What are the motor and sensory innervation patterns of the upper limb, including the brachial plexus branches and their respective muscle and cutaneous distributions?

Sensory and Motor Supply of the Upper Limbs

Brachial Plexus Formation and Organization

The brachial plexus is formed primarily from the ventral rami of spinal nerves C5-T1, with occasional contributions from C4 and/or T2, and provides all motor and sensory innervation to the upper extremity through a systematic organization of roots, trunks, divisions, cords, and terminal branches. 1, 2

Anatomical Structure

The brachial plexus follows a predictable organizational pattern that can be conceptualized as beginning with five nerve roots and terminating in five major nerves 3:

Roots (5):

• Formed by the ventral rami of C5, C6, C7, C8, and T1 1, 2
• These nerve roots pass between the anterior and middle scalene muscles alongside the subclavian artery 1, 4

Trunks (3):

• Superior trunk: formed by C5 and C6 3
• Middle trunk: continuation of C7 3
• Lower trunk: formed by C8 and T1 4, 3
• The trunks are formed as the roots exit between the scalene muscles 1

Divisions (6):

• Each trunk divides into anterior and posterior divisions 1, 2
• Anterior divisions supply flexor compartments 1
• Posterior divisions supply extensor compartments 1

Cords (3):

• Lateral cord: formed from anterior divisions of superior and middle trunks 3
• Medial cord: continuation of the anterior division of the lower trunk 4, 3
• Posterior cord: formed from all three posterior divisions 3
• The cords travel with the subclavian artery and vein in the infraclavicular region 1, 2

Terminal Branches and Their Distributions

The cords form terminal branches at the lateral margin of the pectoralis minor muscle and continue through the axilla into the arm and forearm 1, 2:

From the Lateral Cord:

• Musculocutaneous nerve: provides motor innervation to the anterior arm flexors (biceps brachii, coracobrachialis, brachialis) and sensory innervation to the lateral forearm 3
• Lateral root of median nerve: contributes to median nerve formation 3

From the Medial Cord:

• Ulnar nerve: originates from C8-T1 nerve roots via the lower trunk and medial cord; provides motor innervation to most intrinsic hand muscles and sensory innervation to the medial hand 4, 3
• Medial root of median nerve: joins with lateral root to form the median nerve 3
• Medial cutaneous nerves of arm and forearm: provide sensory innervation to medial arm and forearm 5

From the Posterior Cord:

• Radial nerve: provides motor innervation to all posterior arm and forearm extensors and sensory innervation to the posterior arm, forearm, and dorsal hand 3, 5
• Axillary nerve: provides motor innervation to deltoid and teres minor muscles and sensory innervation to the lateral shoulder 3, 5

Median Nerve (formed from both lateral and medial cords):

• Provides motor innervation to most anterior forearm flexors and thenar muscles 3
• Provides sensory innervation to the lateral palm and lateral 3.5 digits 3

Clinical Significance and Common Pitfalls

Anatomical Variations

Important caveat: Anatomical variations of the brachial plexus are common and can significantly impact clinical presentation and surgical planning 6, 7:

• Variations in cord formation occur frequently, including abnormal fusion patterns of trunks and divisions 6
• Prefixed plexuses (with C4 contribution) and postfixed plexuses (with T2 contribution) alter the typical dermatomal and myotomal distributions 7
• Communications between terminal branches (particularly between musculocutaneous and median nerves) occur in up to 15% of individuals 5, 8

Diagnostic Considerations

Complete brachial plexopathy causes weakness, sensory loss, and flaccid loss of tendon reflexes in all regions innervated by C5-T1 nerve distributions 1:

• Electrodiagnostic studies are essential to confirm clinical diagnosis of plexopathy 1, 2
• Differentiating between preganglionar (nerve root) and postganglionar (plexus) lesions is critical because treatment approaches differ significantly 2, 4
• MRI is the primary imaging modality for evaluating brachial plexus pathology, requiring specialized sequences through the oblique planes of the plexus 1

Pathological Implications

The brachial plexus can be affected by diverse pathologies including trauma, nerve entrapment, neoplasm, inflammatory conditions, infectious processes, autoimmune disorders, hereditary conditions, or idiopathic etiologies 1:

• Tumors invading the lower brachial plexus (C8-T1) present with radicular pain or neurologic findings affecting the ulnar distribution of the hand 4
• Parsonage-Turner syndrome (neuralgic amiotrophy) is a common cause of non-traumatic brachial plexopathy 2
• Primary tumors are most commonly benign schwannomas and neurofibromas, while extrinsic tumors from lung and breast cancer can invade or metastasize to the plexus 2