Brachial Plexus: Definition, Course, Roots, Functions and Associated Injuries

It is a network of nerve fibers that supplies the skin and muscles of the upper limb.

It starts at the base of the neck, through the axillary area and runs through the entire upper limb. The plexus is made up of divisions or branches of the cervical spinal nerves C5, C6, C7, and C8, and the first thoracic spinal nerve, T1.

Brachial plexus divisions

The brachial plexus is divided into five parts; roots, trunks, divisions, ropes, and branches (a good mnemonic for this is R ead T hat D amn C adaver B ook). There are no functional differences between these divisions, they are simply used to help explain the brachial plexus.


The “roots” refer to the anterior branches of the spinal nerves that comprise the brachial plexus. At each vertebral level, paired spinal nerves arise.

They exit the spinal cord through the intervertebral foramina of the spinal column. Each spinal nerve divides into an anterior and a posterior branch.

The brachial plexus roots are formed by the anterior branches of the C5-T1 spinal nerves (the posterior divisions innervate the skin and musculature of the intrinsic muscles of the back).

After their formation, these nerves pass between the anterior and medial scalene muscles to enter the base of the neck.


At the base of the neck, the roots of the brachial plexus converge to form three trunks. These structures are named for their relative anatomical location:

  • Upper trunk: a combination of C5 and C6 roots.
  • Middle trunk: continuation of C7.
  • Lower hoarse t : combination of C8 and T1 roots. The trunks traverse laterally, crossing the posterior triangle of the neck.

Log divisions

Each trunk divides into two branches within the posterior triangle of the neck. One split moves forward (toward the front of the body) and the other back (toward the back of the body).

Thus, they are known as the anterior and posterior divisions. We now have three anterior and three posterior nerve fibers.

These divisions leave the posterior triangle and move to the armpit. They recombine in the brachial plexus cords.


The anterior and posterior divisions have entered the axilla, combining to form three cords, named for their position relative to the axillary artery.

  • The lateral cord is formed by: the anterior division of the upper trunk. The anterior division of the middle trunk.
  • The posterior cord is formed by: the posterior division of the upper trunk.
  • The medial cord is formed by: the anterior division of the lower trunk.

The cords give rise to the main branches of the brachial plexus. Main branches In the axilla and the proximal aspect of the upper limb, the three chords give rise to five main branches.

These nerves continue into the upper limb to provide innervation to the muscles and skin present.

Brachial plexus functions

Motor functions : innervates the brachialis, biceps brachii and coracobrachialis muscles.

Sensory functions : detaches the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral part of the posterior forearm.

Axillary nerve Roots: C5 and C6

Motor functions: innervates the teres minor and deltoid muscles.

Sensory functions : detaches the upper lateral cutaneous nerve of the arm.

Middle nerve Roots: C6 – T1

(It also contains C5 fibers in some individuals).

Motor functions : innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbricals associated with the index and middle fingers.

Sensory functions: detaches the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.

Radial nerve Roots: C5 – T1

Motor functions: innervates the triceps brachii and muscles in the posterior compartment of the forearm (which are primarily, but not exclusively, extensors of the wrist and fingers).

Sensory functions : innervates the posterior aspect of the arm and forearm, and the posterolateral aspect of the hand.

Ulna nerve Roots: C8 and T1

Motor functions: innervates the muscles of the hand (apart from the thenar and two lateral lumbrical muscles), the superior flexor muscle and the medial half of the deep flexor of the fingers.

Sensory functions : innervates the anterior and posterior surfaces of the middle and middle finger and the associated palm area.

Brachial plexus dissection

When the upper limb is dissected, it can be difficult to recognize where the brachial plexus is located – it may look like a mass of nerves.

The important structure to look for is an “M” shape. It is formed by the musculocutaneous, median and ulnar nerves, generally superficial to the axillary artery.

This shape is usually consistent between corpses. It can help you get your bearings, and you can work backwards to identify wires, splits, and branches.

Minor branches

In addition to the five main branches of the brachial plexus, several smaller nerves arise. They do it from the five parts of the brachial plexus, and they are listed below:

  • Dorsal scapular nerve.
  • Long thoracic nerve.
  • Nerve to subclavian.
  • Lateral pectoral nerve.
  • Medial pectoral nerve.
  • Medial cutaneous nerve of the arm.
  • Medial cutaneous nerve of the forearm.
  • Superior subscapular nerve.
  • Thoracodorsal nerve.
  • Inferior subscapular nerve.

Brachial plexus injury

An intact brachial plexus is vital for normal upper limb function. There are two main types of injuries that can affect the brachial plexus.

An upper brachial plexus injury affects the upper roots and a lower brachial plexus injury affects the lower roots.

Erb’s palsy

Erb’s palsy commonly occurs when there is an excessive increase in the angle between the neck and the shoulder, stretching (or even tearing) the C5 and C6 nerve roots.

It can occur as a result of a difficult delivery or trauma to the shoulder.

Affected nerves: nerves derived only from the C5 or C6 roots: musculocutaneous, axillary, suprascapular and nerve to subclavian.

Paralyzed muscles : supraspinatus, infraspinatus, subclavian, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor.

Motor Functions : Movements that are significantly lost or weakened include abduction at the shoulder, lateral rotation of the arm, supination of the forearm, and flexion at the shoulder.

Sensory functions: loss of sensation in the lateral part of the arm, which covers the sensory innervation of the axillary and musculocutaneous nerves.

The affected limb hangs limply, medially rotated by the unopposed action of the pectoralis major. The forearm is pronounced due to the loss of biceps brachii.

The wrist is weakly flexed due to the normal increase in tone of the wrist flexors relative to the wrist extensors.

This position is known as “waiter’s advice” and is characteristic of Erb’s palsy.

Lower Brachial Plexus Injury: Klumpke’s Palsy

An injury to the lower brachial plexus is the result of excessive abduction of the arm (for example, a person catching a branch when falling from a tree). It has a much lower incidence than Erb’s palsy.

Affected nerves : T1 root derived nerves: ulnar and median nerves.

Paralyzed muscles: all the intrinsic muscles of the hand (the flexor muscles in the forearm are also supplied by the ulnar and median nerves, but are innervated by different roots).

The main symptom is a “claw hand,” caused by the unopposed action of the extensor muscles of the fingers.

The lumbar muscles flex the metacarpophalangeal joints and extend the interphalangeal joints, so their paralysis will cause the opposite: extension of the MCP and flexion of the IP joints.