Ulnar nerve

The ulnar nerve is a major peripheral nerve located along the medial side of the upper limb, supplying motor and sensory innervation to specific muscles and skin regions in the forearm and hand.

Origin

The ulnar nerve arises from the medial cord of the brachial plexus, receiving fibers from the eighth cervical (C8) and first thoracic (T1) spinal segments. At its point of origin in the axilla, it lies medial to the axillary artery.

The ulnar nerve in the forearm

1. Course in the forearm

• In the arm, the ulnar nerve descends along the medial side of the brachial artery.

• At the mid-arm region, it pierces the medial intermuscular septum, entering the posterior compartment of the arm.

• At the elbow, the nerve passes behind the medial epicondyle of the humerus, distinct from the median and radial nerves, as it bypasses the cubital fossa.

• It then enters the flexor side of the forearm by passing between the two heads of the flexor carpi ulnaris muscle.

• In the upper forearm, the ulnar nerve descends along the medial forearm, lying superficial to the flexor digitorum profundus and deep to the flexor carpi ulnaris.

• Near the lower forearm, the ulnar nerve deflects slightly laterally and comes to lie between the flexor digitorum profundus and flexor digitorum superficialis muscles.

2. Relationship with the ulnar artery

• In the upper third of the forearm, the ulnar nerve is separated from the ulnar artery by a notable distance.

• As it descends, the nerve lies just medial to the artery for most of its course.

3. Branches of the ulnar nerve in the forearm

a. Articular branches: Supply the elbow joint.

b. Muscular Branches: Innervate the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.

c. Cutaneous Branches:

i. Palmar Cutaneous Branch:

• Arises approximately 5 cm above the wrist.

• Passes superficially over the flexor retinaculum.

• Innervates the skin over the hypothenar eminence, connecting with the median nerve's palmar cutaneous branch.

ii. Dorsal Cutaneous Branch:

• Winds around to the dorsum of the hand after traveling under the flexor carpi ulnaris tendon.

• Pierces deep fascia to innervate the dorsal aspect of the proximal parts of the medial 1½ fingers and adjoining area on the dorsum of the hand, via its two dorsal digital branches: One dorsal digital branch supplies the ulnar side of the little finger; the other dorsal digital branch innervates the adjacent sides of the little and ring fingers.

• Please note that the distal parts of these digits are supplied by dorsal cutaneous branches of the proper palmar (volar) digital nerves of the ulnar nerve.

• A communicating branch connects with the radial nerve's dorsal cutaneous branches.

The ulnar nerve in the palm region

1. Transition into the palm

• As the ulnar nerve crosses the wrist, it passes over the flexor retinaculum (outside the carpal tunnel), lying between the pisiform bone (medially) and the ulnar artery (laterally).

2. Terminal branches in the palm

The ulnar nerve divides into two terminal branches: superficial and deep branches.

i. Superficial Terminal Branch:

• Muscular Branches: Supply the palmaris brevis muscle.

• Cutaneous Branches: These include two palmar digital nerves (medial and lateral), for  the medial one and half fingers.

a. Medial Palmar Digital Nerve: This is a proper palmar digital nerve that innervates the skin of the medial side of the little finger.

b. Lateral Palmar Digital Nerve: This is a common palmar digital nerve, which splits into two proper palmar digital nerves and innervates the adjoining skin of the ring and little fingers.

• A communicating branch is given to the median nerve in the palm.

ii. Deep Terminal Branch:

• Muscular Branches: Supply the hypothenar muscles (abductor digiti minimi, opponens digiti minimi, flexor digiti minimi), the medial two lumbricals, all dorsal and palmar interossei, adductor pollicis, and occasionally the deep head of flexor pollicis brevis.

• Articular Branches: To the wrist joint.