When actor Christopher Reeve was thrown from his horse 10 days ago, he maimed one of the best-protected parts of the body -- the spinal cord -- at its most vulnerable point. Reeve hit the ground head-first, crushing bones in his neck.

"The neck is an incredibly sophisticated piece of handiwork that allows the head to turn 90 degrees sideways and up and down," said Fraser C. Henderson, a neurosurgeon at Georgetown University Medical Center. But that remarkable mobility comes at a price, he said, for the neck is "the weak point of the spine."

Reeve's injury is much worse than a simple fracture of two vertebrae, because it apparently also stretched or sheared the upper spinal cord housed within them, interrupting vital nerve messages between the brain and most of the rest of the body. That left Reeve paralyzed from the neck down and unable to breathe on his own.

The spinal cord, a glistening rope-like bundle of nerves, connects the brain to the rest of the nervous system. It is protected by the bony spinal column, with its 24 vertebrae and other bones stacked in a gentle S-curve between the skull and the tailbone. The delicate fibrous cord is bathed in cerebrospinal fluid and held in place by ligaments and nerve roots inside the spinal column.

The cord itself is elastic -- stretching with every turn of the head and bend of the waist. But it is also exquisitely sensitive to direct pressure.

What's more, the same tight-fitting bony column that guards the spinal cord from the wear and tear of normal life can actually pose a danger if the cord undergoes a traumatic injury and begins to swell. With little or no room to expand, the swelling cord cuts off its blood supply and further damages its fragile tissues.

"Inflammation is a normal response to injury," said John K. Starr, an orthopedic surgeon at George Washington University Medical Center. "But in spinal cord injuries, inflammation becomes your own enemy."

Few spinal injuries sever the cord completely. But fatal or paralyzing nerve loss can result even when the intact cord is bruised, twisted, pinched or stretched -- especially in the upper neck.

"You can have injuries all up and down the spinal cord," said Gary W. Goldstein, a neurologist and president of the Kennedy Krieger Institute in Baltimore, which specializes in study and treatment of nervous system disorders. "But the neck is particularly vulnerable."

There are seven cervical (neck) vertebrae, labeled C1 through C7, each guarding a different set of nerve roots. The higher up a neck injury is, the more life-threatening it is because it may cut off the brain's communication with all nerves below it. Reeve fractured the top two cervical vertebrae -- known as C1 and C2. The nerves that control breathing are below that, mainly in C3 and C4. All the nerves that control movement of the torso, arms and legs are lower.

"There's no more devastating injury to a young person and a family," said Henderson, who specializes in surgery of the brain stem and cervical spine at Georgetown. "From every point of view, it's probably the worst kind of injury because it's so costly in emotion and medical resources."

Some 7,800 severe spinal cord injuries nationwide are reported each year, but experts say that figure understates the actual number. Thousands of others, they say, are not reported because the person dies instantaneously or before reaching a hospital.

The shattering impact of such injuries is compounded by the fact that the victims tend to be young -- about half are under 26. And, in virtually every case, the injury comes without warning. While chronic illnesses such as multiple sclerosis can damage the spinal cord, the vast majority of traumatic cord injuries result from vehicle crashes, falls, gunshots, stabbings or diving into shallow water. Intoxication with drugs or alcohol is often a factor, experts say.

In Reeve's case, the injury came in a freakish accident during an equestrian competition in Culpeper, Va. An accomplished and safety-conscious horseman who was wearing a helmet at the time, Reeve was thrown into the air when his horse stopped abruptly in front of a jump.

Medical experts said Reeve likely would have died if an emergency medical team had not been standing by at Culpeper's Commonwealth Park equestrian center. The team found him unconscious and not breathing and revived him with mouth-to-mouth resuscitation in the ambulance on the way to the hospital, where he was put on an artificial respirator.

While Reeve's own physicians have been guarded in their public statements, saying it is still too early to predict how much bodily control he will lose or recover, other doctors say the outlook for patients with damage that high in the spinal cord is grim.

"This is a horrible injury," said GW's Starr. "Statistically, and individually, his prognosis is dismal." Reducing Damage

Until the past few years, there was no immediate treatment for spinal injuries beyond the basics of trauma care: immobilizing the neck to prevent further injury, restoring pulse and breathing, and replenishing fluids. But several studies have suggested that patients with spinal cord injuries do better if they are promptly given an anti-inflammatory steroid called methylprednisolone.

To be effective, the drug must be given within eight hours of the injury and in massive doses -- about 1,000 times the normal amount of steroids the body uses in a day. Methylprednisolone is thought to reduce nerve damage by slowing inflammation and other harmful biochemical reactions that are part of what doctors call the secondary trauma of a spinal cord injury -- beyond the initial blow.

Surgery cannot repair a severed nerve or heavily damaged spinal cord and restore, for example, the ability to breathe. But surgery is used to remove fragments of bone pressing on the spinal cord, clear a blood clot or realign a fracture. In addition, it may be necessary to stabilize the head and spine of a patient with a broken neck. Some patients also undergo surgery later in their rehabilitation to relieve pain caused by abnormal movement or friction of damaged tissues in the spinal column.

Doctors at University of Virginia Medical Center in Charlottesville operated on Reeve yesterday in an effort to stabilize his broken neck, fusing it with bone grafts from his pelvis and metal screws so that he might be able to sit up and not be confined to lying motionless in bed.

Pneumonia is another threat. Any patient who must be on a respirator indefinitely is likely to develop pneumonia or other lung infections. Lung and respiratory problems are the major cause of death in patients with spinal cord injuries, Henderson said.

For patients who become quadriplegic, paralyzed from the neck down, recent technological advances offer hope of recovering a measure of mobility and control. Newfangled wheelchairs can be operated with a joystick attached to the chin or with "sip-and-puff" control like a drinking straw. Similar controls enable quadriplegic patients to use home computers. An implantable electric "pacer," similar to a heart pacemaker, is used by some patients who cannot breathe on their own; it helps trigger the diaphragm, the "breathing muscle," so that they can wean themselves off a respirator.

The lower the injury in the spinal column, the better chance the person has of regaining some degree of mobility and independence. People with quadriplegia due to an injury to the lower neck -- for example, at the sixth cervical vertebra -- may recover well enough to feed themselves, walk with difficulty in leg braces and drive a vehicle. Some quadriplegic patients can have children.

Yet recovery from spinal cord trauma is a relative term. If the uppermost part of the cord is damaged but the brain stem is intact, as in Reeve's case, the patient may remain conscious while paralyzed and permanently dependent on a respirator. Those cases may be the most devastating to patient and family, Starr said. "Not only are you fully damaged but you remain aware of it, an otherwise normal person trapped inside a useless body."

That's why the tiniest sign of sensation or physical control below the neck -- a wiggle in the toe or a felt pinprick in the skin -- offers hope in a quadriplegic patient.

Reeve's doctor said last Friday that the patient had at least some feeling in the chest, indicating that the spinal cord had not been completely severed.

"Any flicker of function corresponds to a flicker of hope," Starr said. Trying to Make Nerves Grow

The ultimate treatment of a spinal cord injury would be to find a way to make nerves heal like a bone fracture. But nerve fibers are so fragile that they either don't grow back or do so in stunted fashion "in a way that remains a mystery to us," Starr said.

Nerve endings do begin to regenerate after an injury, Goldstein said. "The problem is, they can't find their connections."

Scientists have high hopes they will someday learn how to repair damaged spinal nerves. Experiments in humans are probably years away, but preliminary studies with spine-injured animals have shown that nerve cells can be coaxed to grow across small gaps and reconnect with adjoining nerves.

In one study reported last year, newborn rats overcame hind-leg paralysis after sections of their spinal cords were removed and replaced with transplants from fetal animals. In another, severed spinal nerves in rats regrew after being treated with growth-stimulating biochemicals.

Scientists also recently learned that the spinal cord contains compounds that naturally suppress nerve growth -- perhaps as a safety feature to prevent nerve cell cancer -- and spinal nerve regeneration can be enhanced by giving not only nerve growth factors but also drugs that block this growth-suppressing chemical.

Still, researchers noted that the most promising instances of spinal nerve regeneration have been limited to tests in very young animals, whose capacity to grow new nerves is far greater than that of adults. It could take years, they said, to learn how to spur useful nerve regrowth in adult animals, and even longer before the same might be done in humans.

For patients who survive a spinal cord injury, the initial hospitalization is "a relative blink of an eye," Starr said. "The most significant impact of these injuries unfolds over years and even decades. The loss in both dollars and personal terms is staggering."

Even for the lucky ones, rehabilitation is a long-haul process, with progress measured in tiny symbolic steps.

"I tell people to remember that no matter which body parts may not move, it's really still the same person," Starr said. "They're going to need a lot of help, and most of it's going to have to come from the families."

Still, about half of the patients who were employed at the time of their spinal cord injury eventually return to work. Life expectancy for people with cord injuries, while lower than for the rest of the population, is increasing.

The roster of high-profile people living with severe spinal cord injuries is extensive. It includes soul singer Teddy Pendergrass, quadriplegic since a 1984 car accident; jockey and Triple Crown winner Ron Turcotte, who became paraplegic in a fall at Belmont Park in 1978; and former star football players Dennis Byrd of the New York Jets, Mike Utley of the Detroit Lions and Darryl Stingley of the New England Patriots. Former American League baseball umpire Steve Palermo learned to walk again with ankle braces and canes after getting shot in the lower spine in 1991. Pulitzer Prize-winning syndicated columnist Charles Krauthammer has been paralyzed from the waist down since he broke his neck at age 22 in a diving accident.

At the same time, doctors who specialize in spinal cord injuries say the lack of corrective treatments underscores the importance of preventing such trauma in the first place.

"The key is still prevention," said Peter J. Verdin Jr., an orthopedic surgeon in private practice in Alexandria. "As much as we can do, there's nothing to replace what you lose in a spinal cord injury." Staff writer Rick Weiss contributed to this report. CAPTION: Improvements in technology and drugs allow some patients to live for decades who in years past would quickly have died from the complications of their paralysis. CAPTION: SPINAL CORD INJURIES The effect of spinal cord injuries depends on where the damage occurs and how traumatic it is. For most patients, the closer the injury is to the head, the more severe its effects. That is because an injury to one area of the spinal cord will hinder the activity of all the nerves below it on the cord. The spinal nerves are classified by their location: the cervical or neck area, the thoracic or chest area, the lumbar or lower back area and the sacral or pelvic area.

INCIDENCE

* About 7,800 severe spinal cord injuries are reported each year in the United States, not counting nearly 5,000 cases where people die before reaching the hospital. WHO IS AFFECTED

* Males (82 percent), females (18 percent). Most are between 16 and 30 years old.

LEADING CAUSES 44% Motor vehicle accidents. 24% Acts of violence. 22% Falls. 8% Sports, including diving into shallow water.

* Among victims over age 45, falls are the leading cause. TYPES OF NEUROLOGICAL DAMAGE 18% Total paralysis from the neck down

(quadriplegia). 31% Partial paralysis from the neck down. 28% Total paralysis from the waist down

(paraplegia). 23% Partial paralysis from the waist down. AVERAGE HOSPITAL STAY 79 days overall. 95 days for quadriplegics. 67 days for paraplegics.

OUTLOOK

* About 95 percent of patients are able to return to their own homes or another residence in the community.

* About half eventually return to work.

* Of those who survive the first 24 hours, 85 percent are still alive 10 years later.

* The most common cause of death in a person with a serious spinal cord injury is respiratory failure. Common complications include urinary infections, spasticity, chills and fever, bedsores, muscle or joint pain and pneumonia.

* The spinal cord is a ropelike bundle of nerves that connects the brain to the rest of the nervous system. It is protected by the spine, a column of bones and cartilage. The nerves branch out from the spinal cord through openings in the bony vertebrae and control muscles throughout the body.

CERVICAL NERVES

Control the face, neck, arms and the diaphragm, the muscle necessary for breathing. For example, an injury among the top cervical nerves can leave a patient with paralysis from the neck down. But damage to the fifth cervical nerve allows the patient to use the upper arms but not the hands. These patients can feed themselves with the aid of special equipment.

THORACIC NERVES Control the chest and abdomen area. An injury to one of the higher thoracic nerves can cause paralysis from the chest down. Damage to a lower nerve can result in paralysis from the waist down. For example, an injury to the 12th thoracic nerve leaves a patient with complete trunk control and good abdominal muscles but with paralysis of the legs. Walking may be possible, although difficult, with long leg braces.

LUMBAR NERVES Control the legs and bladder and bowel functions. For example, damage to the fourth lumbar nerve cuts movement in the lower leg. The patient can extend his knees and raise his feet. Walking is possible with short braces.

SOURCE: NATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKE