Brachial Plexus

The Brachial Plexus is formed by the anterior division of the fifth, sixth, seventh, and eighth cervical nerves, and the greater part of the first thoracic nerve. The second thoracic nerve, although not part of the plexus, helps in the innervation of the arm through the intercosto-humeral nerve.

Fig. 65. - The Brachial Plexus.

The nerves forming the plexus appear in the posterior triangle of the neck, and, passing between scalenus medius and anticus with the subclavian artery, they accompany the axillary artery to the shoulder and upper limb.

As the nerves enter the posterior triangle they form the three primary cordsFirst primary cord : Fifth and sixth nerves joined together.

Second primary cord : Seventh nerve alone.

Third primary cord: Eighth cervical and first thoracic joined together.

As soon as the three cords are formed they each divide into anterior and posterior divisions to form the secondary cords, which are named according to their relation to the axillary artery.

Outer cord: Anterior divisions of first and second primary cords.

Inner cord: Anterior division of third primary cord.

Posterior cord : Posterior divisions of all three primary cords.

The nerves supplying the shoulder and arm are derived from these three cord-viz. :

Outer cord: Musculo-cutaneous, outer head of median, external anterior thoracic nerves.

Inner cord: Ulnar, inner head of median, internal anterior thoracic, internal cutaneous, and lesser internal cutaneous nerves.

Posterior cord: Circumflex, three subscapular and musculo-spiral nerves.

Fig. 66. - Cutaneous Nerves of Upper Limb (Posterior Surface).

Fig. 67. - Cutaneous Nerves of Upper Limb (Anterior Surface).

Before the nerves join up to form the cords, a few branches are given off which are called Supraclavicular nerves to distinguish them from the branches derived from the secondary cords which are called the Infraclavicular nerves.

Supraclavicular Nerves

Muscular branches to scaleni, sub-clavius, and longus colli.

Posterior scapular supplies the rhomboids and levator anguli scapulae.

Long thoracic supplies serratus magnus. It pierces scalenus medius and enters the axilla between the artery and serratus magnus. This nerve is also called the respiratory nerve of Bell.

Suprascapular supplies supra- and infraspinatus and articular branches to the shoulder-joint. It passes down to the superior border of the scapula, then through the suprascapular foramen and winds round the great scapular notch.

Infraclavicular Nerves

The anterior set from the inner and outer cords supply the chest and front of the limbs, the posterior set of nerves from the posterior cord supply the shoulder and the back of the limb.

Anterior Thoracic Nerves

The external anterior thoracic nerve arises from the outer cord, and the internal one from the inner cord. They pass down one on each side of the axillary artery, and are finally distributed to pectoralis major and minor.

Musculo-Cutaneous nerve from the outer cord lies first between coraco-brachialis and the axillary artery; it then lies between biceps and brachialis to the bend of the elbow. It becomes cutaneous between biceps and brachio-radialis, and ends by supplying the skin on the outer side of the forearm.

Branches

Muscular to biceps, brachialis anticus and coraco-brachialis (this last nerve is not really a branch of musculocutaneous, but is an independent branch from the sixth and seventh cervical nerves incorporated with it).

Cutaneous

Anterior branch supplies the outer half of the anterior surface of the forearm as far as the ball of the thumb. The posterior branch supplies the upper three-fourths of the outer half of the posterior surface of the forearm.