Brachial Plexus 101: A Chiropractic Student’s Guide to Anatomy and Function

The brachial plexus is a critical network of nerves that plays a vital role in the functionality of the upper limbs. For chiropractic students, understanding this intricate system is essential for diagnosing and treating a variety of conditions that affect the shoulder, arm, and hand. The brachial plexus not only facilitates movement but also provides sensory feedback, making it indispensable for daily activities and overall health.

In this blog, we will delve into the general anatomy of the brachial plexus, explore its various structures, discuss its origins, and explain its innervations and functions. By gaining a comprehensive understanding of the brachial plexus, chiropractic students will be better equipped to answer any and all questions on the NBCE chiropractic board exams but also apply their knowledge clinical practice, ultimately improving patient care and outcomes.

GENERAL ANATOMY OF THE BRACHIAL PLEXUS

The brachial plexus is a complex and vital network of nerves that originates from the spinal cord in the neck and extends into the arm. Let’s go over what a chiro student should know about the anatomy of the brachial plexus.

Location and Structure

The brachial plexus is situated in the neck and upper shoulder region. It is formed by the anterior rami of the lower four cervical nerves (C5, C6, C7, C8) and the first thoracic nerve (T1). These nerves emerge from the spinal cord and converge to form the brachial plexus.

Pathway through the Neck

After emerging from the intervertebral foramina, the roots of the brachial plexus pass between the anterior and middle scalene muscles, located in the lateral neck. This anatomical pathway is significant because any abnormality or compression in this area can affect the brachial plexus, leading to conditions such as thoracic outlet syndrome.

Components of the Brachial Plexus

The brachial plexus is traditionally divided into five key sections: roots, trunks, divisions, cords, and branches. Each section plays a specific role in the formation and function of the plexus:

• Roots: The five roots (C5-T1) are the initial segment of the brachial plexus. These spinal nerve roots pass between the anterior and middle scalene muscles in the neck.
• Trunks: The roots merge to form three trunks:
  • Upper Trunk: Formed by the union of the C5 and C6 roots.
  • Middle Trunk: Continuation of the C7 root.
  • Lower Trunk: Formed by the union of the C8 and T1 roots.
• Divisions: Each trunk splits into an anterior and a posterior division. These divisions are responsible for innervating the anterior (flexor) and posterior (extensor) compartments of the upper limb, respectively.
• Cords: The divisions regroup to form three cords, named based on their position relative to the axillary artery:
  • 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: Formed by the anterior division of the lower trunk.
• Branches: The cords give rise to the major nerves that innervate the upper limb. These include:
  • Musculocutaneous Nerve
  • Axillary Nerve
  • Radial Nerve
  • Median Nerve
  • Ulnar Nerve

Think “MARMU” when you’re learning the branches. Psst, here's a sneak peak at a snippet of our Part I board review course packet covering the Brachial Plexus.

STRUCTURES OF THE BRACHIAL PLEXUS

The brachial plexus is an intricate and well-organized network of nerves, divided into five main sections: roots, trunks, divisions, cords, and branches. Each section has a specific function and structure, contributing to the overall role of the brachial plexus in innervating the upper limb.

Roots

The brachial plexus begins with five roots, which are the anterior rami of the spinal nerves from C5 to T1. These roots emerge from the spinal cord and pass through the intervertebral foramina. The roots of the brachial plexus travel between the anterior and middle scalene muscles in the neck. Each root carries motor and sensory fibers that will eventually supply specific areas of the upper limb.

Trunks

The roots combine to form three trunks:

• Upper Trunk: Formed by the union of the C5 and C6 roots. The upper trunk provides innervation to muscles such as the deltoid and biceps brachii.
• Middle Trunk: A continuation of the C7 root. The middle trunk contributes fibers to both the anterior and posterior compartments of the arm.
• Lower Trunk: Formed by the union of the C8 and T1 roots. The lower trunk primarily innervates the muscles of the forearm and hand.

Divisions

Each trunk splits into an anterior and a posterior division. These divisions are responsible for separating the nerve fibers destined for the flexor (anterior) and extensor (posterior) compartments of the upper limb:

• Anterior Divisions: These divisions innervate the anterior (flexor) muscles of the arm, forearm, and hand.
• Posterior Divisions: These divisions innervate the posterior (extensor) muscles of the shoulder, arm, and forearm.

Cords

The divisions then reorganize into three cords, named based on their position relative to the axillary artery:

• Lateral Cord: Formed by the anterior divisions of the upper and middle trunks. The lateral cord gives rise to the musculocutaneous nerve and part of the median nerve.
• Posterior Cord: Formed by the posterior divisions of all three trunks. The posterior cord gives rise to the axillary nerve and the radial nerve.
• Medial Cord: Formed by the anterior division of the lower trunk. The medial cord gives rise to the ulnar nerve and part of the median nerve.

Branches

The cords give rise to the major terminal branches of the brachial plexus, which are the nerves responsible for the motor and sensory innervation of the upper limb:

• Musculocutaneous Nerve: Innervates the anterior muscles of the arm, such as the biceps brachii and brachialis, and provides sensory innervation to the lateral forearm.
• Axillary Nerve: Innervates the deltoid and teres minor muscles and provides sensory innervation to the skin over the deltoid region.
• Radial Nerve: Innervates the posterior muscles of the arm and forearm, such as the triceps brachii and extensor muscles, and provides sensory innervation to the posterior arm, forearm, and hand.
• Median Nerve: Innervates most of the anterior forearm muscles and some hand muscles, and provides sensory innervation to the palmar side of the thumb, index, middle, and part of the ring fingers.
• Ulnar Nerve: Innervates the intrinsic hand muscles and some forearm muscles, and provides sensory innervation to the palmar and dorsal sides of the little finger and part of the ring finger.

CLINICAL RELEVANCE

The origins of the brachial plexus are clinically significant for several reasons:

• Injury Identification: Understanding the origins helps in identifying the specific level of nerve injury. For example, a lesion at the C5-C6 level (upper trunk) can result in Erb's palsy, characterized by weakness in the shoulder and upper arm.
• Thoracic Outlet Syndrome: This condition involves the compression of the brachial plexus roots as they pass through the thoracic outlet, leading to pain, numbness, and weakness in the arm.
• Surgical Considerations: Knowledge of the brachial plexus origins is crucial during surgeries in the neck and shoulder region to avoid inadvertent nerve damage.

Learning and studying the brachial plexus can be overwhelming. This is why our CAL board reviews have mnemonics, colorful drawings and charts to help you remember everything you need to know about this important network of nerves. Want to check out our review courses? Go here!

We hope this blog post reminded you of some important facts about the brachial plexus!

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