Brachial Plexus

A plexus is a collection or web of nerves. It is a component of the peripheral nervous system and is formed by the spinal nerves when they join to make a web or network of nerves.

The spinal nerves originate from the spinal cord, join to form the nervous plexus, and then different branches of the plexus supply the peripheral body parts. There are four important plexus present in the body. These include cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. In this article, we are going to study the brachial plexus in detail.

Brachial plexus is a web or network of nerve fibers present at the root or proximal part of the upper limb. The spinal nerves arising from the brachial plexus provide the motor and sensory nerve supply to the upper limbs. In this article, we will study the location, formation, branches, distribution, and functions of the brachial plexus. We will also discuss the pathologies associated with the brachial plexus.

Location

Brachial plexus is present at the root of the upper limb i.e. at the junction of the upper limb with the axillary skeleton. Brachial plexus begins in the lower part of the neck and extends to the axilla.

The roots of the brachial plexus arise in the middle part of the neck and pass through the gap between the scalene muscles (small muscle f neck) along with the subclavian artery. These roots then join to form trunks of the plexus in the inferior part of the neck. These trunks extend to the axilla where different branches arise from the brachial plexus.

Formation

Every nerve plexus is formed by the roots arising from the spinal cord. The spinal nerves represent the roots of a plexus. These roots join to form trunks. The trunks then undergo subdivision into anterior and posterior divisions. The divisions of the trunks then join to form the cords which give rise to the branches. We will discuss all these terms with reference to the brachial plexus.

Roots of Brachial Plexus

The roots of brachial plexus are formed by the anterior rami of four cervical nerves and one thoracic nerve. These include anterior rami of cervical nerves C5 to C8 and one thoracic nerve T1. These roots then join in different patterns to form the trunks of the brachial plexus.

Trunks and Divisions of Brachial Plexus

The brachial plexus consists of three trunks. These are;

  • Superior trunk, formed by the roots C5 and C6.
  • Middle trunk, formed by the root C7.
  • Inferior trunk, formed by the roots C8 and T1.

These trunks undergo subdivisions so that each trunk is divided into an anterior division and a posterior division.

Cord and Branches of the Brachial Plexus

The divisions of the trunks join to form three cord of the brachial plexus. The cords of the brachial plexus then give rise to the branches. The cords and branches of the brachial plexus are listed below.

  • Lateral cord: It is formed by the anterior divisions of the superior and middle trunks of the brachial plexus. Thus, the lateral cord has a root value C5-C7. It gives rise to three nerves or terminal branches of the brachial plexus. The nerves arising from the lateral cord are;
  • Lateral Pectoral Nerve
  • Musculocutaneous Nerve
  • Lateral root of the Median Nerve
  • Posterior cord: It is formed by the posterior divisions of the superior, middle and inferior trunks of the brachial plexus. It has the root value C5-T1. The posterior cord gives rise to five important nerves or terminal branches of the brachial plexus. These include;
  • Axillary Nerve
  • Radial Nerve
  • Upper and Lower Subscapular Nerves
  • Thoracodorsal Nerve
  • Medial cord: It is formed by the anterior division of the inferior trunk only. The root value of the medial trunk is C5-T1. This cord also gives rise to five  nerves or terminal branches of the brachial plexus which include;
  • Ulnar Nerve
  • Medial Pectoral Nerve
  • Medial Cutaneous Nerve of Arm
  • Medial Cutaneous Nerve of Forearm
  • Lateral root of the Median Nerve

Additional Branches of Brachial Plexus

There are some terminal branches of the brachial plexus that do not arise from the cords of the plexus. Rather, these nerves arise from the initial parts of the plexus. These include the following nerves;

  • Long Thoracic nerve, arises from the roots of the brachial plexus C5-T1.
  • Dorsal Scapular nerve arises from the C5 root.
  • Suprascapular nerve arises from the superior trunk of brachial plexus before it undergoes subdivision.
  • Subclavian nerve arises from the inferior aspect of the superior trunk of brachial plexus, just opposite to the Suprascapular nerve.

Distribution

In this part of the article, we will discuss the distribution of different terminal branches of the brachial plexus and the areas of the body supplied by them.

The nerves arising from the brachial plexus mainly supply the upper limbs. Some branches do provide nervous supply to the structure present in the neck and the thoracic region. A list of the nerves arising from the plexus and their distribution is as follows.

  • Lateral Pectoral nerve supplies the pectoralis major muscle.
  • Musculocutaneous nerve supplies the biceps brachii and the brachialis muscles. It also gives rise to the lateral cutaneous nerve of the forearm.
  • Axillary nerve supplies the deltoid muscles. It gives rise to the upper lateral cutaneous nerve of the arm that supplies the skin over the deltoid region.
  • Radial nerve supplies the extensor muscles of the arm and forearm. It also supplies the brachialis and brachioradialis muscles. Radial nerve also supplies the skin on the lower lateral side of the arm, posterior side of the arm and forearm, and some area of the skin on the dorsum of the hand.
  • Upper and Lower Subscapular nerves supply the subscapularis muscles.
  • The thoracodorsal nerve supplies the latissimus dorsi muscles.
  • Ulnar nerve is the main nerve of the upper limb. It supplies the flexor muscles of the forearm and hand. It also provides cutaneous supply to the lateral half of the palm.
  • Medial Pectoral nerve supplies pectoralis muscles of the thorax.
  • Medial Cutaneous nerves of Arm and Forearm supply the skin on the medial side of arm and forearm.
  • The median nerve supplies the flexor muscles of the forearm and fingers.
  • Long Thoracic nerve supplies the serratus anterior muscle.
  • Dorsal Scapular nerve supplies the rhomboid and levator scapulae muscles.
  • Suprascapular nerve supplies the supraspinatus and infraspinatus muscles.
  • Subclavian nerve supplies the subclavius muscle.

Clinical Importance

Brachial plexus have profound clinical importance. Any injury to the brachial plexus can impair the upper limb along with other clinical manifestations.

Injuries to the brachial plexus can occur as a result of disease, stretching or wounds in the axilla.

Injury of the superior trunk of the plexus causes characteristic waiter’s tip position of the limb in which the limb hangs by side. The limb is also medially rotated. It mostly occurs when the person is thrown from a motorbike or horse. The upper brachial plexus injuries can also occur while the excessive stretching of the neck of the baby during birth.

Erb-Duchenne palsy is an injury to the superior part of the brachial plexus in which the muscles supplied by the C5 and C6 nerves are paralyzed. In this case, the upper limb hangs with an adducted shoulder, the arm is rotated medially, and the elbow is extended.  It mostly occurs in hikers who carry heavy backpacks for a much longer duration of time. It is sometimes referred to as backpackers palsy.

The prolonged hyperabduction of the arm may cause compression of the cords of brachial plexus. It mostly occurs while performing manual tasks overhead such as painting a ceiling etc. The cord compression injury results in pain radiating down the arm, numbness and paresthesia, and weakness of the hands.

Conclusion

Brachial plexus is a web or network of nerves present at the root of the upper limb.

It begins in the neck and extends to the axilla where the terminal branches of the plexus arise.

The roots of the brachial plexus are formed by C5 to C8 cervical and T1 thoracic spinal nerves.

These roots join to form three trunks of the plexus. The three trunks include;

  • Superior trunk
  • Middle trunk
  • Inferior trunk

Each of these trunks undergo subdivisions so that two divisions arise from each trunk, an anterior division, and a posterior division.

These divisions join to form cords of the plexus such that three cords are formed.

  • Lateral cord formed by the union of anterior divisions of superior and middle trunks
  • Medial cord formed by the anterior division of the inferior trunk
  • Posterior cord formed by the posterior divisions of the superior, middle and inferior trunks.

These trunks give rise to several terminal branches that supply the muscles and skin of the upper limb, axilla and a part of the thorax.

Brachial plexus may undergo injury due to some wound or disease in the axillary region. It can also suffer when the angle between the neck and shoulder is increased. The different clinical manifestations discussed in this article include;

  • Waiter’s tip position of Hand
  • Erb-Duchenne palsy of Backpacker’s palsy
  • Hyperabduction injuries

References

  1. Anatomy, descriptive and surgical: Gray’s anatomy. Gray, Henry. Philadelphia: Courage Books/Running Press, 1974
  2. Clinically Oriented Anatomy. Moore, Keith L. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2010 (6th ed)
  3. Human Neuroanatomy. Carpenter, Malcolm B. Baltimore: Williams & Wilkins Co., 1976 (7th ed)