How does the brachial plexus work?/Brachial Plexus is a network of intertwined nerves that control movement and sensation in the arm and hand. A traumatic brachial plexus injury involves sudden damage to these nerves and may cause weakness, loss of feeling, or loss of movement in the shoulder, arm, or hand. The brachial plexus begins at the neck and crosses the upper chest to the armpit. Injury to this network of nerves often occurs when the arm is forcibly pulled or stretched.
Brachial plexus is a major network of nerves transmitting signals responsible for motor and sensory innervation to the upper extremities. The plexus originates as an extension from the ventral rami of C5 through T1 spinal nerves.[rx][rx][rx][rx]
Five spinal nerves give rise to the formation of 3 trunks which subsequently divide into 6 divisions, located anteriorly and posteriorly. From these divisions, merging of nerves will form 3 cords as the lateral, posterior, and medial cords. Finally, 5 specific nerves will arise from the cords as the terminal branches of the brachial plexus, allowing specific muscles of the upper limb to perform corresponding actions. These terminal branches include the following musculocutaneous, axillary, radial, median, and ulnar nerves[rx][rx][rx][rx][rx]. Aside from these nerves, there are also collateral nerves that are found in the brachial plexus which innervate the proximal limb muscles as they arise proximal to the ventral rami, trunks, and cords. Brachial plexus injuries are regarded as one of the most debilitating injuries afflicting the upper extremity.
Types of Nerve in Brachial Plexus
Collateral nerves of the brachial plexus include the following:[rx]
Dorsal scapular nerve innervating the rhomboids
Long thoracic nerve innervating the serratus anterior
Suprascapular nerve innervating the supraspinatus and infraspinatus
Lateral pectoral nerve innervating the pectoralis major
Medial pectoral nerve innervating both pectoralis major and minor
Upper subscapular nerve innervating the subscapularis
Lower subscapular nerve innervating the subscapularis and teres major
Thoracodorsal nerve innervating the latissimus dorsi
Medial brachial cutaneous nerve innervating the skin of the arm medially
Medial antebrachial cutaneous nerve innervating the skin of the forearm medially
The long thoracic nerve is known for allowing the protraction and superior rotation of the scapula, while the suprascapular nerve for shoulder abduction (by supraspinatus) and lateral rotation of the shoulder (by infraspinatus).
Structure and Function of Brachial Plexus
Nerve fibers from the anterior division of the brachial plexus are contained in the musculocutaneous, median, and ulnar nerves. These nerves innervate the anterior muscles of the upper arm, forearm, and intrinsic muscles[rx][rx][rx][rx]. This innervation mainly provides flexion of the upper limb. Nerve fibers arising from the posterior division, including the axillary and radial nerves, provide innervation to the posterior muscles of the arm and forearm, which in turn allows these compartments to perform the functions of the elbow, wrist, and finger extension
Musculocutaneous nerve – arises from C5 and C6 which innervates all the muscles of the arm anteriorly, enabling motor functions such as flexing the elbow and supination by the biceps brachii[rx]. The median nerve originates from C5 to T1 spinal nerves which primarily innervates the anterior forearm (with a section innervated by the ulnar nerve) and the hand (thenar and central sections). Median nerve allows pronation of the forearm and flexion of the wrist and digits, together with the opposition of the thumb[rx][rx]. The ulnar nerve from C8 to T1 spinal nerves constitutes to the innervation of the anterior forearm (with a section innervated by the median nerve) and the hand (hypothenar and central sections). The central section which involves the palmar and dorsal aspects are responsible for the adduction and abduction of second to fifth digits, respectively. Unlike the median nerve which allows opposition, the ulnar nerve is responsible for the adduction of the thumb [rx].
The axillary nerve – is the result of the network of C5 and C6 spinal nerves which arise toward the deltoid allowing for abduction; and the teres minor for external rotation of the shoulder[rx][rx][rx]. The radial nerve innervates the upper arm and forearm posteriorly, which originates from C5 to T1 spinal nerves. The radial nerve provides the function of extending the wrist, elbow and metacarpophalangeal joints of digits and supination by the supinator muscle.
A pull or a stretch on the plexus results in a spectrum of lesions. Sunderland’s[rx] well-known classification is useful to understand the nature of the injury. Broadly speaking for the surgeon, there are three different kinds of lesions:
- Neuropraxia—reversible rapidly in weeks, rarely reaches the surgeon
- Externally intact looking nerves – (Sunderland type two or three injury — axonogenesis) —not to be resected in the neck but distal transfers may be needed if progress is poor
- A neuroma in continuity—represents a postganglionic lesion (Sunderland Type III and IV axonotomessis) and requires surgical repair after excision of the neuroma. Rarely is the neuroma conductive, if it is neurolysis may suffice
- Rupture—Post Ganglionic lesion (neurotomessis Sunderland types), amenable to intra plexal nerve repair
- Avulsion—Pre Ganglionic lesion, typically that root has to be abandoned as a source of regenerating axon.s
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