Median Nerve

Introduction

Introduction to the Median Nerve in the Arm

The median nerve is a major peripheral nerve of the upper limb, formed from the lateral and medial cords of the brachial plexus (C5-T1). In the arm, it has no branches (except for occasional small vasomotor fibers). It runs from the axilla into the anterior compartment of the arm, initially lateral to the brachial artery, then crosses anterior to the artery at mid-arm to lie medial to it in the distal arm. It enters the cubital fossa deep to the bicipital aponeurosis, then passes between the two heads of pronator teres to reach the forearm. Understanding its course and relations in the arm is critical for diagnosing nerve injuries and avoiding iatrogenic damage during procedures.

Study

Origin and Formation of the Median Nerve

The median nerve is formed by two roots: - Lateral root: from lateral cord of brachial plexus (C5-C7). Carries mostly sensory fibers and fibers to thenar muscles. - Medial root: from medial cord of brachial plexus (C8-T1). Carries mostly motor fibers to hand intrinsic muscles (except hypothenar and interossei). The two roots unite anterior or lateral to the axillary artery, forming the median nerve in the axilla. In the arm, the nerve lies initially lateral to the brachial artery.

Course in the Arm

In the proximal arm: The median nerve enters the arm from the axilla, lying lateral to the brachial artery. It is superficial to the coracobrachialis and brachialis muscles. The ulnar nerve is medial to the artery at this level. At mid-arm (approximately at the insertion of coracobrachialis): The median nerve crosses anterior to the brachial artery, passing from lateral to medial. This crossing is a key landmark. In the distal arm: After crossing, the median nerve lies medial to the brachial artery. Both pass deep to the bicipital aponeurosis at the cubital fossa. The nerve then exits the arm by passing between the two heads of pronator teres (humeral and ulnar heads) to enter the forearm. Relations in the cubital fossa: Lateral to medial: biceps tendon, brachial artery, median nerve. The median nerve is the most medial structure.

Branches in the Arm

The median nerve classically gives off NO muscular or cutaneous branches in the arm. It is purely a conduit to the forearm and hand. Small vasomotor branches to the brachial artery may occur but are clinically insignificant. The first branches of the median nerve occur in the forearm: pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and the anterior interosseous branch. In the cubital fossa, it may give off the articular branch to the elbow joint (inconstant).

Clinical Relevance in the Arm

Median nerve injury in the arm is rare because it is well protected. Causes include: - Supracondylar humeral fracture: Can stretch or compress the median nerve (especially the anterior interosseous branch later, but the main nerve can be injured). - Penetrating trauma (stab wounds) to the medial arm. - Iatrogenic: venipuncture of the basilic vein (median nerve lies deep but can be injured if deep puncture), or during surgical approaches to the brachial artery. - Compressive neuropathies: Rarely, a Struther's ligament (supracondylar process of humerus) can compress the median nerve and brachial artery – median nerve then passes through the ligament of Struthers. Lesion in the arm produces motor loss of all median-innervated muscles (forearm pronators, wrist/finger flexors, thenar muscles) and sensory loss over lateral palm and lateral 3.5 digits. However, because no branches in arm, the clinical picture is similar to a high median nerve lesion (above elbow).

Summary

Key Takeaways – Median Nerve in the Arm

Formed from lateral (C5-C7) and medial (C8-T1) cords of brachial plexus. In arm: No branches. Initially lateral to brachial artery, crosses anterior at mid-arm to become medial. Enters cubital fossa medial to brachial artery, deep to bicipital aponeurosis. Exits arm between heads of pronator teres. Clinical: Injury causes high median nerve palsy – loss of pronation, wrist/finger flexion (except ulnar digits), thenar function, and sensory loss over lateral palm and 3.5 digits.