Cranial nerves originate from the brainstem, and mainly control the functions of the anatomic structures of the head.
Spinal nerves take their origins from the spinal cord. They control the functions of the rest of the body. In humans, there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The naming convention for spinal nerves is to name it after the vertebra immediately above it. Thus the fourth thoracic nerve originates just below the fourth thoracic vertebra. This convention breaks down in the cervical spine. The first spinal nerve originates above the first cervical vertebra and is called C1. This continues down to the last cervical spinal nerve, C8. There are only 7 cervical vertabra and 8 cervical spinal nerves.
The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve the neck and back of head.
The phrenic nerve[?] arises from nerve roots C3, C4 and C5. It innervates the diaphragm, enabling breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible. See myelopathy[?]
The last 4 cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1,combine to form the brachial plexus, a tangled array of nerves, splitting, combining and recombining, to form the nerves that subserve the arm and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable, with little variation between people. See brachial plexus injuries[?].
The brachial plexus first forms three trunks, the superior trunk, composed of the C5 and C6 nerve roots, the middle trunk, made of the C7 nerve root, and the inferior trunk, made of the C8 and T1 nerve roots. The suprascapular nerve[?] is an early branch of the superior trunk. It innervates the suprascapular and infrascapular muscles, part of the rotator cuff. See rotator cuff for rotator cuff injuries
The trunks reshuffle as they traverse towards the arm into cords. There are three of them. The lateral cord is made up of fibers from the anterior and middle trunk. The posterior cord is made up of fibers from all three trunks. The medial cord is composed of fibers solely from the medial trunk.
The lateral cord gives rise to the following nerves
The remainder of the thoracic spinal nerves, T3 through T12, do little recombining. The form the intercostal nerves[?], so named because the run between the [rib]s, inter(between)-costal(ribs). For points of reference, the 7th intercostal nerve terminates at the lower end of the sternum[?], also known as the xyphoid. The 10th intercostal nerve terminates at the umbilicus, aka the belly button.
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