Anatomy, Physiology, and Research

Even though the brain controls the activities of the whole body, it only extends down to the top of the neck. Below that, the spinal cord carries messages between the brain and your body. Your face has a direct connection to the brainstem, so it is independent of your spinal cord.

The spinal cord looks like a long rope about the width of your little finger. It runs from the base of your brain down to the lower part of your back, and it is fragile. Spinal cord injury [SCI] can lead to loss of movement and feeling. It can also affect how your brain controls your internal organs. When your spinal cord is injured, parts of your body below the level of the injury are affected.

The spinal cord is protected by your backbones [your spine]. The backbones are 29 small bones stacked on top of each other. These bones are called vertebrae. To allow your back to bend and to lessen jarring, each vertebra is cushioned from the next by disks. Disks are made of a spongy material and act like shock absorbers. Ligaments hold the vertebrae together and allow your neck and back to twist and bend.

[Figure 1.a.1] The Spinal Column Each vertebra has a hole in it—a hard, bony tunnel through which the spinal cord passes. This is the spinal canal. It protects the spinal cord from damage. The vertebrae and disks, held together by ligaments, are called the spinal column [SEE FIGURE 1.a.1].

Your spine has four sections. The top is the cervical section, which is your neck. The next down is the thoracic section, which extends from your lower neck to your lower ribs. The lumbar section is your lower back, and the sacral section is your tailbone [SEE FIGURE 1.a.2]. Your sacral section is really only one bone, with five nerve pairs coming out through holes in it.

The cervical section contains eight pairs of nerves and seven vertebrae. The nerves numbered C1 through C7 exit from the spine above the corresponding numbered vertebrae, and the C8 nerve pair exits between the C7 and T1 bones on each side. For the thoracic and lumbar sections, each of the numbered nerves lies below the corresponding numbered vertebra. There are 12 thoracic nerves and 5 lumbar nerves on each side.

At the lower end of your spinal cord [below the second lumbar vertebra], the nerves travel long distances before they exit the spine. This is because the spinal cord itself ends much higher, at about the level of the L1 vertebra. The lower lumbar and sacral nerves look like a horse’s tail inside the spinal column. In fact, this area is known as the cauda equina, which means “horse’s tail” in Latin.

[Figure 1.a.2] The Spinal Cord

 

How The Spinal Cord Functions

The spinal cord is the communicating link between the spinal nerves and the brain. The long nerve fibers inside the spinal cord are called the upper motor neurons [UMNs]. They run between the brain and the spinal nerves. The spinal nerves branch out from the spinal cord into the tissues of your body. Spinal nerves are called lower motor neurons [LMNs] [SEE FIGURE 1.a.3].

In movement, the brain sends messages through the spinal cord [UMNs] to the spinal nerves [LMNs]. The LMNs then carry these messages to the muscles to coordinate movements, such as walking. In this way, the brain controls movement.

[Figure 1.a.3] Spinal Nerves


In sensation, nerves in your body collect information and send it up the spinal cord to the rain. This allows you to be aware of feelings, such as heat, cold, touch, or pain.

You may wonder how the spinal cord keeps these messages from getting confused, with all the running back and forth between brain and body. The motor nerves and the sensory nerves carry messages in different nerve fibers. Within the spinal cord, the nerve fibers are combined into groups called spinal tracts. Each tract carries messages one way, either up for sensation or down for voluntary movement. They are similar to  the lanes on a freeway [SEE FIGURE 1.a.4].

What Is a Spinal Nerve and What Does It Do?

Each spinal nerve has two main parts. One part carries information related to movement from the spinal cord to the muscles. It is called a motor nerve. Each motor nerve connects to a specific muscle. Each level of the spinal cord causes movement in a certain group of muscles.

[Figure 1.a.4] Spinal Tracts for Nerves

The other part of the spinal nerve carries messages of feeling, such as heat and cold, from the body to the spinal cord. It is called a sensory nerve. Different types of  sensation or feeling are carried up the spinal cord to the brain. These include pain, touch, heat, cold, vibration, pressure, and knowing where a body part is located in space without looking at it.

Each sensory nerve collects information about feelings from a given body part or area of skin. Each skin area is called a dermatome and matches a specific spinal cord level [SEE FIGURE 1.a.5]. Try to identify areas where you have normal sensation and where you do not.

Spinal Cord Injury

Many types of injuries and diseases can cause spinal cord injury or dysfunction. If the space for the spinal cord [spinal canal] becomes narrowed, the spinal cord can become injured. This can happen when bones in your back or neck are broken, or when ligaments are torn and the vertebrae move in different directions.

Gunshot wounds, stab wounds, or fragments from explosions can directly damage the cord without much breaking of the bones. Infections and tumors near the spine can compress the spinal cord. Sometimes, arthritis can affect the bones and slowly compress the cord. Finally, the blood supply to the spinal cord can be blocked, causing
part of the spinal cord to die. [This is similar to how a stroke affects the brain.]

Damage to your spinal cord can cause changes in your movement, feeling, bladder control, or other bodily functions. The changes depend on where and how badly your spinal cord was injured. The main problem is that the connection between your brain and your body below the injury is impaired or broken.

A numbering system is used to name levels of injury. It’s the same as the system used to name bone and nerve levels in your back. A spinal cord injury is named for the lowest level of the spinal cord that still functions the way it did before the injury. It is important to your rehabilitation that you know your level of injury and how it affects your body. The level of spinal cord injury is not always the same level as where the spine was injured. When the spinal cord injury is at a cervical level, it is called “tetraplegia” or “quadriplegia.” When it is at a lower level [thoracic, lumbar, or sacral], it is called “paraplegia.” Most of the nerve supply to the arm and hand comes from cervical nerve roots. This means that people with tetraplegia have some numbness or weakness in their arms or hands. Paraplegia does not affect the arms or hands.

Complete and Incomplete SCI

If there is no voluntary movement [spasms don’t count—they are involuntary] and no feeling below your spinal cord injury level, you have a complete injury. If you have some feeling or voluntary movement below your injury, you have an incomplete injury. This happens when there is only partial damage to your spinal cord; that is, some nerve fibers are still working across your spinal cord injury site.

UMN and LMN Injuries

Earlier in this section, we discussed the difference between upper motor neurons [UMNs] and lower motor neurons [LMNs]. A complete injury cuts or affects all the UMNs running down the spinal cord. This disrupts the connection between the brain and the parts of the body below the injury. However, the LMNs below your spinal cord injury
are not damaged. Because LMNs carry reflex actions, the reflexes below the level of injury are still working. This is a UMN injury [SEE FIGURE 1.a.6].

So the LMNs are still carrying out reflex actions below the level of injury, but this may cause a problem. In reflexes, the brain normally controls how much your nerves react. In a UMN injury, messages from the brain can’t get past the point of injury, so the LMNs act by themselves, which may cause reflexes without limit. One example is spasticity, which is uncontrolled movement of your arms or legs.

LMN injuries are a different story. This kind of injury is usually at the lower tip of the spinal cord [the cauda equina]. The cauda equina is made up entirely of LMNs, so damage to it impairs reflex actions, although other UMNs and LMNs above the injury are still working [SEE FIGURE 1.a.7]. Spasticity does not occur with LMN injuries because the muscles no longer have any nerve contact to stimulate them.

Stated simply, a UMN injury is one in which the UMN pathway is broken, the LMNs below the injury are intact, and spasticity usually occurs.

[Figure 1.a.6] Upper Motor Neuron Injury


[Figure 1.a.7] Lower Motor Neuron Injury



An LMN injury, usually at the cauda equina, blocks nerve activity in muscles controlled below the injury, and no spasticity develops. It is important to know which kind of injury you have because that will determine how it is managed.

Recovery

Immediately after a spinal cord injury, the spinal cord stops doing its job for a period of time—this is called “spinal shock.” All the reflexes below the level of injury are absent during this period, which may last for a few days to several weeks. The return of reflexes below the level of injury marks the end of spinal shock. Often with an LMN injury, reflexes never return.

If you have an incomplete injury, some feelings and movement may come back. Will this
happen to you? No one can say. If you do regain some feeling and movement, it will likely start happening in the first few weeks after your injury.

Rehabilitation begins immediately. You will learn strengthening exercises, new ways to use the muscles that are still moving, and how to use special equipment to achieve more function. If you recover more feeling or movement, your rehabilitation team will develop new goals with you.

Regeneration Research

The dream of spinal cord clinicians and researchers is to regenerate the injured spinal
cord. Much progress has been made in the past 25 years, and some partial regeneration is possible in animals with spinal cord injury. Spinal cord nerve fibers can regrow if they have the proper stimuli to make the nerves grow and to guide where they go. But these treatments are not yet available for humans. Nerve cells would have to regrow for
much longer distances in humans than they do in rats. Even after axons regrow in animals, they have difficulty making new connections. And some research animals that receive experimental regeneration treatments develop severe nerve pain [neuropathic pain].

Some physicians outside the United States offer experimental treatments to people with SCI. Many of these involve cells and treatments that seem to help small animals recover after SCI. One treatment involves injecting olfactory ensheathing glia [cells that help nerve cells in the nose] or stem cells into the spinal cord, in a surgical procedure that costs tens of thousands of dollars. There is little or no scientific support for using these
treatments in humans. Some complications, like spinal infections, happen frequently after some of these surgeries, and some surgeons do not explain these complications in advance to patients. You should consider waiting until the treatments have a better chance of helping you and less of a chance of causing harm. What are the chances you will lose strength and sensation or develop severe pain? Also, if you receive one of these treatments now, you may not be eligible to have other treatments in the future. When you hear of a new treatment, gather as much information as you can and then discuss it with an SCI physician or SCI researcher to determine whether it is something
you should consider. [See the ICORD resource belowfor a list of factors to consider before participating in an experimental treatment or a clinical trial.]

Resources

The International Collaboration on Repair Discoveries [ICORD] has a free booklet titled Experimental Treatments for Spinal Cord Injuries. It contains a summary of experimental approaches to treat SCI, as well as information that you should know if you are thinking about participating in a clinical trial [www.icord.org/iccp.html].

Leading research journals require scientists to register their clinical trials before they begin. ClinicalTrials.gov provides regularly updated information on federally and privately supported clinical research using human volunteers. The website gives you information on a trial’s purpose, who may participate, locations, and phone numbers for more details. Enter the search term “spinal cord injury” to find studies of spinal cord regeneration, spasticity, pain, and other conditions. www.clinicaltrials.gov