Origin and topography of the main trunk
The radial nerve is formed by fibers from spinal nerves C₅–Th₁ and is a continuation of the posterior cord of the brachial plexus (fasciculus posterior).
As it courses down the arm, the nerve passes through the humeromuscular canal (canalis humeromuscularis) and then enters the cubital fossa (fossa cubitalis). Along this path, muscular and cutaneous branches arise from the main trunk, supplying the triceps brachii and providing sensation to the posterior arm.
Branches before division (at the arm level)
In the arm, several important branches arise from the main trunk of the nervus radialis:
- Muscular branches (rr. musculares). These supply the long, lateral, and medial heads of the triceps brachii muscle (m. triceps brachii), as well as the anconeus muscle (m. anconeus).
- Posterior cutaneous nerve of the arm (n. cutaneus brachii posterior). Responsible for sensory innervation of the skin on the posterior surface of the arm.
- Inferior lateral cutaneous nerve of the arm (n. cutaneus brachii lateralis inferior). Innervates the skin of the lower lateral part of the arm.
- Posterior cutaneous nerve of the forearm (n. cutaneus antebrachii posterior). Provides sensation to the skin of the posterior forearm.
Terminal branches: superficial and deep
Upon reaching the level of the lateral epicondyle of the humerus, the radial nerve divides into two major terminal branches:
- Superficial branch (r. superficialis n. radialis). This is a purely sensory branch. It courses down the forearm deep to the brachioradialis muscle (m. brachioradialis). Descending onto the hand, this branch supplies the skin of the dorsal radial side of the hand, the thumb, and partially the proximal phalanges of digits II and III.
- Deep branch (r. profundus n. radialis), also known as the posterior interosseous nerve (n. interosseus posterior). This branch is predominantly motor and travels to the posterior forearm to innervate the distal group of extensors.
Innervation zones of the deep branch
The deep branch of the radial nerve performs a vast amount of work by innervating the forearm muscles and upper limb joints.
Muscular innervation includes the following muscles:
- Supinator muscle (m. supinator);
- Extensor digitorum (m. extensor digitorum);
- Extensor digiti minimi (m. extensor digiti minimi);
- Extensor carpi ulnaris (m. extensor carpi ulnaris);
- Abductor pollicis longus (m. abductor pollicis longus);
- Extensor pollicis brevis (m. extensor pollicis brevis);
- Extensor pollicis longus (m. extensor pollicis longus).
In addition to muscles, the deep branch gives off articular branches to the elbow joint (articulatio cubiti), wrist joint (articulatio radiocarpalis), and metacarpophalangeal joints (articulationes metacarpophalangeae).
Briefly: answers to topical questions
Which muscles are innervated by the muscular branches of the radial nerve before it divides into terminal branches?
Before dividing into terminal branches, the muscular branches of the radial nerve supply:
- The triceps brachii (m. triceps brachii) — long, lateral, and medial heads.
- The anconeus muscle (m. anconeus).
Which forearm muscles does the deep branch of the radial nerve innervate?
The deep branch of the radial nerve (r. profundus n. radialis) innervates several muscles of the posterior forearm compartment. These include:
- Supinator (m. supinator).
- Extensor digitorum (m. extensor digitorum).
- Extensor digiti minimi (m. extensor digiti minimi).
- Extensor carpi ulnaris (m. extensor carpi ulnaris).
- Abductor pollicis longus (m. abductor pollicis longus).
- Extensor pollicis brevis (m. extensor pollicis brevis).
- Extensor pollicis longus (m. extensor pollicis longus).
Through which anatomical canal does the deep branch of the radial nerve pass to reach the posterior compartment of the forearm?
The deep branch of the radial nerve enters the posterior forearm compartment through the supinator canal (canalis supinatorius). This canal lies between the superficial and deep parts of the supinator muscle (m. supinator). Passing through it, the nerve, together with the muscle, wraps around the neck of the radius and enters the posterior fascial compartment, positioning itself among the posterior forearm muscles to innervate them.
Class and exam questions
Where does the radial nerve divide into superficial and deep branches?
The radial nerve divides into superficial and deep branches at the level of the lateral epicondyle of the humerus.
From which cord of the brachial plexus does the radial nerve emerge?
The radial nerve originates from the posterior cord of the brachial plexus (fasciculus posterior).
Which digits of the hand are innervated by the superficial branch of the radial nerve?
The superficial branch innervates the skin of the thumb, as well as partially the proximal phalanges of digits II and III on the dorsal surface of the hand.
What else to study on this topic
- Topography of the humeromuscular canal (canalis humeromuscularis)
- Boundaries and contents of the cubital fossa (fossa cubitalis)
- Muscles of the posterior forearm compartment
- Anatomy of the posterior cord of the brachial plexus
- Sensory zones of the dorsum of the hand and overlap of innervation
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