Chapter 1
Spinal Cord Anatomy & Physiology

Even though the brain controls the majority of the activities of your body, it only extends down as far as the top of your neck. Beyond that, the spinal cord takes over and acts like telephone wires for messages coming and going between the brain and all the other parts of 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-like cord 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 fairly fragile. Damage to your spinal cord can affect your ability to move or feel. It can also affect the workings of some internal organs. If you are injured at a given level of your spinal cord, parts of your body will be affected at and below that level.

To avoid damage, the spinal cord is protected by bone--specifically, by your back bone or spine. The back bones are 29 small bones stacked one on top of the other. These bones are called vertebrae (VERT-i-bray). Because of all the jarring and bending your back must do, each vertebra (VERT-i-brah) is cushioned from the next by disks. Disks are made of spongy material that act like shock absorbers. Ligaments hold the vertebrae together and allow your neck and back to twist and bend.

Each vertebra has a hole in it, so when vertebrae are stacked together, they provide a hard, bony tunnel through which the spinal cord passes. This is called the spinal column. In this way, the spinal cord is protected from damage. (See figure 1.1.)

There are four sections of your spine. The top is the cervical (SURR-vick-ull) section, which makes up your neck. The next down is the thoracic (thor-ASS-ick) section, which runs to your waist level. The lumbar (LUMM-bar) level is next and is your lower back. And last is the sacral (SAY-crull) part, which is your tailbone. (See figure 1.2.)

FIGURE 1.1. The Spinal Column

There are eight pairs of nerves and seven vertebrae in the cervical section of your spine. In this case, the nerves numbered C1 through C7 are above the corresponding numbered vertebrae. C8 then slips through between the C7 and T1 bones. For the thoracic and lumbar sections, each of the numbered nerves lies below the corresponding numbered vertebra. There are 12 thoracic vertebrae and 5 lumbar vertebrae.

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 spiral cord itself ends much higher than where your tailbone marks the lower end of your spine.

This makes the lower lumbar and sacral nerves look like a horse’s tail inside the spiral column. It is known as the cauda equina (CODD-ah eh-QUINE-ah), which means "horse’s tail" in Latin.

Your sacral section is really only one piece of bone with five nerve pairs coming out through holes in it.

FIGURE 1.2. The Spinal Cord

WHAT THE SPINAL CORD DOES

The spinal cord is the communicating link between the spinal nerves and the brain. The nerves that lie only within the spinal cord itself are called upper motor neurons (UMNs). These run only between the brain and the spinal nerves. The spinal nerves branch out from the spiral cord into the tissues of your body. Spinal nerves are also called lower motor neurons (LMNs). (See figure 1.3.)

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 complicated movements such as walking. In this way, the brain can influence movement.

In sensation, information is collected by nerves in your body and sent up the spinal cord to the brain. This allows conscious awareness of feelings such as heat or cold.

FIGURE 1.3. Spinal Nerves

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 cord itself, the nerve fibers are combined into 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 1.4.)

FIGURE 1.4. Spinal Tracts for Nerves

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 the motor portion of the nerve. Each motor portion of a spinal nerve connects to a specific muscle group. Each level of the spinal cord causes movement in a corresponding group of muscles.

The other part of the spinal nerve carries messages of feelings, such as heat and cold, from the body to the spinal cord. It is called the sensory portion of the 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 portion of the spinal nerve collects information about feelings from a given area of skin. Each area is called a dermatome (DER-muh-tome) and matches a specific spinal cord level. (See figure 1.5.)

FIGURE 1.5. Map of Dermatomes

You might want to make your own map of
dermatome sensation using the blank map
(figure 1.6). Color in the sections where
you have feeling. See if the map can tell
you your level of injury.

FIGURE 1.6.
Your Own Dermatome Map

SPINAL CORD INJURY (SCI)

Excessive movement of vertebrae often causes a spinal cord injury. When bones in your back and neck are broken or when ligaments are torn, the spinal cord can get squeezed between two vertebrae. Sometimes stab wounds or gunshot wounds can damage the cord without breaking bones.

Damage to your spinal cord can cause changes in your movement, feeling, bladder control, or other bodily functions. How many changes there are depends on where your spinal cord was injured and how severely the spinal cord was injured. The main problem is that the connection between your brain and the parts of your body below the injury is impaired.

A numbering system is used to name levels of injury. It is 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 your injury. It is important to your rehabilitation that you know your level of injury and how it affects your body.

Complete and Incomplete SCI

When there is no voluntary movement (spasms don’t count--they are involuntary) or feeling below your spinal cord injury level, you have a complete injury.

If you do 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 preserved across your spinal cord injury site.

Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) Injuries

Earlier in this chapter, we discussed the difference between upper motor neurons (UMNs) and lower motor neurons (LMNs). This section will tell you why it is important that you know this.

FIGURE 1.7. Upper
Motor Neuron Injury

Most spinal injuries damage both UMNs and LMNs. A complete injury cuts or squeezes all the UMNs running down the spinal cord. This disrupts the connection between the brain and the parts of the body below the injury. LMNs below your spinal cord injury are not damaged. Because LMNs carry reflex actions, the reflexes below the level of injury are still in working order. This is a UMN injury. (See figure 1.7.)
The reflex action that the LMNs carry out below the level of injury may still work, but there is one problem. In reflexes, the brain keeps control on how much your nerves react. In a UMN injury, control by the brain no longer exists because messages from the brain can’t get through the point of injury. The LMNs act by themselves, causing reflexes without limit. One example is spasticity (spa-STI-si-ti). Spasticity is the uncontrolled movement of your arms of legs. See more about spasticity in the chapter on Nerves, Muscles & Bones.

FIGURE 1.8. Lower
Motor Neuron Injury

LMN injuries are a different story. This kind of injury is found for the most part at the lower tip of the spinal cord, or the cauda equina. Damage to the cauda equina impairs reflex actions. This is because the cauda equina is made up entirely of LMNs. Other UMNs and LMNs above the injury are still in good shape. (See figure 1.8.)

Spasticity is not found in LMN injuries as it is in UMN injuries, because muscles governed by these LMNs tend to shrink or atrophy (AT-row-fee). This is because these muscles no longer have any nerve contact to stimulate them.

Stated simply, a UMN injury is one where the UMN pathway is broken and the LMNs below the injury are intact and spasticity is noted. An LMN injury, usually at the cauda equina, abolishes nerve contact with muscles controlled below the injury and no spasticity develops. It is important for you to know which type of injury you have, because how your spinal cord injury is managed will differ depending on that fact.

RECOVERY

Immediately after a spinal cord injury, the spinal cord stops doing its job for a period of time called "spinal shock". All the reflexes below the level of injury are absent during this period of several weeks or months. The return of reflexes below the level of injury marks the end of spinal shock. At this time, your doctor can determine if you have a complete or an incomplete injury.

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 in the first few weeks after your injury.

Rehabilitation begins immediately. You will be instructed in strengthening exercises, new styles of movement, and the use of special equipment to work with what you have. If you do get additional recovery of feeling or movement, your rehabilitation team will develop new goals with you.

SCI Self-Care Guide Main Page
Chapter 1     SCI Anatomy & Physiology
Chapter 2     Skin Care
Chapter 3     Circulatory System
Chapter 4     Respiratory Care
Chapter 5     Range of Motion
Chapter 6     Bladder Management
Chapter 7     Bowel Management
Chapter 8     Nutrition
Chapter 9     Medications
Chapter 10   Nerves, Muscles, and Bones
Chapter 11   Autonomic Dysreflexia
Chapter 12   Pressure Sores
Chapter 13   Psychosocial Adjustment
Chapter 14   Sexual Health & Rehabilitation Chapter 15   Community Resources
Chapter 16   Vocational Rehabilitation
Chapter 17   Recreation
Chapter 18   Driver’s Training
Chapter 19   Attendant Management
Chapter 20   Home Modifications
Chapter 21   Approaching Discharge
Chapter 22   Pain after Spinal Cord Injury
Chapter 23   Substance Abuse and SCI
Chapter 24   Exercise
Chapter 25   Alternative Medicine
Chapter 26   Equipment
Chapter 27   Staying Healthy
Glossary

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