The radial nerve arises in the axillary region and descends down along the posterior surface of the humerus. It then passes through the cubital fossa and terminates in the posterior compartment of the forearm, by dividing into two terminal branches: superficial (sensory) and deep (motor).
Due to its length, the radial nerve is the most commonly injured nerve of the upper extremity. The most known presentation of radial nerve palsy is the "wrist drop".
This article will discuss the anatomy and function of the radial nerve.
|Origin||Posterior cord of brachial plexus (C5-T1)|
|Branches||Posterior brachial cutaneous nerve, inferior lateral brachial cutaneous nerve, posterior antebrachial cutaneous nerve, muscular branches, deep branch of radial nerve, superficial branch of radial nerve|
Motor: triceps brachii, anconeus, brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, supinator, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis
Sensory: lower outer aspect and posterior surface of the arm, central and posterior aspect of the forearm, thenar eminence and dorsal aspect of the radial three and half digits of the hand
Origin and course
The radial nerve is the largest terminal branch of the brachial plexus. It originates from the posterior cord along with the axillary nerve, carrying fibers from ventral roots of spinal nerves C5-C8 and T1.
The radial nerve arises in the axilla, immediately posterior to the axillary artery, between coracobrachialis and teres major muscles. It descends obliquely downwards through the posterior aspect of the arm, between the bellies of medial and lateral head of triceps, through a shallow depression on the posterior surface of the humerus (radial groove).
For the most part of its course in the arm, the nerve is accompanied by the brachial artery.
In the distal part of the arm, the radial nerve wraps around the distal humerus and courses anterior to the lateral condyle of humerus, where it penetrates the lateral intermuscular septum. Upon crossing the cubital fossa, the radial nerve terminates by dividing into two terminal branches: superficial (sensory) and deep (motor).
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Branches and innervation
Along its course in the arm region, the radial nerve provides muscular branches that innervate the triceps brachii, anconeus, and brachioradialis muscles. Here, it also gives off two sensory branches (posterior brachial cutaneous nerve and lateral inferior cutaneous brachial nerve) that innervate the skin of the posterior aspect of the arm as well as its inferior lateral aspect.
Upon entering the cubital region, and before its division, the radial nerve provides one more sensory branch called the posterior antebrachial cutaneous nerve. This nerve innervates a strip of skin down the middle of the posterior forearm.
The deep branch, also known as the “motor branch”, provides motor innervation to the posterior compartment of the forearm.
This nerve descends inferiorly through the posterior aspect of the forearm where it penetrates the supinator muscle and emerges as the posterior interosseous nerve. Its main function is to supply the muscles located in the posterior compartment of the forearm (the wrist extensors and the long muscles of the thumb). These muscles include the: extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum, extensor indicis, extensor digiti minimi, extensor pollicis longus, extensor pollicis brevis and abductor pollicis longus.
The superficial branch continues the course of the radial nerve and enters the hand from the radial side. This branch is also known as the "sensory branch" because of its primary role to provide sensation to the thenar eminence and dorsal aspect of the radial 3 and a half digits of the hand.
Radial nerve palsy
The radial nerve is the most commonly injured nerve of the arm. The injuries of this nerve usually occur due to fractures of the humerus. The nerve can also be injured when it is "overused" (e.g. sports-related injuries) or compressed (e.g. improper use of crutches).
Radial nerve injuries have distinct presentations and symptoms, depending on the anatomic location and type of injury.
The most well-known motor presentation of the radial nerve injury is the "wrist drop". It occurs due to the paralysis of the posterior forearm muscles and their inability to extend the wrist.
The sensory loss will depend on the anatomical location of the injury. For example, if the nerve is injured in the axillary region, the sensory loss will be located at the lateral arm and the posterior aspect of the forearm radiating to the radial aspect of the hand and digits. This is seen commonly with "Saturday night palsy" (e.g. when the arm is resting on the chair for a log time compressing the nerve).