Nervous system
The easiest way to think of a nerve is as a collection of fibers. These fibers are neurons. A neuron runs from the spine and affects one muscle1. The nerve may be a collection of fibers coming from different spinal levels and may affect several muscles.
From the third to the eigth cervical vertebrae (C3-C8) and the first thoracic vertebrae (T1) nerves exit the spine and come together forming the brachial plexus. The brachial plexus has a pretty complicated set of divisions.
- The Roots - these come off the spine as mentioned above
- The Trunks - the trunks are clusters of roots. The Superior is from C5-6, the Middle is C7, and the Inferior is C8-T1
- The Divisions - each of the Trunks has an anterior and posterior division
- The Cords - The lateral cord comes from the anterior division of the superior and middle trunks; the posterior cord comes from all three posterior divisions; the medial cord comes from the anterior divison of the inferior trunk
From every level, roots to the cords, nerves exit the plexus and run toward their target muscles. The nerves are often referred to by the level of the spine that contributes to them. The spinal level is especially important when there is damage to the spine. A tumor on the right side of C5 could affect any nerve leaving at that space. Any nerve that is created from a C5 root could be affected. Remember though that there are a number of differnt fibers. In this way one root can affect several muscles, even though one fiber can only effect one muscle, because one root contains many fibers and several nerves.
The table below is created as a reference for anyone needing to know about a specific nerve.
Here is the complete list of branches of the brachial plexus:
| Nerve | Origin | Distribution |
| Supraclavicular branches |
| Dorsal scapular | C5 with contributions from C4 | Innervates rhomboid muscles and levator scapulae |
| Long thoracic | C5-C7 | Innervates serratus anterior |
| Nerve to subclavius | Superior trunk, C4-C6 | Innervates subclavius and sternoclviaular joint |
| Suprascapular | Superior trunk, C4-C6 | Innervates supraspinatus, infraspinatus and glenuhumeral joint |
| Infraclavicular branches |
| Lateral pectoral | Lateral, C5-C7 | Innervates pec. maj., but supplies a loop to med. pect. nerve |
| Musculocutaneous | Lateral, C5-C7 | innervates coracobrachilais, biceps, brachialis; continues as lateral antebrachial cutaneous nerve |
| Median | Later from C6-C7, medial from C8-T1 | Innervates the flexor muscles in forearm (except flexor carpi ulnaris, ulnar half of flexor digitorum profundus) and five hand muscles |
| Medial pectoral | Medial cord, from C8-T1 | Innervates the pec. min. and part of pec. maj. |
| Medial brachial cutaneous | Medial cord, from C8-T1 | Supplies the skin on the medial side of the arm |
| Medial antebrachial cutaneous | Medial cord, from C8-T1 | Supplies the skin on the medial side of the forearm |
| Ulnar | terminal branch of the medial cord, C7-T1 | Innervates one and one half flexor muscles in the forearm, most small muscles of the hand and skin of the hand medial to a line bissecting the 4th digit |
| Upper subscapular | Post cord C5-C6 | Innervates top of subscapularis |
| Thoracodorsal | Post cord C6-C8 | Innervates latissimus dorsi |
| Lower subscapular | post cord C5-C6 | innervates bottom of subscapularis and teres major |
| Axillary | terminal brach of post cord C5-C6 | innervates teres mino and deltoid, shoulder joint and skin over inferior part of deltoid |
| Radial | Terminal branch post C5-T1 | Innervates triceps, anconeus, brachoradialis, extensors of forearm; supplies skin on posterior aspect of arm and forearm via posterior cutaneous nerves |
1 Actually it can affect any organ, but we're focused on the muscles here.
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