Brachial plexus

Introduction

The brachial plexus is a complex network of nerves that originates from the spinal cord and provides motor and sensory innervation to the upper limb. It plays a crucial role in the movement and sensation of the shoulder, arm, forearm, and hand. Understanding its anatomy and function is essential for clinical practice, particularly in the diagnosis and management of nerve injuries.

Anatomy of the Brachial Plexus

Roots

The brachial plexus arises from the anterior rami of spinal nerves C5 to T1. These roots emerge between the anterior and middle scalene muscles and converge to form the trunks of the plexus. Each root carries both motor and sensory fibers that contribute to the innervation of the upper limb.

Trunks

The roots combine to form three trunks:

• Upper trunk: formed by the union of C5 and C6 roots
• Middle trunk: continuation of the C7 root
• Lower trunk: formed by the union of C8 and T1 roots

The trunks lie posterior to the clavicle and divide into anterior and posterior divisions to supply the flexor and extensor compartments of the upper limb, respectively.

Divisions

Each trunk splits into an anterior and a posterior division. The anterior divisions generally supply the anterior (flexor) muscles of the upper limb, while the posterior divisions supply the posterior (extensor) muscles. These divisions then reorganize to form the cords of the brachial plexus.

Cords

• Lateral cord: formed by the anterior divisions of the upper and middle trunks
• Posterior cord: formed by the posterior divisions of all three trunks
• Medial cord: continuation of the anterior division of the lower trunk

The cords are named based on their relationship to the axillary artery and serve as the origin for the terminal branches of the brachial plexus.

Branches / Terminal Nerves

• Musculocutaneous nerve
• Axillary nerve
• Radial nerve
• Median nerve
• Ulnar nerve

These terminal nerves provide both motor and sensory innervation to specific regions of the upper limb.

Minor Branches / Nerve Contributions

• Dorsal scapular nerve
• Long thoracic nerve
• Suprascapular nerve
• Subscapular nerves
• Medial brachial cutaneous nerve
• Medial antebrachial cutaneous nerve

These minor branches contribute to the innervation of smaller muscles and cutaneous areas around the shoulder, thorax, and arm.

Functional Anatomy

Motor Functions

The brachial plexus provides motor innervation to all muscles of the shoulder, arm, forearm, and hand. Each terminal nerve supplies specific muscle groups:

• Musculocutaneous nerve: innervates the biceps brachii, brachialis, and coracobrachialis muscles
• Axillary nerve: supplies the deltoid and teres minor muscles
• Radial nerve: innervates the triceps brachii, brachioradialis, and extensor muscles of the forearm and hand
• Median nerve: supplies most of the flexor muscles in the forearm and the thenar muscles of the hand
• Ulnar nerve: innervates the intrinsic hand muscles not supplied by the median nerve and some forearm flexors

Sensory Functions

The brachial plexus carries sensory fibers that provide cutaneous sensation to the upper limb. The distribution is both dermatomal and peripheral:

• Dermatomes: each root contributes to a specific segment of skin sensation along the arm
• Cutaneous branches: terminal nerves provide sensation to defined areas of the shoulder, arm, forearm, and hand

Reflexes

Several deep tendon reflexes are mediated by the brachial plexus roots:

• Biceps reflex: primarily C5-C6, tests musculocutaneous nerve function
• Brachioradialis reflex: primarily C6, tests radial nerve function
• Triceps reflex: primarily C7, tests radial nerve function

Development and Embryology

The brachial plexus develops during early embryogenesis from the ventral rami of spinal nerves C5 to T1. Growth of the limb buds guides the organization of the plexus into roots, trunks, divisions, cords, and terminal branches. Proper signaling and patterning are critical for the normal formation of the plexus.

• Embryological origin: arises from neural crest cells that form the spinal nerves and their extensions
• Developmental anomalies: may include variations in nerve branching, absence of certain nerves, or abnormal fusion of trunks, which can impact limb function

Clinical Significance

Brachial Plexus Injuries

Injuries to the brachial plexus can result from trauma, birth-related complications, or iatrogenic causes. These injuries are classified based on the location and severity of nerve involvement.

• Etiology: includes blunt or penetrating trauma, obstetric injuries during delivery, and surgical or anesthetic complications
• Types:
  • Upper plexus injuries (C5-C6): often cause weakness in shoulder abduction and elbow flexion
  • Lower plexus injuries (C8-T1): may result in weakness or paralysis of the hand and wrist muscles
  • Pan-plexus injuries: involve all roots and can lead to complete paralysis of the upper limb
• Signs and symptoms: muscle weakness, loss of sensation, abnormal reflexes, and atrophy in affected regions
• Electrodiagnostic evaluation: electromyography and nerve conduction studies help localize the injury and assess severity

Compression Syndromes

Chronic compression of brachial plexus nerves can lead to functional impairment and pain.

• Thoracic outlet syndrome: compression of the brachial plexus between the scalene muscles or clavicle causing pain, numbness, and weakness
• Peripheral nerve entrapments: localized nerve compression, such as by fibrous bands or repetitive activity, leading to sensory and motor deficits

Congenital and Neuropathic Conditions

• Erb-Duchenne palsy: upper plexus injury typically from birth trauma, causing shoulder and arm weakness
• Klumpke’s palsy: lower plexus injury affecting hand and wrist function
• Other congenital neuropathies: rare developmental anomalies affecting nerve branching or muscle innervation

Surgical and Interventional Considerations

• Nerve repair and grafting: surgical reconstruction of damaged nerves to restore function
• Nerve transfers: redirecting functional nerves to reinnervate paralyzed muscles
• Regional anesthesia applications: brachial plexus block for surgical procedures on the upper limb

Imaging and Diagnostic Evaluation

Accurate evaluation of the brachial plexus is essential for diagnosis, planning interventions, and monitoring recovery. Various imaging modalities and diagnostic tests are used depending on the suspected pathology.

• MRI of brachial plexus: provides detailed images of nerve anatomy and surrounding soft tissues
• Ultrasound evaluation: useful for visualizing superficial nerves and guiding interventions
• CT myelography: highlights nerve roots and spinal canal anatomy in cases of suspected root compression
• Electromyography and nerve conduction studies: assess nerve function, detect lesions, and localize injuries

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2018.
3. Netter FH. Atlas of Human Anatomy. 7th ed. Philadelphia: Elsevier; 2019.
4. Birch R. Surgical Disorders of the Peripheral Nerves. 2nd ed. London: Springer; 2017.
5. Chung KC, Mackinnon SE. Handbook of Peripheral Nerve Injury. 1st ed. New York: Thieme; 2018.
6. Burnett MG, Zager EL. Pathophysiology of Peripheral Nerve Injury. Neurosurg Focus. 2004;16(5):1-12.
7. Leis AA. Brachial Plexus Injuries. N Engl J Med. 2000;343(12):936-947.
8. Heckmann JG, Rasche D. Brachial Plexus Anatomy and Lesions. Hand Clin. 2010;26(4):453-467.
9. Wilbourn AJ. Electrophysiologic Evaluation of Brachial Plexus Injuries. Muscle Nerve. 2002;26(1):4-15.
10. Hanna M, Bialocerkowski A. Imaging of the Brachial Plexus. Radiol Clin North Am. 2011;49(6):1075-1090.