The radial nerve is a nerve in the human body that supplies the posterior portion of the upper limb. It innervates the medial and lateral heads of the triceps brachii muscle of the arm, as well as all 12 muscles in the posterior osteofascial compartment of the forearm and the associated joints and overlying skin.
It originates from the brachial plexus, carrying fibers from the posterior roots of spinal nerves C5, C6, C7, C8 and T1.[1]
The radial nerve and its branches provide motor innervation to the dorsal arm muscles (the triceps brachii and the anconeus) and the extrinsic extensors of the wrists and hands; it also provides cutaneous sensory innervation to most of the back of the hand, except for the back of the little finger and adjacent half of the ring finger (which are innervated by the ulnar nerve).
The radial nerve divides into a deep branch, which becomes the posterior interosseous nerve, and a superficial branch, which goes on to innervate the dorsum (back) of the hand.
This nerve was historically referred to as the musculospiral nerve.[2]
The radial nerve originates from the posterior cord of the brachial plexus with root values of C5 to C8 and T1. From the brachial plexus, it travels behind the third part of the axillary artery (part of the axillary artery distal to the pectoralis minor). In the arm, it runs behind the brachial artery and then enters the lower triangular space to reach the radial sulcus on the back of the humerus.[1] It travels downwards together with profunda brachii artery, between the lateral and medial heads of triceps brachii until it reaches the lateral side the arm at 5 cm below the deltoid tuberosity where it pierces the lateral intermuscular septum to reach the anterior compartment of the arm. Then, it descends down to cross the lateral epicondyle of the humerus where the nerve terminates by branching itself into superficial and deep branch which continues into cubital fossa and then into the forearm.[3]
The radial nerve gives out muscular branches to supply the long head, medial head, and lateral head of triceps brachii muscles before and during its course in the radial sulcus. After it emerges out from the radial sulcus, it supplies the brachialis, brachioradialis and extensor carpi radialis longus.[3]
In the forearm, it is divided into a superficial branch (primarily sensory) and a deep branch (primarily motor).[4]
The superficial branch of the radial nerve is widely separated from the radial artery in the upper one third of the forearm, closely related to radial artery in the middle third of the forearm, and in the lower third, it descends in the forearm under the tendon of brachioradialis. It crosses brachioradialis to enter posterior of forearm near the back of the wrist and supply dorsum of hand. It gives sensory supply to dorsal aspect of hand, dorsal aspect of thumb, index finger, middle finger and lateral side of ring finger except the nail beds, which are supplied by proper digital branches of median nerve.[3]
It is commonly believed that the radial nerve provides motor innervation to the long head of the triceps. However, a study conducted in 2004 found that axillary nerve innervated the long head of the triceps in twenty cadavers without any supply from the radial nerve.[7]
Function
The following are branches of the radial nerve (including the superficial branch of the radial nerve and the deep branch of the radial nerve/posterior interosseous nerve).
Cutaneous
Cutaneous innervation of the right upper extremity. Areas innervated by the radial nerve are colored in pink.
Cutaneous innervation by the radial nerve is provided by the following nerve branches:
The superficial branch of the radial nerve provides sensory innervation to much of the back of the hand, including the web of skin between the thumb and index finger.
Motor
Muscles of the posterior forearm. All the labelled muscles (that is, all the visible muscles except the ones on the dorsal hand and one at top left) are innervated by the radial nerve, and represent all muscles innervated by the radial nerve except for the supinator.
Loss of extension of forearm, weakness of supination, and loss of extension of hand and fingers.
Presence of wrist drop, due to inability to extend the hand and fingers.[9][10]
Sensory deficit: Loss of sensation[9] in lateral arm, posterior forearm, the radial half of dorsum of hand, and dorsal aspect of radial 3+1⁄2 digits, excluding their nail beds.
Weakness of supination, and loss of extension of hand and fingers.
Presence of wrist drop, due to inability to extend the hand and fingers.[9][10]
Sensory deficit: Loss of sensation in posterior forearm, the radial half of dorsum of hand, and dorsal aspect of radial 3+1⁄2 digits, excluding their nail beds.
Just below the elbow
Common mechanism of injury: Neck of radius fracture, elbow dislocation or fracture, tight cast, rheumatoid nodules, injections due to tennis elbow, injuring the deep branch of the radial nerve that pierces the radial head, causing posterior interosseous nerve syndrome
Motor deficit:
Weakness in extension of hand and loss of extension of fingers.
^ abcdeKrishna, Garg (2010). "8 - Arm". BD Chaurasia's Human Anatomy (Regional and Applied Dissection and Clinical) Volume 1 - Upper limb and thorax (5th ed.). India: CBS Publishers and Distributors Pvt Ltd. pp. 95, 111, 122, 128. ISBN978-81-239-1863-1.
^Palastanga, Nigel; Field, Derek; Soames, Roger (1994), "Preface", Anatomy and Human Movement, Elsevier, pp. viii, ISBN978-0-7506-0970-8, retrieved 2024-08-23
^de Sèze MP, Rezzouk J, de Sèze M, Uzel M, Lavignolle B, Midy D, Durandeau A (2004). "Does the motor branch of the long head of the triceps brachii arise from the radial nerve? An anatomic and electromyographic study". Surg Radiol Anat. 26 (6): 459–61. doi:10.1007/s00276-004-0253-z. PMID15365769. S2CID10052988.
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