The Brachial Plexus

Roots

The ‘roots’ refer the anterior rami of the spinal nerves that comprise the brachial plexus. These are the anterior rami of 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 spinal nerve then divides into an anterior and a posterior ramus. The roots of the brachial plexus are formed by the anterior rami of spinal nerves C5-T1 (the posterior divisions innervate the skin and musculature of the intrinsic back muscles).

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

By TeachMeSeries Ltd (2025)

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

By TeachMeSeries Ltd (2025)

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.

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Trunks

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

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

The trunks traverse laterally, crossing the posterior triangle of the neck.

Divisions

Each trunk divides into two branches within the posterior triangle of the neck. One division moves 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. They recombine into the cords of the brachial plexus.

Cords

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

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.

By TeachMeSeries Ltd (2025)

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.

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.

By TeachMeSeries Ltd (2025)

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”).

By TeachMeSeries Ltd (2025)

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 associated with 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.

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

Radial Nerve

See more detailed information here.

• Roots: C5 – T1.
• Motor Functions: Innervates the triceps brachii, and the 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.

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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.

By TeachMeSeries Ltd (2025)

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

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 can just look 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, usually superficial to the axillary 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.

By TeachMeSeries Ltd (2025)

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

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:

Clinical Relevance

Upper Brachial Plexus Injury (Erb’s Palsy)

Erb’s palsy refers to an injury to the upper roots of the brachial plexus (typically C5-6). It most commonly occurs as a result of a stretching injury during a difficult vaginal delivery.

• Nerves affected – the peripheral nerves derived from C5-6 roots are most affected. This includes the musculocutaneous, axillary, suprascapular, and nerve to subclavius.
• Muscles affected – supraspinatus, infraspinatus, subclavius, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor.
• Motor functions affected – abduction at shoulder, lateral rotation of arm, supination of forearm, and flexion at shoulder.
• Sensory functions affected – sensation over the lateral aspect of upper limb (C5-6 dermatomes).

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. The wrist is weakly flexed due to the normal tone of the wrist flexors relative to the weakened wrist extensors. This is position is known as ‘waiter’s tip’ and is characteristic of Erb’s palsy.

By TeachMeSeries Ltd (2025)

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