2.1. Brachial Plexus Anatomy
The brachial plexus is formed from the ventral roots of C5 through T1 (Fig. 1). It has three trunks (upper, middle, and lower), three cords (medial, lateral, and posterior), and a number of
From: The Clinical Neuropysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ
terminal nerves, the most substantial being the median, ulnar, and radial nerves. The ventral rami of C5 and C6 merge to form the upper trunk, C7 ventral ramus forms the middle trunk and C8-T1ventral rami join to form the lower trunk. The anterior divisions of the upper and middle trunks form the lateral cord; the anterior division of the lower trunk forms the medial cord; and posterior divisions from all three trunks form the posterior cord. Terminal branches from the cords are responsible for the motor and sensory innervation of the upper extremity and shoulder girdle. These terminal branches include the musculocutaneous nerve (lateral cord), median nerve (lateral and medial cord), ulnar nerve (medial cord), and radial and axillary nerves (posterior cord). Several smaller nerves also arise directly from the plexus.
Useful points to remember are:
• The long thoracic nerve (C5,6,7) and dorsal scapular nerve (C5,6) arise directly from the nerve roots and therefore their involvement implies a very proximal lesion, usually of the roots themselves.
• The most likely differential diagnostic consideration for an upper trunk or lateral cord brachial plexus lesion is a C5-6 radiculopathy; for a lower trunk or medial cord lesion is a C8-T1 radiculopathy and for a middle trunk lesion is a C7 radiculopathy.
• The lateral cord contribution to the median nerve is mainly sensory and derived from C5 and C6 ventral rami through the upper trunk.
• The medial cord contribution to the median nerve is mainly motor and originates from C8 and T1 ventral rami through the lower trunk.
Brachial plexopathies present with a variety of clinical syndromes depending on the anatomical part of the plexus involved. In terms of frequency, the upper trunk is the most often involved, likely due to its superficial location, with trauma being the most frequent cause. Less common but potentially more serious (due to the incidence of infiltrating tumors in this region) are lesions of the lower trunk; isolated middle trunk lesions are rare. Regardless of etiology or the portion of the plexus involved, axonal loss is the underlying pathophysiology in the majority of brachial plexus lesions.
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