Video: Main nerves of the upper limb

Since the internet became available to the masses in the 90s, we've been since obsessed with getting and exchanging information faster and quicker. It seems there is no end to our data demand. Of course, transmitting information at high speed is what nerves have been doing in organisms for hundreds of millions of years. Perhaps, you've experienced the rapid speed at which sensory information travels in your upper limb after accidentally hitting your funny bone off the table. This phenomenon is actually your ulnar nerve – one of the main nerves of the upper limb, responding to the clumsy injury. Are you curious to learn more about the innervation of the upper limb? Fasten your seatbelt as we explore the main nerves of the upper limb.

What exactly are you buckling up to learn about? Well, we're going to discuss the main nerve supplying the upper limb and identify each of their origins from the brachial plexus. During our voyage, we will also travel distally and discover the regions and structures of the upper limb that each nerve innervates. As we arrive at our final stop, we will take a look at a clinical note on the nerves of the upper limb to help consolidate our knowledge. So, are you ready to explore the nerves of the upper limb? Excellent! Let's get started with where it all begins – the brachial plexus.

The brachial plexus is a network of interlacing nerves that provide complete motor and sensory innervation to the upper limb. In this schematic representation of the brachial plexus, we can see that it is formed by the anterior or ventral rami of the cervical spinal nerves C5 to C8 and most of the anterior ramus of spinal nerve T1. These anterior rami coalesce into the plexus that courses inferiorly and laterally in the neck, crosses over the first rib, and enters the axilla. While traveling through the neck, the brachial plexus is accompanied anteriorly by the subclavian artery. In the axilla, the plexus surrounds the axillary artery. The brachial plexus is traditionally organized into different components medially to laterally including the roots, trunks, divisions, cords, and main terminal nerve branches. You can remember this arrangement with the handy mnemonic, Read That Damn Cadaver Book.

The roots of the brachial plexus represent the anterior rami of spinal nerves C5 to C8 with an additional contribution from the anterior ramus of spinal nerve T1. Similarly, to trees, roots give rise to trunks of which the brachial plexus contains three. The C5 and C6 roots merge to form the superior trunk while the C7 root itself gives rise to the middle trunk and the C8 and T1 roots form the inferior trunk. Each trunk subsequently gives rise to anterior and posterior divisions which continue on as the cords of the brachial plexus.

The three cords of the brachial plexus result from the combination of the divisions. The anterior divisions of the superior and middle trunks combine to form the lateral cord. The posterior divisions of all three trunks produce the posterior cord. Finally, the continuation of the anterior division of the inferior trunk gives rise to the medial cord. The three cords finally terminate as five terminal branches, which form the main nerves of the upper limb and include the musculocutaneous nerve emanating from the lateral cord, the axillary and radial nerves splitting off the posterior cord, the ulnar nerve continuing off the medial cord, and lastly, the median nerve arising from both the lateral cord and medial cord.

Now that we've described the general structure of the brachial plexus, it's time to look at the main focus of this tutorial – the terminal branches of the brachial plexus.

The musculocutaneous nerve is a terminal branch of the lateral cord of the brachial plexus and carries nerve fibers from the anterior rami of the C5 to C7 spinal nerves. The musculocutaneous nerve provides motor innervation to the flexor muscles in the anterior compartment of the arm including the biceps brachii muscle. The musculocutaneous nerve also supplies sensory innervation to the skin on the lateral side of the forearm via the lateral cutaneous nerve of the forearm.

Moving on to the next nerve of the upper limb, we meet the axillary nerve or the circumflex nerve. This nerve emanates off the posterior cord of the brachial plexus and carries fibers from the C5 and C6 anterior rami. This nerve passes posteriorly around the surgical neck of the humerus where it is especially susceptible to damage in the event of a fracture of this bone. The axillary nerve provides motor innervation to the deltoid muscle and teres minor muscle and sensory innervation to the skin overlying the deltoid region, otherwise, known as the regimental badge area.

Now let's look at the radial nerve, which is the other terminal branch of the posterior cord. With nerve fiber contributions from the anterior rami of all five levels, C5 to T1, the radial nerve is the largest branch of the brachial plexus. After its origin, the radial nerve leaves the axilla and enters the posterior compartment of the arm. During its course, the radial nerve is responsible for the innervation of the extensor muscles in the posterior compartment of the arm and forearm. The radial nerve also provides sensory innervation to the skin of the posterior aspect of the arm and forearm. In the hand, the radial nerve gives off digital branches through which it innervates the skin of the dorsal aspect of the thumb, index, middle, and radial half of the ring finger.

It's time to look at the ulnar nerve. The ulnar nerve is a continuation of the medial cord and carries fibers from the anterior rami of C8 to T1. After its origin, the ulnar nerve courses down the medial aspect of the arm then it descends posterior to the medial epicondyle of the humerus where it is subcutaneous and easily palpable. It's in this region that the ulnar nerve is commonly referred to as the funny bone. After that, the ulnar nerve enters and descends in the anterior compartment of the forearm along the ulna. Here it provides motor innervation to muscles not supplied by the median nerve, specifically, the flexor carpi ulnaris muscle and the medial or ulnar half of the flexor digitorum profundus muscle. In the hand, the ulnar nerve gives off digital branches. Here it is responsible for the innervation of most of the intrinsic hand muscles and the skin of the little finger and medial half of the ring finger.

The last terminal branch, the median nerve, arises from both lateral and medial cords and has nerve contributions from the ventral rami of C6 to T1. This median nerve is so-called because it runs approximately down the middle of the arm and forearm. In the forearm, it provides motor innervation to almost all of the flexor muscles of the wrist and hand located in the anterior compartment of the forearm. As the median nerve enters the hand, it passes deep to a band of connective tissue called the flexor retinaculum. In the hand, the median nerve supplies some of the intrinsic hand muscles. Here it gives off digital branches supplying sensory innervation to the skin of the palmar and distal dorsal surfaces of the thumb, index, and middle fingers and the radial half of the ring finger.

All right, it's time to get clinical.

The median nerve enters the hand passing through the carpal tunnel which is located anteriorly at the wrist and formed by the carpal bones and the flexor retinaculum. Carpal tunnel syndrome results from the compression of the median nerve within the carpal tunnel. Nerve injury may be a result of increased pressure on the median nerve caused by overuse, swelling of the surrounding tendons and tendon sheaths like in rheumatoid arthritis, and cysts of the carpal joints. Signs and symptoms include pain, numbness, and tingling or paresthesia in the distribution of the median nerve. Weakness and muscle atrophy of the hand may also occur. This condition is generally treated by making a small incision in the flexor retinaculum to release the pressure on the nerve.

Before we bring today's tutorial to an end, let's reinforce what we learned with a handy summary.

We began this tutorial with an overview of the brachial plexus and explored its general organization. We started with the roots which are formed by the anterior rami of spinal nerves C5, C6, C7, C8, and T1. Next, we reviewed how the roots give rise to the three trunks of the brachial plexus. Then we covered how each trunk divides into anterior and posterior divisions which continue on to form three cords. Finally, the cords give rise to the five terminal branches. The musculocutaneous nerve emanates from the lateral cord, the axillary and radial nerves both arise from the posterior cord, the ulnar nerve is a continuation of the medial cord, and finally, the median nerve arises from both the lateral and medial cords.

We subsequently talked about the five terminal branches individually. The musculocutaneous nerve innervates the flexor muscles in the anterior compartment of the arm and terminates as the lateral cutaneous nerve of the forearm innervating the skin of this region. The axillary nerve innervates both the deltoid and teres minor muscles and the skin over the deltoid region. The radial nerve and its branches innervate all the muscles in the posterior compartments of the arm and forearm as well as the skin of the posterior aspect of the arm and forearm and the skin of the lateral surface of the dorsum of the hand. The ulnar nerve in its branches innervates two muscles of the forearm – the flexor carpi ulnaris muscle and the medial half of the flexor digitorum profundus muscle – and most of the intrinsic muscles of the hand. It also innervates the skin over the palmar surface of the little finger, medial half of the ring finger, and the skin over the dorsal surface of the medial part of the hand. The median nerve innervates most of the muscles in the anterior compartment of the forearm. In the hand, it innervates some of its muscles as well as the skin of the palmar and distal dorsal surfaces of the thumb, index, and middle fingers and the radial half of the ring finger. Finally, we dove into our clinical section where we discussed a condition known as carpal tunnel syndrome.

Terrific work, everyone! I hope you enjoyed today's tutorial and happy studying!