Brachial Plexus & Upper Limb Muscles

• The lecture covers the brachial plexus and muscles of the upper limb.
• Spinal nerves exit the spinal cord via the intervertebral foramen to innervate peripheral structures.
• Nerves split into anterior and posterior branches called primary rami.
• Posterior primary rami innervate muscles of the back.
• Anterior primary rami of spinal nerves innervate muscles and skin in the upper limb by way of the brachial plexus.
• The brachial plexus consists of spinal nerves C5, C6, C7, C8, and T1.
• "Plexus" means "braid" in Latin, describing how roots combine and divide to form peripheral branches.
• Terminal nerves typically receive contributions from multiple roots, providing overlap.
• The brachial plexus passes laterally to the cervical spine, between the anterior and middle scalene muscles.
• The plexus travels inferiorly and laterally, passing under the clavicle to end in the axilla (armpit).
• A neurovasculature bundle including a vein and artery runs alongside it into the upper limb.
• The axilla is where the end of the plexus and the beginning of the terminal branches are found.
• The axilla is bordered by serratus anterior, brachialis, pectoralis major, and subscapularis muscles.
• The mnemonic 5-3-6-3-5 represents the brachial plexus structure: five roots, three trunks, six divisions, three cords, and five terminal branches.
• "Real Texans Drink Cold Beer" is a mnemonic for roots, trunks, divisions, cords, and branches.
• MARMU represents the five terminal nerves in lateral-to-medial order: musculocutaneous, axillary, radial, median, and ulnar.
• C5, C6, C7, C8, and T1 are the anterior rami of the spinal nerves that form the roots.
• The superior trunk is formed from C5 and C6.
• The middle trunk is formed from C7 alone.
• The inferior trunk is formed from C8 and T1.
• Each of the three trunks divides into an anterior and a posterior division, resulting in six divisions.
• The lateral cord is formed by the anterior divisions of the superior and middle trunks.
• The posterior cord is formed by all three posterior divisions.
• The medial cord is formed by the remaining anterior division of the inferior trunk.
• Cords are named based on their position relative to the axillary artery (lateral, medial, posterior).
• The musculocutaneous nerve originates from the lateral cord.
• The axillary and radial nerves originate from the posterior cord.
• The median nerve originates from contributions of the lateral and medial cords.
• The ulnar nerve is from the medial cord.
• Each terminal nerve has root contributions traceable back through the cords, divisions, trunks, and spinal nerve roots.
• Extrinsic back muscles attach to the spine but insert onto the scapula or humerus to move it.
• The trapezius is a large, flat, triangular muscle in the superficial back.
• The trapezius originates broadly from the external occipital protuberance, ligamentum nuchae, and spinous processes of T1-T12.
• It inserts onto the spine of the scapula, acromion, and lateral third of the clavicle.
• It is innervated by cranial nerve XI, the spinal accessory nerve.
• Upper trapezius fibers extend/laterally flex the neck, rotate the head contralaterally, elevate the scapula, and assist in upward rotation.
• Middle trapezius fibers retract (adduct) the scapula.
• Lower trapezius fibers depress the scapula and assist in upward rotation.
• Rhomboids (major and minor) lie beneath the trapezius.
• The rhomboid minor runs from the spinous processes of C7-T1 to the medial border of the scapula.
• The rhomboid major runs from T2-T5 to the medial border of the scapula.
• Both rhomboids are innervated by the dorsal scapular nerve and retract (adduct) the scapula.
• The rhomboids assist in scapular elevation and cause downward rotation of the scapula.
• The levator scapula originates on the transverse processes of C1-C4 and inserts on the medial border of the scapula.
• It is innervated by the dorsal scapular nerve.
• Levator scapula can extend and laterally flex the neck, and rotate the neck ipsilaterally, or elevate and downwardly rotate the scapula.
• The pectoralis minor originates on ribs 3-5 and inserts on the coracoid process of the scapula.
• It is innervated by the medial pectoral nerve.
• Pectoralis minor protracts (abducts), depresses, and downwardly rotates scapula; with the scapula fixed, it elevates the ribs during forced inhalation.
• The subclavius is a small muscle from the first rib to the inferior surface of the clavicle.
• Subclavius stabilizes the clavicle at both the acromioclavicular and sternoclavicular joints.
• Serratus anterior originates on ribs 1-9 and inserts on the deep surface of the medial border of the scapula.
• It is innervated by the long thoracic nerve (C5-C7).
• Serratus anterior powerfully protracts the scapula (pulling it forward) and rotates the glenoid upward (upward rotation).
• It anchors the scapula against the rib cage.
• Weakness or paralysis of the serratus anterior causes scapular winging and difficulty with actions like pushing or raising the arm overhead.
• Pectoralis major originates broadly 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 is innervated by the medial and lateral pectoral nerves.
• Pectoralis major flexes, adducts, and medially rotates the arm and can horizontally adduct the arm.
• Latissimus dorsi originates from T7-L5, the sacrum, iliac crest, and thoracolumbar fascia and inserts into the floor of the bicipital groove of the humerus.
• It is innervated by the thoracodorsal nerve.
• Latissimus dorsi extends, adducts, and medially rotates the arm at the shoulder, and can cause an anterior tilt of the pelvis.
• Scapular movements include elevation, depression, protraction (abduction), retraction (adduction), and upward and downward rotation.
• Muscles can work synergistically or as antagonists (e.g., trapezius vs. rhomboids and levator scapula for upward vs. downward rotation).

Lecture on the brachial plexus and upper limb muscles. Covers spinal nerves, primary rami, and the path of the brachial plexus from the cervical spine to the axilla. Explains how spinal nerve roots combine and divide into peripheral branches.