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Upper Limb - Innervation
Major Terminal Nerves
- Median nerve
- Ulnar nerve
- Radial nerve
Median Nerve
- Innervates the lateral flexors (below the elbow), derived from C6 – T1
- C6 and C7 are mostly sensory / C8 and T1 are mostly motor.
- Does not innervate any of the muscles of the upper arm or provide any of its sensory coverage.
Superficial forearm group
- C6 and C7 roots -> innervate pronator teres (PT) and flexor carpi radialis (FCR).
- C7, C8 roots -> innervate flexor digitorum superficialis (FDS) and palmaris longus (PL).
- Anterior interosseous nerve branch innervates the anterior interosseous group:
- C7–T1 roots -> innervate pronator quadratus (PQ), flexor pollicis longus (FPL), and flexor digitorum profundus 2 and 3 (FDP 2,3).
- When the anterior interosseous nerve is injured in isolation (anterior interosseous syndrome or neuropathy), patients exhibit the "OK" sign (or more aptly, the not "OK" sign); wherein, they are unable to form a circle with the thumb and index finger. Neither the thumb's interphalangeal joint nor the index finger's distal interphalangeal joints will flex.
Recurrent motor branch of the thumb innervates the thenar group.
- C8, T1 roots -> innervate abductor pollicis brevis (APB), opponens pollicis (OP), and flexor pollicis brevis (FPB).
The terminal motor group.
- C8, T1 roots -> innervate first and second lumbricals.
- The digital sensory branches provide distal sensory innervation via digital sensory branches.
Median neuropathies
Proximal
- Along the distal humerus, supracondylar fracture can injure the median nerve (and also the ulnar nerve) and affect all of the downstream components.
- The humeral and ulnar heads of the pronator teres muscle can ensnare the median nerve as it exits the cubital fossa. All of the downstream components are affected except the superficial forearm group because innervation to the superficial forearm group lies proximal to the entrapment site.
- Anterior interosseous syndrome (see above)
Distal
- Carpal tunnel at the wrist
- Most common entrapment site of the median nerve.
- The median palmar cutaneous nerve, enters the hand superficial to the carpal tunnel, so it's spared in carpal tunnel syndrome
- A laceration of the palm can injure the recurrent branch of the thumb in isolation.
Clinical Effect of Median Neuropathies
- Median nerve injury produces lateral flexor weakness and thenar eminence (thumb pad) wasting, so-called "ape hand".
Ulnar nerve
- Innervates the medial flexors below the elbow.
- The C8, T1 nerve roots supply all of the ulnar nerve-innervated muscles (so we omit the roots with each group; they are all C8, T1).
Forearm muscle group
- Comprises flexor carpi ulnaris (FCU) and flexor digitorum profundus 4 and 5 (FDP 4, 5).
- As the ulnar nerve enters the hand, it splits into the superficial sensory division, which is purely sensory except that it provides motor innervation to palmaris brevis, and the deep branch, which is purely motor and innervates muscle groups across the hand.
The deep branch innervates:
The hypothenar group:
- Comprises abductor digiti minimi (ADM), opponens digiti minimi (ODM), and flexor digiti minimi (FDM).
The intrinsic hand group
- Comprises lumbricals 3 and 4, the palmar interossei, and dorsal interossei muscles; notably, the first dorsal interosseous provides finger abduction of the second digit and is easy to test and inspect.
The thenar group
- Comprises flexor pollicis brevis (FPB) and adductor pollicis (AP).
Ulnar neuropathies
Proximal
- Cubital tunnel — the most common ulnar nerve entrapment site.
- Supracondylar fracture or medial epicondylar compression.
- All of the motor and sensory components of the ulnar nerve lie distal to the distal humerus and cubital tunnel; therefore, all of the components of the ulnar nerve are affected.
Distal
- At the wrist - Guyon's canal (aka Guyon's tunnel) formed from the palmar carpal ligament.
- The vast majority of Guyon canal entrapments only affect the deep, motor branch, which passes between the hook of the hamate and the pisiform bone.
- Proximal to Guyon's canal, the ulnar nerve derives the palmar and dorsal ulnar cutaneous nerves, which are unaffected in Guyon canal entrapment.
- Fracture of the hook of the hamate
Clinical Effect of Ulnar Neuropathies
- Weakness of the medial flexors and a more restricted clawing of the hand, so-called ulnar claw, with clawing of digits 4 and 5.
- Versus Klumpke's palsy, which affects both median and ulnar nerve distal musculature.
- 4th and 5th digit clawing forms the shape of a Pope's blessing, the so-called: Benediction sign.
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".