This is an excerpt from Kinetic Anatomy 4th Edition With HKPropel Access by Robert S. Behnke,Jennifer L. Plant & Jennifer L. Plant.
Spinal nerves have both anterior and posterior rami (branches or arms). The anterior rami form the brachial plexus (figure 6.3), which is typically divided into two parts based on their position relative to the clavicle: the supraclavicular (above the clavicle) and the infraclavicular (below the clavicle) parts.
The dorsal scapular nerve (C5 anterior and posterior rami) innervates the levator scapulae and rhomboid major and minor muscles (figure 6.3). The long thoracic nerve (C5, C6, and C7 anterior and posterior rami; figures 6.2 and 6.3) innervates the serratus anterior muscle. Not illustrated are nerves to the scalene and longus colli muscles of the cervical spine (C2-C8) and a communicating nerve (C5) to the phrenic nerve of the cervical plexus, which are discussed in chapter 10 on the spinal column. Two nerves coming from one of three groups of nerves of the brachial plexus, known as the upper, middle, and lower trunks, are the subclavian nerve (C4, C5, and C6), innervating the subclavian muscle, and the suprascapular nerve (C4, C5, and C6; figures 6.2 and 6.3), innervating the supraspinatus and infraspinatus muscles of the rotator cuff.
Three large cords of nerves (lateral, medial, and posterior) are formed in the brachial plexus from divisions of the upper, middle, and lower trunks of spinal nerves (see figure 6.3). The infraclavicular nerves arising from the lateral cord are the lateral anterior thoracic nerve (C5, C6, and C7), innervating the pectoralis major muscle; the lateral aspect of the median nerve (C5, C6, and C7), innervating most of the anterior forearm muscles and some in the hand; and the musculocutaneous nerve (C4, C5, and C6; figure 6.4), innervating the anterior arm muscles.
Applying pressure between the triceps brachii and the biceps brachii on the medial aspect of your arm may produce a tingling sensation in your hand because you are compressing the median nerve (figure 6.5).
There are five major nerves of the brachial plexus that arise from the medial cord (see figure 6.3). The medial anterior thoracic nerve (C8 and T1) innervates the pectoralis major and minor muscles (see figure 6.3). Joining the previously mentioned lateral aspect is the medial aspect of the median nerve (C8 and T1), which innervates most of the anterior muscles of the forearm and some of the hand (see figures 6.4 and 6.5). The medial cutaneous nerve of the forearm (medial antebrachial cutaneous nerve; T1) is not a motor nerve and innervates the skin and fascia on the medial surface of the arm. The ulnar nerve (C8 and T1) innervates the flexor carpi ulnaris and the muscles of the medial aspect of the hand not innervated by the median nerve (see figure 6.4).
The ulnar nerve is easily located between the medial epicondyle of the humerus and the olecranon process of the ulna (figure 6.6). Sudden pressure on this area often produces a tingling sensation in the forearm and is referred to as “hitting your funny bone.”