Here four of the five sections will be mentioned in the order of their occurrence from the most proximal point to the most distal. The last remaining section which is the nerve branches themselves will be discussed separately below, along with their regions of innervation.
The roots are the first section of the plexus which arise from the ventral rami of the last four cervical spinal nerves and the first thoracic spinal nerve.
The trunks make up the second section. The superior trunk is comprised of the ventral rami from the fifth and sixth cervical spinal nerves. The middle trunk comes from the seventh cervical spinal nerve and the inferior trunk arises from the ventral rami of the eighth cervical spinal nerve and the first thoracic spinal nerve.
The divisions are the bifurcations of the trunks and are the third section of the brachial plexus. There are two anterior divisions and a posterior division. The first anterior division is that of the superior and middle trunks that form the lateral cord. The second anterior division is that of the inferior trunk which continues as the medial cord. The posterior division occurs from all three trunks and becomes the posterior cord.
As it happens, the cords follow on as the fourth section from the trunks. The lateral, medial and posterior cords now divide into their subsequent nerve branches and continue distally down the upper limb to innervate the various anatomical structures listed below.
Nerve Branches and Innervation
Each nerve branch will now be discussed, with reference to the spinal roots whose fibers it carries, the structures it innervates and if it becomes a cutaneous nerve. The nerves will be grouped according to the section from which they arose.
Three nerves arise from the roots, the first of which is the dorsal scapular nerve that carries fibers from the fourth and fifth cervical spinal nerves and innervates the rhomboid muscles and the levator scapulae muscle. The fifth to the seventh cervical spinal nerves create the long thoracic nerve which innervates the serratus anterior. The final nerve from the roots is the branch to the phrenic nerve which stems from the fifth cervical spinal nerve.
Two nerves arise from the superior trunk and they are nerve to the subclavius muscle and the suprascapular nerve which innervates the supraspinatus and the infraspinatus muscles. Both nerves arise from the fifth and sixth cervical spinal nerves. The lateral cord produces three nerves which all stem from the fifth to the seventh cervical spinal nerves.
The lateral pectoral nerve innervates the pectoralis major muscle and the pectoralis minor muscle by communicating with the medial pectoral nerve. The musculocutaneous nerve innervates the coracobrachialis muscle, the brachialis muscle and the biceps brachii muscle before continuing on to become the lateral cutaneous nerve of the forearm. The lateral root of the median nerve gives fibers to the median nerve.
The posterior cord has five nerve branches starting with the upper subscapular nerve, which arises from the fifth and sixth cervical vertebrae and innervates the upper part of the subscapularis muscle. The last three cervical spinal nerves form the thoracodorsal nerve otherwise known as the middle subscapular nerve which innervates the latissimus dorsi. The lower subscapular nerve has the same origins as the upper subscapular nerve and innervates the lower part of the subscapularis muscle and the teres major muscle. The axillary nerve originates from the fifth and sixth spinal roots and is comprised of two branches. The anterior branch innervates the deltoid muscle and its overlying skin while the posterior branch innervates the teres minor muscle and the deltoid muscle before continuing into the upper lateral cutaneous nerve of the arm. Lastly, the radial nerve arises from all of the spinal roots and innervates the triceps brachii, the supinator muscle, the anconeus muscle, the extensor muscles of the forearm and the brachioradialis muscle. It then proceeds to become the posterior cutaneous nerve of the arm which innervates the skin of the posterior arm.
The medial cord is the last place that nerve branches arise from and the spinal roots that contribute to these branches are eighth cervical root and the first thoracic root, with the exception of the medial root of the median nerve, which stems from the sixth and the eighth cervical roots and contributes fibers to the median nerve before innervating portions of the hand that aren’t covered by the radial or ulnar nerves. The medial pectoral nerve innervates the pectoralis major and minor muscles. The medial cutaneous nerve of the arm and the medial cutaneous nerve of the forearm innervate the front and medial skin of the arm and the medial skin of the forearm respectively. They are the only two nerves of the brachial plexus that serve entirely as cutaneous nerves. Finally, the ulnar nerve, which innervates the flexor carpi ulnaris muscle, the two medial bellies of the flexor digitorum profundus and the intrinsic hand muscles save the thenar muscles and the two most lateral lumbrical muscles. It also innervates the medial side of the hand, the medial one and a half fingers on the palmar side and medial two and a half fingers on the dorsal side.
An injury to the brachial plexus can cause loss of cutaneous feeling and movement in the upper limb. The regions that are affected depend on which nerve fibers were damaged, whether the injury was due to trauma or some sort of infection and how high up the plexus it occurred. Time and rest is the only treatment available with a strict physiotherapy regime to strengthen and regain a range of motion once symptoms have completely disappeared.