Lying posterior to the vessels this nerve inclines backwards away from them and pierces the medial intermuscular septum in the lower third of the arm, accompanied by the superior ulnar collateral artery and a branch of the radial nerve to the medial head of triceps (the ‘ulnar collateral’ nerve). It gives no branch in the arm; its branch to the elbow joint comes off as it lies in the groove behind the medial epicondyle of the humerus.
The surface marking of the nerve is along a line from the medial bicipital groove behind coracobrachialis to the point behind the medial epicondyle of the humerus where it is readily palpable.
The nerve courses through the lower part of the extensor compartment and disappears into the forearm by passing between the humeral and ulnar heads of origin of flexor carpi ulnaris. It lies in contact with the bone in the groove behind the medial epicondyle, then lies against the medial ligament of the elbow joint, to which it gives a twig of supply.
The ulnar nerve enters the forearm from the extensor compartment by passing between the humeral and ulnar heads of origin of flexor carpi ulnaris. It is more easily compressed against the medial surface of the coronoid process than against the humerus where it lies behind the medial epicondyle. In the forearm the nerve lies under cover of the flattened Aponeurosis of flexor carpi ulnaris with the ulnar artery id its radial side. This neurovascular bundle lies on flexor digitorum profundus. Branches of supply are even to flexor carpi ulnaris and the ulnar half (usually) of flexor digitorum profundus. The branch to flexor carpi ulnaris contains C7 fibers brought to the ulnar nerve in the axilla; the branch to flexor digitorum profundus contains C8 and T1 fibers.
The ulnar nerve and ulnar artery emerge from beneath the tendon of flexor carpi ulnaris just proximal to the wrist and pass across the flexor retinaculum into the hand. Before emerging each gives off a dorsal branch which passes medially between the tendon of flexor carpi ulnaris and the lower end of the ulna. The dorsal branch of the ulnar nerve supplies the dorsum of the hand and of the ulnar one and a half fingers proximal to their nail beds. The dorsal branch of the ulnar artery enters the posterior carpal arch.
The surface marking of the nerve is along a line from the medial epicondyle of the humerus to the radial side of the pisiform bone.
Ulnar nerve in the hand
The ulnar nerve leaves the forearm by emerging from beneath the tendon of flexor carpi ulnaris and passes distally on the flexor retinaculum alongside the radial border of the pisiform bone. Here it divides into superficial and deep branches. The superficial branch supplies palmaris brevis and is then carried on as the digital nerves to the ulnar one and a half fingers. The deep branch passes deeply into the palm between the heads of origin of flexor and abductor digiti minimi and through the origin of opponens digiti minimi to lie on the hook of the hamate, whose distal border it indents with a shallow groove. Passing down to the interossei, it arches deeply in the palm within the concavity of the deep palmar arch. It gives motor branches to the three hypothenar muscles, the two lumbricals on the ulnar side, all the interossei and both heads of adductor pollicis. Compare the ulnar nerve in the hand with the lateral plantar in the foot. The cutaneous distribution is identical, but in the foot the superficial branch of the lateral plantar nerve, unlike that of the ulnar, supplies three muscles, and the deep branch supplies three instead of only two lumbricals.