The vast majority of nerves that send information to and from the brain relay that information through the spinal cord, which is wrapped in the protective bone of vertebrae, between which openings (foramina) permit the passage of nerve roots. Those nerves go on to form the peripheral nervous system.
Sometimes these nerve roots can be compressed by a bony growth from the vertebrae, or the slipping of one vertebra over the other narrows the opening (foramen) through which a nerve runs. When this happens, the nerve root is injured in what is called a radiculopathy. The resulting symptoms vary, depending on the location of the nerve root.
Anatomy of the Spinal Column
While there is some variation between individuals, for the most part we all have seven vertebrae in our neck, called cervical vertebrae. Below these are the thoracic vertebrae (attached to ribs), and then five lumbar vertebrae. The last lumbar vertebra is attached to the sacrum, a large bone that helps make up the pelvic circle.
The vertebrae are usually abbreviated down to a number and letter, counting from the top of the spine to the bottom. For example, C5 means the fifth cervical vertebra from the top of the spine. T8 means the 8th thoracic vertebra down from C7 (the last cervical vertebra).
In general, nerve roots are named after the bone above them. For example, the nerve root that exits between the 4th and 5th lumbar vertebrae would be the called L4. The cervical nerves are different, though: although there are only 7 cervical vertebrae, there are 8 cervical nerves, the first of which exits above the first cervical vertebra. So in the neck, the nerves are labeled after the vertebrae below them. In order to be clear, it's usually best to specify nerve roots by referencing both vertebrae, e.g. (C7-T1), but most physicians do not do this in everyday practice.
The spinal cord itself actually only descends to L1 in adults, where it ends in a structure called the conus medullaris. The nerves continue to dangle down from this point, though, floating in a sac of cerebrospinal fluid. This collection of nerves is called the cauda equina, Latin for "horse's tale," which the loose nerves somewhat resemble until they exit out the foramina between the lumbar vertebrae.
Each nerve root that exits that spinal cord carries messages from the brain to make particular muscles move, and receives messages from particular areas of skin. Because of this fact, it's possible to deduce at which level a radiculopathy is occurring, based on the symptoms experienced. Furthermore, radiculopathies are almost always painful, whereas many other nerve problems are not.
Many radiculopathies are caused by subtle shifts in the skeletal architecture of the vertebral column. The thoracic vertebrae are prevented from shifting much because they are anchored by the ribcage. For this reason, the most noticeable radiculopathies occur in the cervical and lumbar spine.
The nerves that branch off the spinal cord in the neck exit the vertebral foramina to form an intermixing pattern called the brachial plexus. From there, the nerves go on to innervate the skin and muscles of the arm. For practical purposes, the most important nerve roots in the arm are C5, C6, and C7. It's worth knowing that about 20 percent of all cervical radiculopathies involve two or more levels.
C5: The deltoid (the shoulder muscle that lifts the arm from the body) is innervated by nerves coming from C5. In addition to shoulder weakness, this radiculopathy may lead to numbness in the shoulder and upper arm.
C6: A C6 radiculopathy can lead to weakness in the biceps and wrist extensors. In addition, there may be sensory abnormalities in the index and middle fingers, as well as part of the forearm.
C7: Almost half (46 percent) of all cervical radiculopathies involve this nerve root. The main weakness is in the triceps muscle that straightens the arm. There may also be some sensory loss in part of the hand, such as the ring finger.
The nerves that exit the neural foramina in the lumbar spine go on to form the lumbar plexus, a complex anastamosis of different nerves. From there, these nerves go on to innervate the skin and muscles of the leg.
L4: The iliopsoas, which flexes the hip, may be weak, as may the quadriceps that extend the leg at the knee. The knee and part of the lower leg may also be numbed.
L5: The ability to raise the point of the foot off the floor may be diminished, and the top surface of the foot may be numb. This nerve root is involved in about 40 to 45 percent of lumbosacral radiculopathies.
S1: The ability to point the foot towards the floor (as if you were going to stand on tiptoe) is weakened, and there may be numbness of the small toe and sole of the foot. This nerve root is involved in about 45 to 50 percent of lumbosacral radiculopathies.
We've just reviewed the anatomy of the nerves that exit the spinal cord. While we've discussed some of the symptoms, we haven't even begun to explore the many different causes of neuropathy, or their treatment. While most back pain goes away on its own, if weakness is developing, its a sign that more aggressive therapy may be called for.
Alport AR, Sander HW, Clinical Approach to Peripheral Neuropathy: Anatomic Localization and Diagnostic Testing. Continuum; Volume 18, No 1, February 2012
Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002