Brachial Plexus and Upper Limb Muscles Lecture

Today's lecture is going to be about the **brachial plexus and muscles of the upper limb.** Prior to this point, we've talked about spinal nerves. Remember that spinal nerves exit off the spinal cord and they exit the intervertebral foramen as they go to innervate structures in the periphery. We pr...

Today's lecture is going to be about the **brachial plexus and muscles of the upper limb.** Prior to this point, we've talked about spinal nerves. Remember that spinal nerves exit off the spinal cord and they exit the intervertebral foramen as they go to innervate structures in the periphery. We previously talked about the fact that these nerves split into anterior and posterior branches called primary rami. The posterior primary rami typically go to innervate the muscles of the back, and we talked about that when we looked at the layers of muscles in the back. Now we're going to be looking at anterior primary rami of spinal nerves, and specifically, we're going to be looking at the spinal nerves that are responsible for innervating the majority of muscles and skin in the upper limb. This is called the brachial plexus. This structure is shown to you here, and we're going to go through it step by step and talk about how it forms and what it's responsible for innervating in the upper limb. This initial picture just shows you an overview of the plexus, starting with the spinal nerves that help form it: C5, C6, C7, C8, and T1. These five roots (anterior primary rami) will form the brachial plexus. In Latin, the word "plexus" means "braid," so you can see these five roots combine and divide in various ways to ultimately give us peripheral branches that innervate different structures in the upper limb. The key takeaway is that each terminal nerve typically receives contributions from more than one root, providing overlap or "backup" in case one of the roots is damaged. This is what the brachial plexus looks like in situ, meaning in the body. Initially, the roots pass just laterally to the cervical spine and travel between our scalene muscles. Think back to when we palpated the scalene muscles---those dense muscles of the neck. The brachial plexus passes between the anterior and middle scalene. From there, it travels inferiorly and laterally, passing underneath the clavicle. Eventually it ends up in the axilla (the armpit). Running alongside the brachial plexus (shown in yellow here) we also see a blue structure (vein) and a red structure (artery). This neurovasculature bundle travels together into the upper limb. The axilla, or armpit, is an important anatomical area where we find the end of the plexus and the beginning of the terminal branches. It's bordered by muscles like serratus anterior, brachialis, pectoralis major, and subscapularis. Now let's build the plexus from start to finish. A handy mnemonic is 5-3-6-3-5, indicating five roots, three trunks, six divisions, three cords, and five terminal branches. Another mnemonic is "Real Texans Drink Cold Beer" (roots, trunks, divisions, cords, branches). And the final mnemonic, MARMU, helps you remember the five terminal nerves in lateral-to-medial order: musculocutaneous, axillary, radial, median, and ulnar. Starting with five roots: C5, C6, C7, C8, and T1. These are the anterior rami of the spinal nerves. Next, five becomes three by forming three trunks: Superior trunk (from C5 and C6) Middle trunk (from C7 alone) Inferior trunk (from C8 and T1) Then, each trunk divides into an anterior and a posterior division, so three trunks become six divisions (three anterior + three posterior). Those six divisions combine to form three cords: Lateral cord, formed by the anterior divisions of the superior and middle trunks Posterior cord, formed by all three posterior divisions (of the superior, middle, and inferior trunks) Medial cord, formed by the remaining anterior division of the inferior trunk These cords are named according to their position relative to the axillary artery (lateral, medial, posterior). Finally, the cords give rise to five terminal branches (MARMU): 1. Musculocutaneous nerve (from the lateral cord) 2. Axillary nerve (from the posterior cord) 3. Radial nerve (from the posterior cord) 4. Median nerve (from contributions of the lateral and medial cords) 5. Ulnar nerve (from the medial cord) Each of these terminal nerves has root contributions traceable back through the cords, divisions, and trunks to the specific spinal nerve roots. Next, we shift our focus to some muscles of the upper limb, starting with extrinsic back muscles that actually move or stabilize the scapula/upper limb. Recall from an earlier lecture that extrinsic back muscles attach to the spine but do not move it; instead, they use the spine as an attachment point and insert onto the scapula or humerus. First is trapezius, a large, flat, triangular muscle in the superficial back. It has a very broad origin on the external occipital protuberance, ligamentum nuchae, and spinous processes of T1 to T12. It narrows significantly and inserts onto the spine of the scapula, the acromion, and the lateral third of the clavicle. Innervated by cranial nerve XI (the spinal accessory nerve), trapezius has three sets of fibers: Upper fibers: can extend/laterally flex the neck, rotate the head contralaterally, elevate the scapula, and assist in upward rotation of the scapula.\ Middle fibers: retract (adduct) the scapula.\ Lower fibers: depress the scapula and assist in upward rotation of the scapula. Next are the rhomboids, major and minor, beneath trapezius. Rhomboid minor runs from the spinous processes of C7--T1 to the medial border of the scapula (at the spine), and rhomboid major runs from T2--T5 to the medial border of the scapula (below the spine). Both are innervated by the dorsal scapular nerve. They retract (adduct) the scapula, assist in scapular elevation, and cause downward rotation of the scapula. Also in that layer is levator scapula, a tubular muscle in the neck that originates on the transverse processes of C1--C4 and inserts on the medial border of the scapula (above the spine). It's also innervated by the dorsal scapular nerve. Because the neck is mobile, levator scapula can extend the neck, laterally flex the neck, and rotate the neck ipsilaterally if it pulls on its origin. Or it can elevate and downwardly rotate the scapula if it pulls on its insertion. Flipping to the front (anterior chest), we have pectoralis minor. It originates on ribs 3, 4, and 5 and inserts on the coracoid process of the scapula. Innervated by the medial pectoral nerve, it protracts (abducts) the scapula, depresses it, and downwardly rotates it. With the scapula fixed, it can also elevate the ribs during forced inhalation. Subclavius is a small muscle from the first rib to the inferior surface of the clavicle. It helps stabilize the clavicle at both the acromioclavicular and sternoclavicular joints. Serratus anterior is a broad muscle with a sawtooth appearance, originating on ribs 1--9 and inserting on the deep (anterior) surface of the medial border of the scapula. Innervated by the long thoracic nerve (C5--C7), it powerfully protracts the scapula (pulling it forward) and rotates the glenoid upward (upward rotation). It also anchors the scapula against the rib cage. Weakness or paralysis of serratus anterior (often from long thoracic nerve injury) causes the scapula to "wing," popping off the rib cage, and makes actions like pushing or raising the arm overhead difficult. Moving to the larger chest muscle, pectoralis major has a broad origin on the clavicle, sternum, and costal cartilages of ribs 1--7 and inserts on the lateral lip of the bicipital groove of the humerus. It's innervated by the medial and lateral pectoral nerves. Pectoralis major flexes, adducts, and medially rotates the arm; it also can horizontally adduct the arm. Latissimus dorsi is a broad muscle of the lower back, originating from T7--L5, the sacrum, the iliac crest, and the thoracolumbar fascia. It inserts into the floor of the bicipital groove of the humerus. Innervated by the thoracodorsal nerve, latissimus dorsi extends, adducts, and medially rotates the arm at the shoulder, and it can also cause an anterior tilt of the pelvis via its attachment to the sacrum and ilium. Finally, we see a summary diagram of scapular movements: elevation, depression, protraction (abduction), retraction (adduction), and upward and downward rotation. It's helpful to identify which muscles are responsible for each motion and how some muscles can work synergistically while others act as antagonists (e.g., trapezius vs. rhomboids and levator scapula for upward vs. downward rotation).