Radial nerve

The radial nerve is the largest branch of the posterior cord of the brachial plexus, arising from the C5–T1 nerve roots. It plays a crucial role in motor and sensory innervation of the upper limb, especially the posterior compartments of the arm and forearm.

Origin and course in the axilla

• The radial nerve originates from the posterior cord of the brachial plexus in the lower part of the axilla.

• It descends posteriorly and lies behind the third part of the axillary artery before entering the arm.

Course in the arm

1. Proximal Arm

In the upper arm, the radial nerve initially runs close to the brachial artery. It soon separates from the artery and enters a triangular space, bordered by the humerus (laterally), long head of the triceps brachii (medially), and teres major muscle (superiorly).

2. Spiral groove

After passing through the triangular space, the radial nerve continues into the posterior compartment of the arm, coursing through the spiral groove of the humerus, where it lies between the lateral and medial heads of the triceps muscle.

Relationship with nearby structures:

The radial nerve is accompanied by the profunda brachii artery in the spiral groove. Here, it gives off:

• The posterior brachial cutaneous nerve (before entering the spiral groove), a.k.a. posterior cutaneous nerve of the arm,

• The inferior lateral brachial cutaneous nerve (within the groove), a.k.a. inferior lateral cutaneous nerve of the arm,

• The posterior antebrachial cutaneous nerve (within the groove), a.k.a. posterior cutaneous nerve of the forearm.

After exiting the spiral groove, the radial nerve pierces the lateral intermuscular septum approximately 5 cm distal to the deltoid insertion to enter the anterior compartment of the arm.

3. Distal arm and cubital fossa

In the lower arm, the radial nerve lies in front of the cubital fossa, positioned in the groove between:

• Brachialis (medially),

• Brachioradialis and extensor carpi radialis longus (laterally).

At this level, the nerve innervates these three muscles.

4. Division at the lateral epicondyle

At the lateral epicondyle of the humerus, at the upper border of the supinator muscle, the radial nerve divides into:

• The superficial branch (sensory), and

• The deep branch (motor).

Motor innervation in the arm

Before the Spiral Groove:

• The radial nerve innervates the long head and medial head of the triceps brachii.

Within the Spiral Groove:

• It innervates the lateral head and the medial head of the triceps brachii. The branch to the lateral head also innervates the anconeus.

Distal Arm:

• As the radial nerve descends in the intermuscular groove, it provides branches that innervate a) the brachioradialis, b) the extensor carpi radialis longus, and c) the brachialis (proprioceptive fibers).

5. The superficial branch of the radial nerve

• The superficial branch is a purely sensory continuation of the radial nerve.

• Course: It descends in the anterior forearm, underneath the brachioradialis, running alongside the radial artery in the middle third of the forearm. In the distal forearm, it turns laterally and passes under the tendon of the brachioradialis to reach the dorsum of the hand.

• Innervation: The superficial branch gives dorsal digital branches that supply a) the skin over the dorsum of the hand, b) the dorsal proximal parts of the thumb, index, middle, and the lateral half of the ring finger.

• Note that the distal dorsal aspects of these fingers are innervated by the median nerve.

• Clinical significance: Injury to this branch causes sensory loss over the dorsum of the hand.

6. The deep branch of the radial nerve

Course and Key Relations:

• The deep branch passes between the two heads of the supinator muscle, winding around the lateral side of the radius.

• Before piercing the supinator, it innervates the extensor carpi radialis brevis and the supinator muscle (it also gives branches to the supinator while traversing between its two heads).

7. Posterior compartment of the forearm (Posterior interosseous nerve)

After emerging from the supinator muscle, the deep branch is called the posterior interosseous nerve, which travels in the posterior compartment of the forearm between the superficial and deep extensor muscle groups. It gives off:

Short branches to the:

Extensor digitorum, Extensor digiti minimi, and Extensor carpi ulnaris.

Long branches

• Lateral branch: Supplies the abductor pollicis longus and extensor pollicis brevis.

• Medial branch: Supplies the extensor pollicis longus and extensor indicis.

Articular and sensory branches

• Articular branches: These supply the elbow joint, the wrist joint, the distal radioulnar joint, and the intercarpal and intermetacarpal joints.

• Sensory branches: These are given to the radius, ulna, and interosseous membrane.