Brachial plexus

The brachial plexus is a sophisticated network of nerves originating from the anterior primary rami of the C5, C6, C7, C8, and T1 spinal nerves. This intricate structure extends from the neck into the axillary region. It serves as the primary source of motor, sensory, and autonomic innervation for the upper extremity, including the scapular region, arm, forearm, and hand.

Origins in the Neck: Roots of the Brachial Plexus

The brachial plexus begins with the ventral rami of spinal nerves C5 through T1, which are referred to as the plexus's "roots." These rami/roots come together and intermingle to form the later structures within the plexus.

The branches given off from the roots include:

• Nerve branches to the segmental muscles of the neck, including the longus colii (C2-C6), and the anterior (C4-C7), middle (C3-C7), and posterior (C5-C7) scalene muscles.

• Dorsal Scapular Nerve (C5): This nerve originates from the C5 root. It travels alongside the dorsal scapular artery and provides motor innervation to the levator scapulae, rhomboid major, and rhomboid minor muscles.

• Long thoracic nerve (C5, C6, C7): Arising from the C5, C6, and C7 roots, this nerve innervates the serratus anterior muscle. It often courses with the lateral thoracic artery.

• In addition, the C5 root contributes to the formation of the phrenic nerve (C3, C4, C5), which is essential for diaphragm function.

Trunks of the brachial plexus

As the brachial plexus passes through the posterior triangle of the neck, its roots combine to form three trunks. These trunks emerge between the anterior and middle scalene muscles in the neck:

• Superior Trunk: Formed by the union of the C5 and C6 roots.

• Middle Trunk: A continuation of the C7 root.

• Inferior Trunk: Created by the convergence of the C8 and T1 roots.

Two nerves arise directly from the superior trunk:

• Suprascapular Nerve (C4, C5): This nerve innervates the supraspinatus and infraspinatus muscles as it travels through the suprascapular notch.

• Nerve to Subclavius (C5, C6): This nerve provides innervation to the subclavius muscle.

The middle and inferior trunks typically do not give rise to any branches.

Divisions of the brachial plexus

As the brachial plexus crosses over the first rib and passes through the axillary inlet en route to the axilla, each trunk divides into an anterior and a posterior division just beneath the clavicle:

• The anterior divisions innervate the flexor compartments of the upper limb.

• The posterior divisions innervate the extensor compartments.

Cords of the brachial plexus

Upon entering the axilla, the divisions of the brachial plexus reorganize to form three cords, which encircle the axillary artery. These cords are named based on their relative positions to the second part of the axillary artery: the lateral cord, medial cord, and posterior cord.

1. Lateral cord

• Formed by the union of the anterior divisions of the superior and middle trunks.

• It carries nerve fibers from spinal nerve roots C5, C6, and C7.

2. Medial cord

• Derived from the anterior division of the inferior trunk.

• It contains fibers originating from C8 and T1.

3. Posterior cord

• Formed by the convergence of the posterior divisions of the superior, middle, and inferior trunks.

• It includes fibers from all five nerve roots, spanning C5 to T1.

Branches of the brachial plexus

The cords give off several branches:

1. Lateral cord branches

The lateral cord gives rise to the following branches:

• Lateral pectoral nerve (C5, C6, C7): This nerve innervates the pectoralis major muscle, contributing to its motor function.

• Musculocutaneous nerve (C5-C7): Formed entirely from the lateral cord, this nerve supplies the anterior compartment of the arm, including the biceps brachii, brachialis, and coracobrachialis muscles. It also provides lateral cutaneous sensation to the forearm through the lateral cutaneous nerve of the forearm.

• Lateral root of the median nerve (C5, C6, C7): This branch combines with the medial root (from the medial cord) to form the median nerve, a major mixed nerve of the forearm and hand.

2. Medial cord branches

• Medial pectoral nerve (C8, T1): This nerve innervates both the pectoralis major and pectoralis minor muscles.

• Medial cutaneous nerve of arm (C8, T1): Responsible for providing sensory innervation to the skin of the medial arm.

• Medial cutaneous nerve of forearm (C8, T1): This nerve supplies sensory innervation to the skin of the medial forearm.

• Ulnar nerve (C8, T1): The ulnar nerve innervates some of the forearm muscles (e.g., flexor carpi ulnaris and the medial half of flexor digitorum profundus) and most intrinsic hand muscles (e.g., the hypothenar muscles, interossei, and medial lumbricals). It also provides sensation to the medial 1½ fingers (the little finger and half of the ring finger) and the corresponding part of the hand.

• Medial root of the median nerve (C8, T1): Combines with the lateral root (from the lateral cord) to form the median nerve, which has both motor and sensory functions (described below).

Notes on the Median nerve (C5-T1)

The median nerve is formed by contributions from the lateral and medial cords.

• Motor innervation: It supplies most muscles in the anterior forearm (except those innervated by the ulnar nerve) and the thenar muscles in the hand and lateral lumbricals.

• Sensory innervation: It provides sensation to the palmar surface of the lateral 3½ fingers (thumb, index, middle, and half of the ring finger).

3. Posterior cord branches

The posterior cord gives rise to the following branches:

• Upper subscapular nerve (C5, C6): Innervates the subscapularis muscle.

• Lower subscapular nerve (C5, C6): Supplies the subscapularis and teres major muscles.

• Thoracodorsal nerve (C6, C7, C8): Innervates the latissimus dorsi muscle.

• Axillary nerve (C5, C6): Supplies the deltoid (responsible for shoulder abduction) and the teres minor (a rotator cuff muscle). It also provides sensation to the lateral shoulder via the superior lateral cutaneous nerve of the arm.

• Radial nerve (C5-T1): Innervates all extensor muscles of the arm and forearm, including the triceps brachii and the extensors of the wrist and fingers. It also supplies sensory input to the posterior arm, forearm, and hand, including the dorsal aspect of the lateral 3½ fingers (except the fingertips).

Summary tables - Brachial plexus in a nut-shell

Table 1: Supraclavicular components

Table 2: Infraclavicular components