Ulnar Nerve

Introduction

Introduction to the Ulnar Nerve in the Arm

The ulnar nerve is a major peripheral nerve of the upper limb, arising from the medial cord of the brachial plexus (C8-T1). It is often called the 'funny bone' nerve due to its superficial position at the elbow. In the arm, the ulnar nerve runs from the axilla into the anterior compartment, medial to the brachial artery, then pierces the medial intermuscular septum at mid-arm to enter the posterior compartment. It descends posterior to the medial epicondyle (in the cubital tunnel) to reach the forearm. In the arm, it gives off no muscular branches (except for a small branch to the medial head of triceps inconstant), but it gives an articular branch to the elbow joint and a cutaneous branch (palmar and dorsal branches later). Its course in the arm makes it vulnerable to injury in humeral shaft fractures, medial epicondyle fractures, and external compression.

Study

Origin and Course in the Arm

The ulnar nerve arises from the medial cord (C8-T1) of the brachial plexus, receiving fibers from C8 and T1 (with possible minor contribution from C7 via medial cord). It descends in the axilla medial to the axillary artery. In the proximal arm, it lies medial to the brachial artery and anterior to the medial head of triceps. At approximately the mid-arm (level of the coracobrachialis insertion), the ulnar nerve pierces the medial intermuscular septum to enter the posterior compartment of the arm. It then runs on the medial head of triceps, passing behind the medial epicondyle of the humerus (in the cubital tunnel) to enter the forearm. The nerve is accompanied by the superior ulnar collateral artery (branch of brachial artery) throughout its course in the arm.

Branches in the Arm

The ulnar nerve typically gives no muscular branches in the arm to the triceps (though some individuals have a small branch to the medial head, clinically insignificant). Branches in the arm: - Articular branch to the elbow joint: Arises near the medial epicondyle, supplies the elbow capsule. - Vascular branches to the brachial artery (sympathetic). - It does NOT give cutaneous branches until the forearm (dorsal and palmar cutaneous branches). The first motor branch is in the forearm to the flexor carpi ulnaris and the ulnar half of flexor digitorum profundus.

Clinical Relevance in the Arm

Ulnar nerve injury in the arm can result from: - Humeral shaft fractures (especially at the distal third near medial epicondyle). - Medial epicondyle fractures (common in children, can entrap the nerve). - Cubital tunnel syndrome (compression behind medial epicondyle) – often from repetitive elbow flexion, not strictly an 'arm' lesion but involves the transition. - Gunshot or stab wounds to the medial arm. - Compression during prolonged surgery or coma ('postoperative ulnar neuropathy'). A lesion in the arm (above the elbow) produces a complete ulnar nerve palsy: weakness of all ulnar-innervated muscles (FCU, FDP to ring/little, hypothenar, interossei, lumbricals 3&4, adductor pollicis), sensory loss over the ulnar 1.5 digits (little and ulnar half of ring) both dorsally and palmarly, and characteristic 'claw hand' deformity (hyperextension at MCP, flexion at IP of ring and little fingers). The claw is less severe if the lesion is at the wrist because FDP is spared – but in an arm lesion, FDP to ring/little is paralyzed, so the claw is less prominent? Actually, classic teaching: low ulnar palsy (wrist) causes claw hand because FDP is spared (unopposed pull). High ulnar palsy (arm) also causes claw but with less flexion deformity? Wait: The claw deformity in ulnar palsy occurs due to loss of lumbricals (unopposed pull of FDP and extensor digitorum). If FDP is also paralyzed (high lesion), the claw may be milder because the flexor pull is lost. However, the classic 'claw hand' is more associated with low lesions. But high lesions still cause interossei and lumbrical loss, and the ring/little may still hyperextend at MCP if the extensor digitorum is unopposed. Medically, both present with some clawing. We'll keep it accurate.

Summary

Key Takeaways – Ulnar Nerve in the Arm

Origin: Medial cord of brachial plexus (C8-T1). Course: Medial to brachial artery in proximal arm → pierces medial intermuscular septum (mid-arm) → posterior compartment → behind medial epicondyle. Branches in arm: Articular to elbow; no muscular branches (except inconstant to triceps). Clinical: Vulnerable to medial epicondyle fractures, humeral shaft fractures, and cubital tunnel compression. Injury causes ulnar claw hand, sensory loss over ulnar 1.5 digits, and weakness of all ulnar-innervated muscles.