The radial nerve is an essential component of the correctly functioning upper limb. It innervates essentially all the muscles on the posterior aspect of our arms, and is therefore a large nerve. When we look at our evolutionary lineage, we must ask ourselves why we have such powerful elbow extensors (triceps brachii). Such questions are always worth considering and can act as a clue to our evolutionary history. The nerve itself is relatively easy to identify behind the axillary artery, and this is also where we can find its smaller associated nerve, the axillary nerve. This article will cover the anatomical details as well as the clinical relevance of the nerve.
Course and Muscular Innervation
The radial nerve is the nerve of the posterior arm. This is the case for both the upper arm (three heads of the triceps) as well as the forearm (wrist and finger extensors).
The nerve originates from the posterior cord of the brachial plexus (from the ventral rami of C5-T1). The posterior cord runs posterior to the axillary artery, and goes from the anterior to posterior triangle via the triangular interval (bound by the teres major superiorly, the long head of triceps medially and the lateral head of the triceps laterally) and descends in the arm, by spiraling around the humerus. The profunda brachii (the deep branch of the brachial artery) supplies the upper arm muscles with blood, and also runs with the nerve through this triangular space and continues into the spiral groove. The nerve supplies the lateral and medial heads of triceps brachii. It has been thought that the third head (the long head) was also supplied by the radial nerve, however research has shown that the axillary nerve in fact supplies the long head of triceps brachii.
The radial nerve follows the same path as the profunda brachii branch of the brachial artery. It leaves the spiral or radial groove in the bone as it descends. It supplies brachioradialis when it emerges anterior to the lateral epicondyle, which is a supinator of the forearm. This is clinically useful in patients who have an issue with the musculocutaneous nerve, and therefore whose major elbow flexors (biceps brachii and brachialis, are denervated). The brachioradialis crosses the elbow joint and attaches to the radial styloid process. Hence the elbow can still be flexed to a significant degree.
The anconeus muscle (a small vestigial muscle that sits on the posterior aspect of the elbow and abducts the ulna in pronation of the forearm) is supplied by the radial nerve.
At the level of the elbow, the nerve then passes into the supinator muscle, where it divides into a deep and superficial branch. The deep branch becomes the posterior interosseus nerve, and descends down the posterior forearm, to supply all of the wrist extensor muscles. The superficial branch goes on to supply sensation to the back of the hand. These include extensor carpi radialis longus and brevis, extensor carpi ulnaris, extensor digitorum, extensor indicis and extensor digiti minimi. It also innervates the long thumb muscles, i.e. extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
The radial nerve gives sensory branches all the way along its course. It supplies sensation to the lower outer aspect of the upper arm (via its C5, C6 nerve roots) via the lateral cutaneous nerve of the arm, gives sensation to the posterior surface of the arm via the posterior cutaneous nerve of the arm, as well as the central region of the posterior aspect of the forearm via the posterior cutaneous nerve of the forearm (again C5-C6 nerve root), and finally the superficial branch of the radial nerve that supplies sensation to the thenar eminence and dorsal aspect of the radial 3 and half digits of the hand.
Why do we need such powerful elbow extensors? The answer can be deduced from our evolutionary ancestors, who were tree climbers, and needed powerful arm extensors for balance and stability. Why for instance does the triceps brachii muscle have three heads? The answer has been revealed by histological study, which reveals that each head has a different muscle fibre composition. The medial head is made of type 1 fibres i.e. it is functionally useful for sustained long contractions of little power. The long head was found to be composed of type 2b fibres, hence is useful for short powerful contractions. Finally, the lateral head is Type 2a i.e. it is a mixed muscle, and like a 400metre runner, is both useful for short fast contraction and more sustained contraction. Depending on the movement type, each head has a different level of importance.
Saturday Night Palsy/Crutch palsy
The radial nerve arises as the terminal branch of the posterior cord of the brachial plexus. It sits behind the axillary artery, and hence is vulnerable to compression injuries. If someone is drunk, they often rest their armpit on chairs and fall asleep. When they wake up, they suffer from wrist drop, as the radial nerve has been compressed for a long time and is hence not functioning. Thus, the phenomenon has been termed Saturday night palsy.
A humeral fracture can damage any of the 5 major branches of the brachial plexus (axillary, radial, ulnar, musculocutaneous and median) as all the nerves run close to the humerus once they arise from the brachial plexus. The radial nerve however is particularly vulnerable, as it runs in the spiral grove, and is therefore tightly adherent to the humerus along the majority of its course in the arm. A fracture is therefore highly likely to disrupt the nerve. Symptoms include wrist drop (as the wrist extensors aren’t functioning), inability to extend the elbows, as the triceps become denervated, and diminished sensation on almost the entire posterior surface of the arm and forearm.
If the wrist is compressed with tight jewelry over its lateral aspect, the superficial branch of the radial artery will be compressed beneath the tendinous insertion of brachioradialis. This results in the loss of sensation of the dorsal aspect of the radial 3 and half fingers, as well as the centre of the forearm posteriorly. There is no motor deficit as the motor branches have all been given off well before the wrist.