A standard spinal nerve supplies a strip of skin (dermatome) and a strip of muscle (myotome).
At the top and bottom of the spinal column, a collection of spinal nerves connect and divide to form a plexus of nerves (plexus is Latin for braid). The brachial plexus is the most complicated of these.
It is formed from the anterior primary rami of spinal nerves C5 to T1 (and on occasion C4 or T2), and is responsible for providing the motor, sensory and sympathetic supply to virtually the whole arm.
The nerves enter the plexus as roots and combine and divide along its course, changing terminology each time, until they finally leave the plexus as peripheral nerves or branches. The plexus runs a virtually straight course from its origin in the base of the neck, under the clavicle, through the axilla and into the arm medial to the humerus. Throughout its course it is closely, but variably, related to the axillary artery.
The brachial plexus lies superficially in the root of the neck and can be palpated with the head turned away. This makes it easy to find for regional anaesthesia, but vulnerable to trauma during birth, penetrating or distracting injuries, or under general anaesthesia.
The Plexus Comprises:
- Branches – these are end nerves which arise from any part of the plexus (apart from the divisions)
NB: Roots, Trunks, Divisions, Cords, and Branches – Real Teenagers Drink Cold Beer
While it is helpful to use diagrammatic representation to learn the parts of the plexus, it important to relate this to the real anatomy to understand its relations and to be able to apply knowledge clinically.
The brachial plexus is formed from the spinal nerves C5 – T1. The anatomy is variable. Occasionally, when C4 is a major contributor the plexus is termed ‘prefixed’ and when T2, it is termed ‘post-fixed’.
Its five roots emerge from the intervertebral foramina, between scalenus anterior and scalenus medius, and pass inferolaterally across the first rib where they meet the subclavian artery as an anterior relation.
There are four peripheral nerves or branches given directly from the roots:
- Nerve to scalene muscles (segmental) – not shown
- Nerve to subclavius (C5, C6)
- Nerve to rhomboid (Dorsal Scapular Nerve: C5) passes posteriorly
- Nerve to serratus anterior (long thoracic nerve: C5, C6, C7) passes inferiorly
The trunks are formed by a combination of the roots as they pass inferolaterally over the first rib superoposterior to the subclavian artery. Here they are palpable just above the clavicle.
- C5 and C6 combine to form the upper trunk
- C7 continues as the middle trunk
- C8 and T1 combine to form the lower trunk
The upper trunk gives off the only branch – the suprascapular nerve, which passes posteriorly through the suprascapular notch to innervate supra- and infraspinatus.
As the plexus passes beneath the clavicle and enters the axilla, each trunk gives off an anterior and posterior division. These divisions combine to form the cords of the plexus.
They surround the axillary artery and are named according to their relative position to the artery.
All three posterior divisions combine to form the posterior cord, which lies behind the axillary artery on the posterior wall of the axilla.
Branches leaving the posterior cord include: (MNEMONIC STAR)
- Subscapular nerve (upper and lower) supplying subscapularis and (lower) teres major
- Thoracodorsal nerve (aka middle subscapular nerve) supplies latisimus dorsi
- Axillary nerve supplying teres minor and deltoid nerves,
- Radial nerve is the terminal branch of the posterior cord, which passes inferiorly between the long and medial heads of triceps into the posterior compartment of the arm.
The anterior divisions of the upper and middle trunks combine lateral to the axillary artery to form the lateral cord. The lateral cord gives
- lateral pectoral nerve, which supplies the the pectoralis major
- musculocutaneous nerve, which passes through coracobrachialis,
- gives a contribution to the median nerve.
The anterior division of the lower trunk alone continues as the medial cord lying medial to the artery. This gives the
- medial pectoral nerve supplying the pectorals,
- the median cutaneous nerves of the arm and forearm,
- ulnar nerve
- contribution to the median nerve.
The plexus produces branches that mostly supply muscles and skin surrounding the shoulder girdle and several larger peripheral nerves:
- Seen here, emerging posteriorly from the posterior cord, the axillary nerve supplies the deltoid and teres minor – abducting and externally rotating the arm at the shoulder. It also carries sensation from the ‘regimental badge’ region of skin overlying deltoid (superior lateral cutaneous nerve) and from the capsule of the shoulder joint.
- Damage to the nerve can occur on dislocation of the shoulder or a fracture of the neck of the humerus, and sensation must be checked before reduction.
- Once the axillary nerve has split from the posterior cord of the plexus, it becomes the radial nerve. It descends the arm posteromedially, innervating triceps as it passes. It then travels in the spiral radial sulcus of the humerus, passing anterolaterally to pass anterior to the lateral epicondyle.
- It then enters the forearm, crossing the antecubital fossa and forming two branches – the superficial (primarily sensory) and deep (primarily motor). The superficial branch descends under the brachioradialis and provides sensation to the dorsum of the thumb and lateral 1.5 fingers. The deep branch descends in the posterior compartment and innervates the extensors of the wrist.
- Injury causes the classic ‘Saturday night palsy’ (sleeping slumped over the back of a chair) – producing a wrist drop and paraesthesia over the thumb.
- The musculocutaneous nerve emerges from the lateral cord of the plexus and pierces the coracobrachialis. It innervates flexors of the arm and then gives a sensory branch – the lateral cutaneous nerve of the forearm.
- Isolated injury is rare, but will produce weakness of flexion and numbness over the lateral forearm.
- The ulnar nerve derives from the medial cord and passes posterior to the medial epicondyle of the humerus where it can be palpated and damaged. It passes through the cubital tunnel and enters the anterior compartment of the forearm and descends with the ulnar artery deep in the medial aspect of the forearm. It enters the hand superficial to the carpel tunnel in the ulnar tunnel and can be palpated again against the pisiform bone lust lateral to the hypothenar eminence.
- The ulnar nerve innervates the flexor carpi ulnaris and half of the flexor digitorum profundus, as well as muscles of the hypothenar eminence, the lateral lumbricals and adductor pollicis. The adductor pollicis is interrogated during peripheral nerve stimulation, when assessing the depth of neuromuscular blockade. The sensation to both dorsal and palmar sides of the medial aspect of the hand is carried by the ulnar nerve.
- Injury can occur at many places along its length and produces the characteristic claw hand with hyperextension of the metacarpophalangeal joints (MCP) and flexion of proximal interphalangeal joints (PIP) and distal interphalangeal joints (DIP).
- The median nerve is formed from the lateral and median cords of the plexus. It travels with the artery as it descends the arm, initially anteriorly and then medially to it as it enters the antecubital fossa.
- It then passes between the heads of pronator teres, descending the forearm amongst the flexors innervating as it goes. It gives off a palmar branch before entering the carpal tunnel. In the hand it innervates the muscles of the thenar eminence, the 1st and 2nd lumbricals, and provides sensation to the lateral palmar aspect.
- The median can be damaged anywhere along its course, typically iatrogenically in the antecubital fossa causing weakness of pronation, and pathologically in the carpal tunnel where it presents as carpel tunnel syndrome with numbness/tingling in the lateral fingers and wasting of the thumb.