 We humans have the unique ability to interact with the world around us using our upper limbs. Precise motor maps created in the brain enable us to move specific muscle groups in discrete ways needed to play this music. Sensory information from our fingers also allows us to explore the world through touch. But how do you connect the brain with peripheral receptors and muscles? How do nerves get to where they need to be? Our upper limbs evolve to position our hands in space for use as motor and sensory tools. This requires precise muscle movements and precise control over those muscles. But how does such an intricate tool develop and what are its components? We're now going to explore the evolution and development of the brachial plexus, the nerve bundle that makes this unique human experience possible. During development, the precursors of the musculoskeletal system, the somites migrate into what will be a limb. Each somite is associated with its own spinal nerve. Muscles are organized into compartments that share similar functionality. For example, here in the upper limb we have a flexor and an extensor compartment. Parts of somites can migrate into more than one of these muscle compartments and they take their nerves with them. But look at this, there's only one nerve in each compartment. So that means that each one of these peripheral nerves contains fibers from more than one spinal nerve. If this mixing and matching of fibers does not occur within the brachial plexus, the complexity of our movements would never develop. Let's take a look at the formation of this plexus in more detail. Here we have spinal nerves C5 to T1 emerging from the spinal cord. The anterior ramai of these spinal nerves or roots will form the brachial plexus. We will now recombine these roots into trunks, divisions, cords, and finally peripheral nerve branches. Let's do this step by step in this diagram. First, let's take the upper two ramai from C5 and C6 and combine them into a superior trunk. C7 forms its own middle trunk. C8 and T1 combine to form the inferior trunk. Each of these trunks then divides to give rise to an anterior and posterior division. These divisions recombine to form the three cords. The medial cord is simply an extension of the anterior division of the inferior trunk. Notice it contains fibers only from C8 and T1. The lateral cord is formed by the anterior divisions of both the upper and middle trunk merging, and you'll notice that it contains fibers from C5, C6, and C7. And look at this. The posterior cord contains fibers from all spinal nerves because it is a combination of all posterior divisions. The terminal nerves for the upper limb are a result of one last recombination of these cords. The muscular cutaneous nerve emerges directly from the lateral cord. It carries fibers from C5, C6, and C7. It will innervate the flexor compartment of the arm. The median nerve is the result of branches from both the lateral and medial cord combining. It contains fibers from C6, C7, C8, and T1. It will go on to innervate the flexors of the forearm and parts of the hand. The ulnar nerve emerges from the medial cord. It contains fibers from C8 and T1, and interestingly from C7 joining here from the lateral cord. It will go on to innervate the intrinsic muscles of the hand. The radial nerve is an extension of the posterior cord. It contains all spinal levels involved in the brachial plexus. It innervates all extensors of the upper limb. The auxiliary nerve branches off the posterior cord as well and innervates muscles around the shoulder. As you can see, the brachial plexus allows for fibers from various spinal levels to recombine into each one of these peripheral nerves associated with the different specific muscle compartments of the arm. Now let's take a look at this in the anatomy lab. This is a dissection showing the right brachial plexus. This is medial, this is lateral, this is superior, and this is inferior. The brachial plexus forms posterior to the axillary artery, so I'm going to remove the artery so that we can clearly see the various parts. The roots of the brachial plexus are clearly visible here. C5, C6, C7, C8, and T1. C5 and 6 join to form the superior trunk here, and C7 becomes the middle trunk. C8 and T1 join to become the inferior trunk. Each of these trunks divides into an anterior and posterior division that then recombine to form three cords around the axillary artery. The medial cord is here, the lateral cord here, and the posterior cord is posterior to the artery. From these three cords arise the major peripheral nerves, musculocutaneous, median, and ulnar. You'll notice these together with the medial and lateral cords form an M over the anterior aspect of the axillary artery. The musculocutaneous nerve innervates the flexor compartment of the arm. The median nerve innervates mainly the flexor compartment of the forearm. The ulnar nerve innervates mainly intrinsic muscles of the hand. The posterior cord is posterior to the artery, so we have to move structures laterally to see the nerves that arise from the posterior cord. Here is the axillary nerve and the radial nerve, which are the major branches of the posterior cord. The radial nerve innervates the extensor compartments of both the arm and the forearm. It's this whole amazing evolution of the control of our upper limb that allows this violinist to create such beautiful music.