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C h e c k o u t A B M P 's l a t e s t n e w s a n d b l o g p o s t s . Av a i l a b l e a t w w w. a b m p . c o m . 87 The nerve roots in the thoracic region don't blend together to form a plexus, but simply innervate local muscles and other tissues in the area. The remaining two plexuses are in the lumbar and sacral areas. The lumbar plexus has contributions from the T12 nerve root and the first four lumbar vertebrae (L1–L4). Fibers from the lumbar plexus have sensory innervation to regions of the pelvis and thigh, and motor branches that focus on pelvic and thigh muscles. The sacral plexus has nerve fibers that emerge from lumbar and sacral nerve roots from L4–S4. Fibers from the sacral plexus innervate the pelvic region, but also emerge to form the body's largest nerve, the sciatic nerve, which runs the entire length of the lower extremity and has terminal branches all the way into the plantar surface of the foot and toes. One of the more fascinating aspects of nerve fibers is that individual nerve cells run the entire length of the nerve. That means that for the nerve fibers within the sciatic nerve that eventually go into the foot, there are individual cells extending from the spinal cord all the way to the foot. These are exceptionally long cells! The importance of these long nerve cells and how they relate to various pain complaints is illustrated by impairment of axoplasmic flow, which is discussed later. Most of these major nerve trunks carry both motor and sensory fibers and are referred to as mixed nerves. Some smaller peripheral nerves may carry exclusively motor or exclusively sensory fibers. A peripheral nerve has fibers that originate from multiple nerve roots. Similarly, fibers from one single nerve root may blend into and terminate in different peripheral nerves. Having fibers from different nerve roots within a peripheral nerve reduces the likelihood of complete nerve dysfunction if a single nerve root is injured or damaged. However, it also makes identifying certain types of nerve injury more challenging for the clinician. TERMS OF NERVE INJURY There are two terms that are used to describe nerve injuries and that indicate the location of the pathology. The first is radiculopathy, which is a nerve pathology that occurs at the nerve root level. A common radiculopathy is the herniated nucleus pulposus, or herniated disc, in which the disc presses on the nerve root. Earlier, we noted that nerve roots contain fibers that eventually make up different peripheral nerves. Therefore, pressure on a nerve root can affect fibers that make up more than one peripheral nerve. Consequently, pressure on a single nerve root can produce sensory symptoms along a wide area of the extremity. The area of skin supplied by fibers from a single nerve root is referred to as that nerve root's dermatome (Image 4). A radiculopathy or nerve root dysfunction can produce symptoms anywhere along that nerve root's dermatome. Farther along its length, each individual peripheral nerve innervates a particular region of skin with its sensory fibers. The region of skin innervated by fibers from a single peripheral nerve is referred to as that nerve's cutaneous innervation. The cutaneous Nerve root dermatome. Nerve plexus. 3 4 Brachial plexus