Anatomy, Shoulder and Upper Limb, Median Nerve

Excerpt

The innervation of the upper limb is divided into anterior (ventral) divisions that supply the anterior muscles (such as biceps brachii, brachialis, pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis and profundus, and pronator quadratus). These muscles are innervated by C5-T1 spinal nerves.

The spinal cord is divided into white columns (myelinated axons) and gray horns (cell bodies of motor neurons). The sensory neurons have their cell bodies in the posterior (dorsal) root ganglia. They have a peripheral process that innervates the organs of sensation in the skin and a central process that connects the posterior root ganglia to the spinal cord posterior (dorsal) columns for sensory function. The cell bodies for motor function lie in the anterior (ventral) gray horns. The posterior roots serve sensation. The anterior roots are motor in function. The posterior and anterior roots join to form the mixed spinal nerve. Those that serve the upper limb are derived from spinal cord segments C5-T1. The entirety of the spinal cord gives off eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal mixed spinal nerve.

The mixed spinal nerves join to form the brachial plexus (Latin brachium = arm). The mixed spinal nerves give off posterior and anterior branches. The anterior branches from C5-T1 are termed roots of the brachial plexus. Those from C5 and C6 join to form the superior trunk. Those from C7 form the middle trunk. Those from C8 and T1 form the superior trunk. The posterior nerves are involved in the innervation of the paraspinal muscles.

Each trunk then divides into anterior and posterior divisions. The anterior divisions from C5, C6, and C7 join to form the lateral cord. Those from the anterior divisions of C8 and T1 form the medial cord. The posterior divisions of C5-T1 join to form the posterior cord.

The median nerve is one of the five terminal divisions of the brachial plexus. The convergence of the lateral and the medial cords form this nerve, and it has contributions from all anterior rami of C5-T1.