A pain in the back need not be a pain in the neck.

The most common complaint that doctors hear is that of back pain. Sometimes the patient knows what precipitated it--lifting a heavy object, for example--but most of the time, it just "came on." Depending upon its cause, the pain may be localized to a small area, or it may radiate more distantly.

One of the most common varieties is sciatica--named after the sciatic nerve. Technically, the sciatic nerve is a bundle of nerves, enclosed in a single sheath, that comes off the spine in the lower back. Passing through the buttocks down the thigh to the knee, it divides into two branches, one on the back of the calf and the other on the lateral (outside) part of the lower leg, and from there into the foot.

Pressure on, or other irritation of, the sciatic nerve, usually at or near its point of origin from the spinal column, may cause pain in the center of the back, in the buttocks, or all the way down to the foot. Other characteristic symptoms include numbness, tingling, and/or weakness along its path.

irritation of the sciatic nerve may be due to pressure from a so-called "slipped disk"--more correctly, a herniated intervertebral disk. The disk is made of a semisoft, fibrous material that provides padding between the vertebrae along the spinal column.

A capsule that holds the disk in place may bulge and press 'upon an adjacent nerve. In the lower back, the sciatic nerve is the most vulnerable.

The sciatic nerve also may be irritated by pressure from one of the muscles comprising the muscle group in the buttocks that controls motion of the thigh. This piriformis muscle, which rifts the leg sideways, may become inflamed because of overexertion or injury and press upon the sciatic nerve. Spasm in any of the muscles of the buttocks, whether from injury, a fall, or whatever, also may irritate it.

The precise cause of sciatic nerve pain may be difficult to establish, but simple treatment often brings relief, whatever its cause. Such ordinary medication as aspirin or ibuprofen may do the trick, with or without a day or two of bed rest. A heating pad also may help, as well as exercise to strengthen the muscles of the abdomen and back, which keep the bones of the spine and hip in proper balance.

An attack of sciatica or other acute back pain may come on suddenly, from the most innocuous of movements--slight bending or just sitting in one position for an extended period. The slightest movement may then cause knife-like pain, virtually immobilizing the patient.

If this happens, lying flat on one's back---whether on the floor, a couch, or in bed--usually will bring some relief. Taking the strongest painkiller at one's disposal may produce a quick and complete cure.

What seems to happen is that the motion that initiated the pain causes the adjoining muscles to go into instant spasm to prevent further motion. However, this spasm causes further irritation of the nerve, which sets up an instantaneous cycle of nerve pain followed by muscle spasm and more nerve pain. The medication relieves the nerve pain, allowing the muscle to relax, thus breaking the cycle--often with complete relief.

Medical attention is recommended for pain that lasts more than a couple of days. Doctors often prescribe muscle relaxants to break the spasm cycle. Rarely, and only as a last resort, does chronic back pain require surgery. If recommended, a second or even third opinion is strongly advised.