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The skin of the back is thick, with increasing thickness toward the nape of the neck. The cutaneous innervation of the back is segmentally innervated through the dorsal rami of spinal nerves. The spinous processess of the vertebrae and other osteologic landmarks are palpable, which enable localization of spinal levels through surface landmarks.
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Cutaneous Innervation
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The skin of the back is segmentally innervated by cutaneous nerves that originate from the dorsal rami (Figure 1-1A). Dorsal rami contain both motor and sensory neurons as they branch from each level of the spinal cord and course posteriorly in the trunk. The motor neurons terminate in the deep back muscles (e.g., erector spinae muscles), where they cause muscle contraction. The sensory neurons, however, continue on and terminate in the skin where they provide cutaneous sensations such as pain, touch, and temperature at each dermatomal level of the back (Figure 1-1B). There is some segmental overlap of the peripheral sensory fields from adjacent dermatomes.
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- The dorsal ramus of C1 carries only motor neurons to the suboccipital muscles.
- The dorsal ramus of C2 carries only sensory neurons to the back of the scalp.
- The lateral aspects of the back are innervated by lateral cutaneous nerves, which are derived from the ventral rami segments of spinal nerves at each level (Figure 1-1A).
- S5 and Co 1 only carry sensory neurans.
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Osteology and Surface Anatomy
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The following structures are easily palpated through the skin (Figure 1-1A):
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- Cervical vertebrae. The first prominent spinous process that is palpable is C7 (vertebra prominens). Cervical spines 1 to 6 are covered by the ligamentum nuchae, a large ligament that courses down the back of the neck and connects the skull to the spinous processes of the cervical vertebrae.
- Thoracic vertebrae. The most prominent spine is the T1 vertebra; other vertebrae can be easily recognized when the trunk is flexed anteriorly. Thoracic vertebrae have long spines that point downward so that each spinous process is level with the body of the inferior vertebra. The spines can be counted downward from C7 and T1, or from a line joining the iliac crests at the L4 vertebral level and then counting upward from that site.
- Lumbar vertebrae. The body of L3 is approximately ...