USMLE Guide: Anatomy of the Brachial Plexus

Introduction

The brachial plexus is a complex network of nerves that originates from the spinal cord in the neck region and provides innervation to the upper limb. Understanding the anatomy of the brachial plexus is crucial for medical professionals, especially for those preparing for the United States Medical Licensing Examination (USMLE). This guide aims to provide a comprehensive overview of the anatomy of the brachial plexus, highlighting key concepts and important clinical correlations.

Anatomical Components

The brachial plexus consists of five main nerve roots: C5, C6, C7, C8, and T1. These nerve roots emerge from the spinal cord and combine to form three trunks: upper, middle, and lower. The trunks then divide into anterior and posterior divisions, which further give rise to various cords and terminal branches.

1. Trunks:

   • Upper Trunk: Formed by the union of C5 and C6 nerve roots.
   • Middle Trunk: Comprised of the C7 nerve root.
   • Lower Trunk: Formed by the union of C8 and T1 nerve roots.

2. Divisions:

   • Anterior Division: Derived from the anterior aspect of each trunk.
   • Posterior Division: Derived from the posterior aspect of each trunk.

3. Cords:

   • Lateral Cord: Composed of the anterior divisions of the upper and middle trunks (C5-C7).
   • Medial Cord: Composed of the anterior division of the lower trunk (C8-T1).
   • Posterior Cord: Composed of all the posterior divisions from the three trunks.

4. Terminal Branches:

   • Musculocutaneous Nerve: Arises from the lateral cord and innervates the anterior arm muscles.
   • Median Nerve: Formed by contributions from the lateral and medial cords, it supplies the majority of the flexor muscles in the forearm and some intrinsic hand muscles.
   • Ulnar Nerve: Arises from the medial cord and supplies intrinsic hand muscles and the ulnar half of the flexor digitorum profundus.
   • Axillary Nerve: Arises from the posterior cord and innervates the deltoid and teres minor muscles.
   • Radial Nerve: Arises from the posterior cord and provides innervation to the extensor muscles of the arm and forearm.

Clinical Correlations

Understanding the anatomy of the brachial plexus is crucial for diagnosing and managing various clinical conditions. Here are some important clinical correlations related to the brachial plexus:

1. Brachial Plexus Injury: Trauma, such as shoulder dislocation or stretching during birth, can lead to brachial plexus injury. The most commonly affected nerve roots are C5 and C6, resulting in Erb-Duchenne palsy, characterized by weakness or paralysis of the shoulder and elbow muscles.

2. Thoracic Outlet Syndrome: Compression of the brachial plexus and/or subclavian vessels as they pass through the thoracic outlet can result in thoracic outlet syndrome. Symptoms include pain, paresthesias, and weakness in the upper limb.

3. Median Nerve Compression: Compression of the median nerve at the wrist (carpal tunnel syndrome) can cause numbness, tingling, and weakness in the hand and fingers supplied by the median nerve.

4. Ulnar Nerve Compression: Compression of the ulnar nerve at the elbow (cubital tunnel syndrome) or wrist can lead to sensory and motor deficits in the ulnar distribution of the hand.

Conclusion

Understanding the anatomy of the brachial plexus is essential for medical professionals, particularly those preparing for the USMLE. Familiarizing yourself with the nerve roots, trunks, divisions, cords, and terminal branches is crucial for diagnosing and managing various clinical conditions. Stay aware of the clinical correlations discussed to facilitate accurate diagnoses and appropriate management of brachial plexus-related pathologies.