The Central Nervous System

Because of the significance of the central nervous system, it's important to have a thorough understanding of its role in multiple sclerosis (MS). And since almost every chapter addresses topics that are the result of damage to the central nervous system, knowledge of it can be a powerful tool in managing your MS more effectively.

The human body is made up of not one, but three nervous systems. Although, by category, these three systems are separate, actually they are interdependent and interrelated. In order for the body to function properly, it takes all three nervous systems functioning properly, and in harmony.

As you read this, millions of bits of information from sight and sound enter your nervous system and must be analyzed to determine an appropriate response. Almost all of it is discarded as "unimportant" at the moment. While all of this is going on, the other senses are active too, bombarding the nervous system and demanding a response. At the same time, the body's organs and functions must be constantly monitored and directed. This is all accomplished by the teamwork of the body's three nervous systems. Following is a description of each.

• The Central Nervous System (CNS) is composed of the brain and spinal cord. Impulses originating in the brain are sent to various parts of the body via the spinal cord. The brain is protected and encased by the skull, while the spinal cord is protected and encased by the cervical, thoracic and lumbar vertebrae. The spinal cord is a direct continuation of the brain stem that begins at the upper border of the vertebra and ends at the lower end of the first vertebra.
• The Peripheral Nervous System (PNS) is basically an extension of the central nervous system. The Peripheral Nervous System connects the central nervous system with all the tissues of the body. Thirty one pairs of spinal nerves exit the spine and form a very complex network of nerves reaching out to every part of the body. Messages or signals are relayed from the tissues of the body back to the brain and vice versa. A healthy nervous system allows the unobstructed transmission of these nerve signals as they travel back and forth from the brain to the tissues of the body
• The Autonomic Nervous System (ANS), also known as the "involuntary" nervous system controls activities of the body unconsciously. The Autonomic Nervous System includes all the nerve cells, or neurons, located outside the spinal cord and brain stem. The ANS itself is divided into two separate entities: the sympathetic and the parasympathetic. The sympathetic division sends impulses that speed up or enhance (running for instance) whereas the parasympathetic slows down (digestion for instance). These two systems working together regulate the majority of the body's involuntary functions. Examples of involuntary control are heart rate, respiration, blood circulation and digestion.

Imagine you're sitting at home watching The History Channel. Great program. Clear reception. Suddenly, there's static. Then you start seeing fragments of the Cartoon Channel mixed in with MTV. Then, more static.

The next morning while going through the newspaper, you read about some construction crew digging a trench and accidentally hitting the cable. The accompanying photos show that the insulation around the cable has been scraped off, which resulted in a short-circuit. Similarly, with MS, demyelination is the "scraped" part of the "cable." Myelin is the protective insulation or coating that allows nerve impulses to travel properly throughout the nervous system. Depending on where the plaques of demyelination are located, several systems in the body may be affected. Among some of the more common:

• Vision
• Coordination
• Strength
• Speech
• Sensation
• Bladder control
• Sexual function

Although other diseases can cause demyelination in places other than the brain, spinal cord and optic nerves, this is not the case with MS. When it comes to demyelination within the central nervous system, multiple sclerosis is the likely culprit.

In a healthy body, the immune system will recognize an "invader" and send the appropriate response team to attack it. Unfortunately, for reasons not fully understood, in persons with MS, the immune system appears to seemyelin as an enemy, and attacks it as such, in a process called an autoimmune response.

Concerning multiple sclerosis, any damage to the myelin sheath results in some disruption of the impulse or message that is being transmitted, much as would occur in our cable/telephone analogy. With MS, overactive, misguided cells of the immune system enter the CNS, causing inflammation in the brain and spinal cord. This inflammation causes damage to the myelin. Wherever myelin is destroyed, a plaque, or lesion forms, with a gradual buildup of hardened scar tissue (sclerosis) at the site. These plaques occur in different locations throughout the central nervous system, leading to the name multiple sclerosis. While many of these scars, or plaques, may not affect certain systems, others can cause an interruption of the signals the CNS is trying to send.

We can use the analogy of a telephone system to demonstrate how nerve impulses travel from one nerve cell to the next in a healthy nervous system. The "call" is made from the nerve cell body, and the phone itself is the nucleus of the nerve cell. The signal then travels through the "cable," or axon.

As these "calls" are made, they travel from the axon of one nerve cell (the pre-synaptic neuron) through the synaptic cleft (where neurotransmitters are located) and on to the receptors of the dendrites of the next nerve cell (post-synaptic neuron).

The axon doesn't actually touch the dendrite (receptor) of the next neuron. Neurotransmitters make the connection from the pre-synaptic neuron (a microscopic distance) across the synaptic cleft to connect with the post-synaptic neuron. Staying with the telephone/cable analogy, the minute space of the synaptic cleft can be seen as the "relay station," and the neurotransmitters are the "operators." The message that is sent to the next neuron is stimulated by a nervous impulse, which produces yet another one. The condition of the membrane determines whether the impulse is sent on or stopped.

There are several types of neurotransmitters, and each one is unique. Some primary functions include:

• Memory
• Muscle function
• Pain reduction (and transmission)
• Sleep (facilitation and inhibition)
• Excitement and depression

Just as there are different types of neurotransmitters, there are different types of neurons. As their names indicate, each one focuses on a specific "call." They are:

• Motor neurons
• Sensory neurons
• Interneurons

The motor neuron's job is to send messages to muscles and glands. You'll realize this after a long work out. Think sweat!

Sensory neurons send messages from the eyes, nose, and ears to the central nervous system.
These neurons are responsible for warning you ahead of time of imminent danger. For instance, when you're about to step on broken glass, sensory neurons cause you to stop in your tracks. These neurons cause a reaction without you having to think about it ahead of time.

Interneurons send messages, or connect sensory neurons with motor neurons. Interneurons are responsible for the reflex factor.

The power source of the central nervous system is the brain and spinal cord. The commands for our reactions and bodily functions begin here. Three primary components:

• Cerebrum
• Cerebellum
• Brain stem

The cerebrum, the largest part of the brain controls many things, including the thought process, voluntary movement, and the interpretation of the senses, such as sight, smell and touch. It also is responsible for the type of personality you have.

The cerebellum, the second largest part of the brain is responsible for muscle coordination, posture and the sense of balance.

The brain stem, as small as it is, has the huge responsibility of controlling heartbeat, blood pressure and respiration.

As we've learned by now, each system of the body has its own unique responsibility. But it has to work in unison with other systems to function properly. In this case, it's the teamwork of the brain and spinal cord. Many people think of the spinal cord as just a series of vertebrae, and an occasional source of pain. However, it is the primary source of communication within the central nervous system. It transmits motor and sensory impulses from the body to the brain as well as from the brain to the body.

The spinal cord runs through the vertebral canal, which protects it from injury. And, like the brain, it has both gray matter and white matter. In multiple sclerosis, the spinal cord, along with the brain are where lesions or plaques are most often located, preventing proper communication with the rest of the body via spinal nerves, or nerve roots.

The spinal cord is responsible for most of the reflex reactions. Spinal reflexes respond more quickly since they don't have to send messages to the brain and wait for a response. A good example is the withdrawal reflex. When danger is sensed, this reflex kicks in without you even thinking about it, and basically saves you from hurting yourself. This is called an involuntary motor response.

The opposite of this is something called voluntary muscle movement. This conscious reaction is the most obvious of the nervous system. Voluntary muscle movement begins as a nervous impulse travels from the brain to the spinal cord, then to a spinal nerve, and finally to a peripheral nerve. When the nerve is stimulated, it releases a chemical that stimulates the muscle, causing it to contract.

The nervous system is very complex. A single abnormality can have a major impact on everything else. It's easy to see how scars on nerve cells can make things crazy.

Research continues to find out why, in multiple sclerosis, the body attacks the myelin surrounding nerve cells. Until that question is answered, you, your doctor (deciding on the therapy that is best for you) and your knowledge are the best weapons in your fight against MS.