb

A terminal branch of the medial cord of the brachial plexus (C8–T1). It runs down the inner side of the arm, passes in a groove behind the medial epicondyle of the humerus — the 'funny bone' — and continues down the forearm to enter the hand beside the pisiform bone. It supplies one and a half forearm muscles, most of the small muscles of the hand, and the skin of the little finger and the inner half of the ring finger.

How to spot them

▸Passes in a shallow groove behind the bony knob on the inner side of the elbow, where it lies just under the skin
▸Runs down the inner side of the forearm beside the ulnar artery
▸Enters the hand through a small tunnel beside the pisiform bone, over the flexor retinaculum, rather than through the carpal tunnel
▸Gives no branches in the upper arm

What to know

Knocking it behind the medial epicondyle of the humerus produces a tingling shock down the little finger — the 'funny bone'

Ulnar nerve — it lies unprotected in the groove behind the medial epicondyle.

The only major nerve of the arm that is not protected by muscle or bone as it passes the elbow

Ulnar nerve — hence its vulnerability.

After a lesion, the ring and little fingers curl into a claw while the index and middle fingers are largely spared, because the inner two lumbricals fail

Ulnar nerve — the 'claw hand' (main en griffe).

Gripping a sheet of paper between the thumb and index finger, the patient bends the thumb tip to compensate for a weak adductor pollicis

Ulnar nerve — Froment's sign.

Supplies the skin of the little finger and the inner half of the ring finger, on both the palm and back surfaces

Ulnar nerve — the medial one and a half digits.

Passes with the ulnar artery through a small fibro-osseous tunnel at the wrist beside the pisiform, named after a French surgeon

Ulnar nerve — Guyon's canal.

Supplies flexor carpi ulnaris and half of flexor digitorum profundus, plus the hypothenar muscles, interossei and adductor pollicis

Ulnar nerve — most of the intrinsic hand muscles except the LOAF group.

Key ideas

Compression at the elbow (cubital tunnel syndrome) is the second most common entrapment neuropathy of the arm, after carpal tunnel syndrome.
The ulnar nerve supplies the muscles that make fine finger movements; the median nerve supplies the ones for coarse grip.

Quick-fire facts

•Arises from the medial cord of the brachial plexus, roots C8 and T1 (with a variable contribution from C7)
•Gives off no branches in the arm; in the forearm it supplies flexor carpi ulnaris and the medial half of flexor digitorum profundus
•Passes behind the medial epicondyle of the humerus through the cubital tunnel
•Enters the hand via Guyon's canal, superficial to the flexor retinaculum, beside the pisiform
•Supplies most of the intrinsic hand muscles: all interossei, the hypothenar muscles, adductor pollicis, and the medial two lumbricals
•Its lesion produces claw hand, Froment's sign and wasting of the small muscles of the hand
•Sensation to the little finger and medial half of the ring finger, both palm and back
•The 'funny bone' is not a bone but the ulnar nerve; the pun is on 'humerus'

Drill it — 7 cards

1 / 7new card

Sources

TeachMeAnatomy — Ulnar nerve
OpenStax — 13.4 The Peripheral Nervous System — Anatomy and Physiology 2e