The Brachial Plexus

Original Author: Oliver Jones
Last Updated: July 4, 2017
Revisions: 42
Fig 1.0 - Proximal portion of the brachial plexus, in the neck.

Fig 1- Proximal portion of the brachial plexus, in the neck.

The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and enters the upper arm.

The plexus is formed by the anterior rami (divisions) of the cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.

In this article, we shall look at the anatomy of the brachial plexus – its formation and anatomical course through the body.

The brachial plexus is divided into five parts; roots, trunks, divisions, cords and branches (a good mnemonic for this is Read That Damn Cadaver Book). There are no functional differences between these divisions – they are simply used to aid explanation of the brachial plexus.


Roots

The ‘roots’ refer the beginning of the brachial plexus. They are formed by the spinal nerves C5, C6, C7, C8 and T1.

At each vertebral level, paired spinal nerves arise. They leave the spinal cord via the intervertebral foramina of the vertebral column.

Each nerve then divides into anterior and posterior nerve fibres. The roots of the brachial plexus are formed by the anterior divisions of spinal nerves C5-T1 (the posterior divisions go on to innervate the skin and musculature of the trunk).

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

Fig 1.0 - The spinal cord outflow at each vertebral level. The anterior rami of vertebral levels C5-C8 and T1 make up the roots of the brachial plexus

Fig 2 – The spinal cord outflow at each vertebral level. The anterior rami of vertebral levels C5-C8 and T1 make up the roots of the brachial plexus.


Trunks

At the base of the neck, the roots of the brachial plexus converge, forming three trunks. These structures are named by their anatomical position:

  • Superior trunk: A combination of C5 and C6 roots.
  • Middle trunk: A continuation of C7.
  • Inferior trunk: A combination of C8 and T1 roots.

The trunks begin to move laterally, crossing the posterior triangle of the neck.


Divisions

Within the posterior triangle of the neck, each trunk divides into two branches. One division travels anteriorly (toward the front of the body) and the other posteriorly (towards the back of the body). Thus, they are known as the anterior and posterior divisions.

We now have three anterior and three posterior nerve fibres. These divisions leave the posterior triangle and pass into the axilla region. They recombine in the next part of the brachial plexus.


Cords

Once the anterior and posterior divisions have entered the axilla, they combine together to form three nerves. These nerves are named by their position relative to the axillary artery.

Fig 1.1 - Diagrammatic representation of the Brachial Plexus. For simplicity, the smaller branches of the brachial plexus are not shown. The posterior divisions are shown in yellow, and anterior divisions in black.

Fig 3 – Diagrammatic representation of the Brachial Plexus. For simplicity, the smaller branches of the brachial plexus are not shown. The posterior divisions are shown in yellow, and anterior divisions in black.

The lateral cord is formed by:

  • The anterior division of the superior trunk
  • The anterior division of the middle trunk

The posterior cord is formed by:

  • The posterior division of the superior trunk
  • The posterior division of the middle trunk
  • The posterior division of the inferior trunk

The medial cord is formed by:

  • The anterior division of the inferior trunk.

The cords give rise to the major branches of the brachial plexus.


Major Branches

In the axilla and the proximal aspect of the upper limb, the three cords give rise to five major branches. These nerves continue into the upper limb to provide innervation to the muscles and skin present. In this section, we shall concentrate on these five nerves.

Note: these are only brief notes on the function of the nerves –  for more detailed information click on the title to visit their respective pages.


Musculocutaneous Nerve

See more detailed information here.

Roots: C5, C6, C7.

Motor Functions: Innervates the brachialis, biceps brachii and coracobrachialis muscles.

Sensory Functions: Gives off the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral portion of the posterior forearm.

Fig 1.2 - The derivation of the musculocutaneous nerve from the brachial plexus

Fig 4 – The derivation of the musculocutaneous nerve from the brachial plexus


Axillary Nerve

See more detailed information here.

Roots: C5 and C6.

Motor Functions: Innervates the teres minor and deltoid muscles.

Sensory Functions: Gives off the superior lateral cutaneous nerve of arm, which innervates the inferior region of the deltoid (“regimental badge area”).

Fig 1.4 - The derivation of the axillary nerve from the brachial plexus.

Fig 5 – The derivation of the axillary nerve from the brachial plexus.


Median Nerve

See more detailed information here.

Roots: C6 – T1. (Also contains fibres from C5 in some individuals).

Motor Functions: Innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbricals that move the index and middle fingers.

Sensory Functions: Gives off 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.

Fig 1.5 - The derivation of the median nerve from the brachial plexus

Fig 6 – The derivation of the median nerve from the brachial plexus


Radial Nerve

See more detailed information here.

Roots: C5-C8 and T1.

Motor Functions: Innervates the triceps brachii, and the extensor muscles in the posterior compartment of the forearm.

Sensory Functions: Innervates the posterior aspect of the arm and forearm, and the posterior, lateral aspect of the hand.

Fig 1.6 - The derivation of the radial nerve from the brachial plexus.

Fig 7 – The derivation of the radial nerve from the brachial plexus.


Ulnar Nerve

See more detailed information here

Roots: C8 and T1.

Motor Functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus.

Sensory Functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and associated palm area.

Fig 1.7 - The derivation of the ulnar nerve from the brachial plexus.

Fig 8 – The derivation of the ulnar nerve from the brachial plexus.

Fig 1.6 - The 'M' shape, consisting of the ulnar, median and musculocutaneous nerves.

Fig 9 – The ‘M’ shape, consisting of the ulnar, median and musculocutaneous nerves.

Practical Relevance: Dissecting the Brachial Plexus

When dissecting the upper limb, it can be difficult to recognise what part of the brachial plexus you are at – it just looks like a mass of nerves.

The important structure to look for is an ‘M’ shape. This is formed by the musculocutaneous, median and ulnar nerves, arranged around the brachial artery.

This shape is usually consistent between cadavers. It can help you get your bearings, and you can work backwards to identify the cords, divisions and branches.

Minor Branches

In addition to the five major branches of the brachial plexus, there are a number of smaller nerves that arise. They do so from all five parts of the brachial plexus, and are listed below:

Roots Trunks Lateral cord Medial cord Posterior cord
Dorsal scapular nerve

 

Long thoracic nerve

 

 

Suprascapular nerve

 

Nerve to subclavius

 

 

Lateral pectoral nerve

 

 

 

 

Medial pectoral nerve

 

Medial cutaneous nerve of arm

 

Medial cutaneous nerve of forearm

Superior subscapular nerve

 

Thoracodorsal nerve

 

Inferior subscapular nerve

Clinical Relevance: Injury to the Brachial Plexus

An intact brachial plexus is vital for the normal function of the upper limb. There are two major types of injuries that can affect the brachial plexus. An upper brachial plexus injury affects the superior roots, and a lower brachial plexus injury affects the inferior roots.

Upper Brachial Plexus Injury – Erb’s Palsy

Erb’s palsy commonly occurs where there is excessive increase in the angle between the neck and shoulder – this stretches (or can even tear) the nerve roots, causing damage. It can occur as a result of result of a difficult birth or shoulder trauma.

  • Fig 1.8 - The waiters' tip position, characteristic of Erb's palsy.

    Fig 10 – The waiters’ tip position, characteristic of Erb’s palsy.

    Nerves affected: Nerves derived from solely C5 or C6 roots; musculocutaneous, axillary, suprascapular and nerve to subclavius.

  • Muscles paralysed: Supraspinatus, infraspinatus, subclavius, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor.
  • Motor functions: The following movements are lost or greatly weakened – abduction at shoulder, lateral rotation of arm, supination of forearm, and flexion at shoulder.
  • Sensory functions: Loss of sensation down lateral side of arm, which covers the sensory innervation of the axillary and musculocutaneous nerves.

The affected limb hangs limply, medially rotated by the unopposed action of pectoralis major. The forearm is pronated due to the loss of biceps brachii. This is position is known as ‘waiter’s tip’, and is characteristic of Erb’s palsy.

Lower Brachial Plexus Injury – Klumpke Palsy

A lower brachial plexus injury results from excessive abduction of the arm (e.g person catching a branch as they fall from a tree).  It has a much lower incidence than Erb’s palsy.

  • Nerves affected: Nerves derived from the T1 root – ulna and median nerves.
  • Muscles paralysed: All the small muscles of the hand (the flexors muscles in the forearm are supplied by the ulna and median nerves, but are innervated by different roots).
  • Sensory functions: Loss of sensation along medial side of arm.

The metacarpophalangeal joints are hyperextended, and the interphalangeal joints are flexed. This gives the hand a clawed appearance.

Prosection Images

Prosection 1 – The ‘M’ shape of the brachial plexus, formed by the ulnar, median and musculocutaneous nerves.

Prosection 2 – Branches of the posterior cord of the brachial plexus.

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Quiz

Question 1 / 10
Which of the following nerve roots does not contribute to the brachial plexus?

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Question 2 / 10
Which of the following nerve roots contributes to the superior trunk of the brachial plexus?

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Question 3 / 10
Which group of muscles does the musculocutaneous nerve innervate?

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Question 4 / 10
Which of the following muscles does the axillary nerve innervate?

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Question 5 / 10
Which of the following nerve roots does not contribute to the median nerve?

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Question 6 / 10
Which group of muscles does the median nerve innervate the majority of?

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Question 7 / 10
Which area does the radial nerve not provide sensory innervation to?

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Question 8 / 10
Which of the following is a minor branch of the roots of the brachial plexus?

Quiz

Question 9 / 10
What deformity occurs in Erbs palsy?

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Question 10 / 10
Which root is typically damaged in Klumpkes palsy?

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