Upper Limb - Radial Nerve
Radial nerve
- Supplies the extensors and is derived from the C5–C8 nerve roots via the posterior cord.
- Does provide important motor and sensory branches to the upper arm.
Proximal segment
- Innervates the triceps muscle, which is supplied primarily by C6 and C7 but also by C8; it provides elbow extension.
- Provides the proximal radial nerve sensory branches: the posterior cutaneous nerves to the arm and forearm and the lower lateral cutaneous nerve of the arm.
How to use the triceps as a localizing tool in radial nerve palsy ->
- The axilla lies proximal to the triceps and the posterior cutaneous nerves.
- The radial nerve can be compressed within the axilla: all of its components are affected here; this commonly occurs from the use of crutches.
In radial nerve compression within the axilla, the triceps is weak.
- The spiral groove lies distal to the triceps.
- The radial nerve passes through the spiral groove when it wraps around the humerus in the mid-shaft
- This is an important radial nerve compression site, so called "Saturday night palsy" because it classically develops from hanging an arm on the back of a bar-chair and compressing the nerve (but being unaware of the arm paresthesias because of drunkenness).
In spiral groove compression, the triceps is strong because triceps innervation lies proximal to the spiral groove.
Elbow
- The radial nerve branches into the:
- Superficial radial nerve, which is solely sensory.
- Deep branch, which travels a short distance before it becomes the posterior interosseous nerve.
Deep branch of radial nerve
- C5–C8 roots -> innervate brachioradialis (C5, C6), brachialis [the musculocutaneous nerve innervates the C5 and C6 portion and the radial nerve innervates the small C7 portion], extensor carpi radialis longus and brevis (ECR) (C6, C7), and anconeus (C6 – C8).
- Brachioradialis provides elbow flexion in mid-pronation/supination.
Posterior interosseous nerve
- C6, C7 roots -> innervate the supinator muscle; it provides outward rotation (supination) of the forearm.
- The posterior interosseous nerve passes through an arch within the proximal supinator, called the arcade of Frohse, so indicate that it is a potential posterior interosseous nerve entrapment site.
- Forearm supination is often preserved in radial neuropathy because of the unaffected musculocutaneous-innervated biceps brachii.
- C7, C8 roots -> innervate:
- The distal wrist extensor: extensor carpi ulnaris (ECU)
- Thumb extensors: extensor pollicis longus (EPL) and extensor pollicis brevis (EPB), and also abductor pollicis longus (APL), which as its name says abducts (not extends) the thumb in the plane of the palm.
- The finger extensors: extensor digitorum communis (EDC), and extensor digiti minimi (EDM), extensor indicis proprius (EIP), note that the "P" in our abbreviation is for proprius, the index finger, not pollicis (the thumb).
Radial Neuropathies
See above for details of each of the following.
- Axilla (crutches)
- Spiral groove (Saturday night palsy)
- Supinator muscle (Arcade of Frohse)
Clinical Effect of Radial Neuropathies
- Extensor muscle weakness with "wrist drop".