At each level of the spinal column, spinal nerves branch off from either side of the spinal cord and exit between a pair of vertebrae, to innervate a specific part of the body. The area of skin innervated by a specific spinal nerve is called a dermatome, and the group of muscles innervated by a single spinal nerve is called a myotome. The part of the spinal cord that was damaged corresponds to the spinal nerves at that level and below. Injuries can be cervical 1–8 (C1–C8), thoracic 1–12 (T1–T12), lumbar 1–5 (L1–L5), or sacral (S1–S5). A person's level of injury is defined as the lowest level of full sensation and function. Paraplegia occurs when the legs are affected by the spinal cord damage (in thoracic, lumbar, or sacral injuries), and tetraplegia occurs when all four limbs are affected (cervical damage).