The Anatomy of the Brachial Plexus

The brachial plexus is a network of nerves that originate in the spinal cord in the neck, travel down the neck (via the cervicoaxillary canal) and into the armpit. It contain the nerves that, with only a few exceptions, are responsible for sensation (sensory function) and movement (motor function) of the arms, hands, and fingers. As the nerves travel from the neck to the axilla they are subject to injury, especially injuries that forcefully move the head away from the shoulder.

Understanding the anatomy (roots, trunks, divisions, cords, and terminal branches) of the brachial plexus is important in determining the site of an injury and designing treatment.

Anatomy

The brachial plexus consists of a network of nerve roots, cords, and branches that share common functions. There is one brachial plexus on each side of the body that carries the nerves to each arm. The anatomy can be confusing at first, but is easier to conceptualize by breaking it down into five different regions.

Structure

The brachial plexus is made up of nerve cells that make up the different sections of the brachial plexus. Nerves are made up of axon fibers that transmit information to and from the brain. Nerve cells are surrounded by supportive cells called neuroglia. These cells secrete the substance myelin that lines the nerves and ensures that messages can travel rapidly to and from the brain.

Location and Sections

The brachial plexus arises from nerve roots that emerge from the spinal cord, travel down through the neck (the cervicoaxillary canal), over the first rib, and into the armpit. In the neck region, it lies in a region referred to as the posterior triangle.

There are five distinct anatomic sections to the brachial plexus that vary in their location as well as well as make-up.

Roots (5): The brachial plexus begins when five nerves exit the lower cervical and upper thoracic spinal cord (from the ventral rami).

• C5-C8: Four nerve roots that exit from the lower section of the cervical spinal cord
• T1: The first nerve exiting the thoracic spinal cord

The roots of the brachial plexus leave the spinal cord and pass behind the scalenus anterior muscle. They then emerge between the anterior and middle scalene muscles along with the subclavian artery.

Trunks (3): Shortly after the five nerves exit the spinal cord, they merge to form three nerve trunks.

• Superior (formed by the merging of C5 and C6)
• Medial (from C7)
• Inferior (branches of C8 and T1)

The nerve trunks travel across the inferior (lower) part of the posterior triangle of the neck. At this point they pass laterally around the subclavian artery and over the first rib.

Divisions (6): The three trunks separate into an anterior (sensory division) and posterior (motor) division forming six divisions.

These divisions are found behind the clavicle (collarbone). (The roots and trunk are found above the clavicle (supraclavicular) and the cords and branches beneath (infraclavicular).

Cords (3): The six divisions then merge into three cords. These cords lie near the axillary artery and are named according to their relationship with the artery, whether lateral, medial, or posterior.

• Lateral cord: Formed by the merging of the anterior branches of the superior and medial trunk
• Medial cord: A continuation of the anterior branch of the inferior trunk
• Posterior cord: Formed by the merging of the posterior branches of all three trunks

Terminal Branches: The three cords next give rise to five major nerves of the upper extremity (other nerves originate along different points in the brachial plexus and are discussed below). Understanding the origin of these nerves (and their function) can be very helpful in identifying the possible site of an injury to the brachial plexus.

• The musculocutaneous nerve
• The axillary nerve: The axillary nerve emerges from the brachial plexus and travels to the surgical neck of the humerus
• The radial nerve: The radial nerve is the largest branch of the brachial plexus. It emerges from the brachial plexus and travels along the radial groove of the humerus
• The median nerve: The median travel emerges from the brachial plexus and travels down the arm anterior to the elbow
• The ulnar nerve: The ulnar nerve emerges from the brachial plexus and travels posterior to the medial epicondyle of the humerus

The lateral cord gives rise to the musculocutaneous nerve. The posterior cord gives rise to the radial nerve and the axillary nerve. The medial cord gives rise to the ulnar nerve. The medial and lateral trunk merge to give rise to the median nerve.

Other Branches: A number of other "pre-terminal" nerves emerge at various points along the brachial plexus.

Branches from the roots:

• Dorsal scapular nerve
• Long thoracic nerve
• A branch to the phrenic nerve

Branches from the trunks:

• Suprascapular nerve
• Nerve to the subclavius

Branches from the cords:

• Upper subscapular nerve
• Lower subscapular nerve
• Thoracodorsal nerve

Variations

There are many potential variations in the brachial plexus. One of the most common includes a contribution from either C4 or T2 in the spine. Communication between the medial and ulnar nerves is also common. There are a number of other variations in the formation of the trunks, divisions, and cords.

Function

The brachial plexus innervates both of the upper extremities (the arms and hands), and is responsible for sensation and movement of the upper arms, forearms, hands, and fingers with two exceptions:

• The trapezius muscle (the muscle you use when you shrug your shoulder), which is innervated by the spinal accessory nerve.
• Sensation to an area near the armpit that is instead innervated by the intercostobrachial nerve (this nerve is sometimes damaged when lymph nodes are removed from the armpit during breast cancer surgery).

Motor Function

The five terminal branches of the brachial plexus have the following motor functions:

• Musculocutaneous Nerve: This nerve supplies muscles responsible for flexing the forearm.
• Axillary nerve: This nerve innervates the deltoid muscle and teres minor and is involved in many movements of the arm around the shoulder joint (shoulder anterior flexors). When injured, a person would be unable to bend their elbow.
• Ulnar nerve: This nerve innervates the medial flexors of the wrist, hand, and thumb muscles. including all interosseus muscles. If injured, a person may demonstrate an "ulnar claw hand," with an inability to extend the fourth and fifth digits.
• Median nerve: The median nerve innervates most of the flexor muscles of the forearm, as well as the thumb.
• Radial nerve: This nerve innervates the triceps muscle, the brachioradialis, and the extensor muscles of the forearm.

Tracing the nerves back to the cords, the lateral and medial cords give rise to the terminal branches that innervate flexors, the muscles on the anterior side of the body. The posterior cord, in turn, results in innervation of the extensors.

Sensory Function

The five terminal branches are responsible for sensation of the entire upper extremity with the exception of a small area in the armpit:

• Musculocutaneous nerve: This nerve is responsible for sensation from the lateral side of the forearm.
• Axillary berve: This nerve is responsible for sensation around the shoulder.
• Ulnar nerve: The ulnar nerve supplies sensation to the pinky finger and the lateral half of the ring finger.
• Median nerve: The median nerve transmits sensory input from the thumb, index finger, middle finger, and medial half of the ring finger, as well as the palmer surface of the hand and the upper dorsal surface.
• Radial nerve: This nerve is responsible for sensory input from the back of the hand on the thumb side, as well as the posterior forearm and arm.

Autonomic Function

The brachial plexus also contains nerves that serve autonomic functions, such as controlling the diameter of blood vessels in the arm.

Associated Conditions

There are a number of medical conditions and injuries that can result in damage or dysfunction of the brachial plexus at some point in its course. These can include:

• Trauma: This can range from severe trauma such as a car accident, to injuries in contact sports (stinger football injury).
• Childbirth injuries: Brachial plexus injuries are not uncommon during childbirth, occuring in roughly 1.5 per 1000 live births. Even though conditions such as breech presentation, shoulder dystocia, and large for gestational age babies increase risk, over half of the time no risk factors are present
• Cancer: Both local and metastatic tumors can lead to damage to the brachial plexus. Pancoast tumors, a type of lung cancer that begins at the apex of the lung can encroach on the brachial plexus. Metastases from breast cancer (a complication of metastatic breast cancer) can also damage the plexus. In some cases, a tumor may secrete substances that cause brachial plexus neuropathy (paraneoplastic syndromes).
• Radiation to the chest: Radiation for cancer may damage the brachial plexus
• Complications of medical treatments: Surgery to the neck region (neck dissection), central lines, and some anesthetic procedures have the potential to damage the brachial plexus.
• Infections, inflammation, and toxins

Mechanism

With trauma, injury to the brachial plexus is most likely to occur when a person's neck is stretched away from the shoulder on the affected side.

Degrees of Damage

When damage to the brachial plexus occurs, doctors use different terms to describe the degree of damage.

• Avulsion: An avulsion is when a nerve is torn completely away from the spinal cord. In addition to weakness and loss of sensation in the arm, people with an avulsion may develop a droopy eyelid Horner's syndrome) that suggests damage to the lower brachial plexus
• Rupture: When a nerve is torn, but not at the level of the spinal cord, it is referred to as a rupture. Symptoms will depend on the level of the rupture.
• Neuroma: When scar tissue accumulates around the nerve, it can compress the nerve resulting in a lack of or poor conduction of impulses
• Neuropraxia: With neuropraxia, the nerve is stretched but not torn.

Symptoms

Symptoms of a brachial plexus injury (or compression, such as with a tumor) depend on the severity. Severe injuries can result in complete loss of sensation and paralysis of the arm. Lesser injuries may result in some loss of sensation and weakness.

Injuries that do not completely disrupt the brachial plexus may cause parasthesias, tingling and burning that has been likened to an electric shock sensation. This may be accompanied by pain that can be very severe.

Injuries are sometimes separated and described as upper trunk or lower trunk injuries, depending on the spinal nerve roots affected.

Upper Trunk Injuries (Erb Duchenne Palsy)

Upper trunk injuries involve damage to C5-C6. They occur most commonly with trauma or childbirth, and usually involve forceful separation of the head from the shoulder. A person with this type of injury will present with his arm hanging by his side with the arm rotated medially and the forearm pronated (waiter tip hand).

Lower Trunk Injury (Klumpke's Palsy)

Lower trunk injuries (C8-T1) may occur with tumors (such as Pancoast tumors of the lung), childbirth, a cervical rib, and other causes. With trauma, these often include abduction of the arm (movement away from the body) while holding an object and falling. These spinal nerves eventually emerge as the radial, ulnar, and median nerves giving rise to classic symptoms. A person wtih Klumpke's palsy will be unable to flex or extend his forearm and all fingers will have a clawed appearance.

Diagnosis

A number of different diagnostic studies may be done depending on symptoms and what type of injury is suspected. These may include:

• Ultrasound: Ultrasound is a good test when looking for brachial plexus symptoms unrelated to trauma, such as cancer metastases, fibrosis, neuropathy due to inflammation, and more. It is less helpful in the setting of trauma.
• MRI/CT/CT myelogram: To assess structural damage/trauma
• Electromygraphy (EMG): With an EMG, small needles are placed in muscles to study conduction
• Nerve conduction studies: In these studies, electrodes are applied to the skin which deliver a small electric shock

Treatment

The treatment of brachial plexus injuries depends on the degree as well as other factors. Potential treatments for severe injuries include nerve grafts or transfers or muscle transfers. Regardless of the type of treatment, however, studies suggest that treatment should be performed early on after an injury, or within three to six months to have the best outcome.