| Last Modified: November 1, 2001
The Anatomy of the Central Nervous System
The central nervous system (CNS) is made up by the brain and spinal cord. This complex system controls both things that we intentionally think about and do, like walking and talking, and essential body functions that occur without specific thought on our part, such as breathing and digesting food. The CNS is also involved with the five senses of seeing, hearing, touching, tasting, and smelling, as well as emotions, thoughts, and memory.
The brain is a soft, spongy organ that is made up of nerve cells and tissue. It is divided into three major sections: the cerebrum, the cerebellum, and the brainstem.
The cerebrum is the largest part of the brain, and is divided into two halves, called the right and left hemispheres. The right hemisphere controls the left side of the body and the left hemisphere controls the right side of the body. Each hemisphere is further divided into sections called lobes. There are four lobes in each hemisphere: the frontal, parietal, occipital, and temporal, and each lobe is responsible for certain functions.
The cerebellum is much smaller than the lobes of the brain, and sits at the back of the brain under the cerebrum. It is responsible for balance and coordination and controls complex actions like walking and talking.
The third part of the brain, the brainstem, connects the brain to the spinal cord. It controls some of the most important and necessary body functions, such as breathing and maintaining body temperature and blood pressure. It also controls hunger and thirst.
The spinal cord is made up of bundles of nerve fibers, called vertebra. It starts at the base of the brain and extends a little more than halfway down the back. Spinal nerves connect the brain with other nerves throughout the body and carry messages back and forth between the brain and the rest of the body.
In order to protect the central nervous system from injury or damage, several protective barriers exist. Three thin membranes, called meninges, cover the entire brain and spinal cord forming a thin protective layer. In addition, a thin, watery fluid, called cerebrospinal fluid (CSF), cushions the brain and spinal cord and offers further protection. CSF is produced in four hollow spaces in the brain, called ventricles, and flows through the ventricles and in the spaces between the meninges. It also brings nutrients from the blood to the brain and removes waste products from the brain. The bony structures of the skull and vertebra provide the final layer of protection for the central nervous system.
Cancer of the Brain and Spinal CordCancer can occur in any part of the brain or spinal cord. Cancer cells are abnormal cells that divide too often and without any order. In 1997, about 18,000 new brain tumors were diagnosed, a 50% increase from only ten years ago. They are rare tumors, representing only 1.5% of all cancers reported in the United States.
The causes of central nervous system tumors are not known, and scientists cannot explain why brain tumors develop in healthy adults. Certain factors, however, have been identified that may increase a person's chance of developing a brain tumor. For example, workers in the oil refining, rubber manufacturing, and drug manufacturing industries have higher rates of certain types of brain tumors. Researchers are also studying families in whom multiple members have developed the same type of brain tumor to see whether heredity plays a role. They are also looking at the connection between viral infections and exposure to radiation and the development of brain tumors. There is no research to suggest that head injuries cause or increase a person's risk for developing a brain tumor. Because most patients diagnosed with a brain tumor have no identifiable risk factors, it is believed that brain tumors result from a number of factors acting together.
Tumors which start in the brain are called primary brain tumors and are classified according to the kind of cell from which the tumor seems to originate. The most common primary brain tumor in adults comes from cells in the brain called astrocytes that make up the blood-brain barrier and contribute to the nutrition of the central nervous system. These tumors are called gliomas (astrocytoma, anaplastic astrocytoma, or glioblastoma multiforme) and account for 65% of all primary central nervous system tumors. The following table explains other types of brain tumors, the cells from which the tumors most likely come, and the functions of those cells.
Cancer from other parts of the body can spread to the brain and cause secondary tumors through a process called metastasis. Although it is possible for cancer from anywhere in the body to spread to the brain, it happens most often with cancers of the breast and lung. The cells of a metastatic brain tumor resemble the cells of the organ where the tumor started, not brain cells. For example, if a tumor starts in the breast and spreads to the brain, the cells of the brain tumor will resemble abnormal breast cells, not abnormal brain cells.
Signs and Symptoms of Brain TumorsThe symptoms of both primary and metastatic brain tumors depend mainly on the location in the brain and the size of the tumor. Since each area of the brain is responsible for specific functions, the symptoms will vary a great deal. Tumors in the frontal lobe of the brain may cause weakness and inability to move on one side of the body, known as paralysis, mood disturbances, difficulty thinking, confusion and disorientation, and wide emotional mood swings. Parietal lobe tumors may cause seizures, numbness or paralysis, difficulty with handwriting, inability to perform simple mathematical problems, difficulty with certain movements, and loss of the sense of touch. Tumors in the occipital lobe can cause loss of vision in half of each visual field, visual hallucinations, and seizures. Temporal lobe tumors can cause seizures, perceptual and spatial disturbances, and inability to understand simple of multi-step commands, known as receptive aphasia. If a tumor occurs in the cerebellum, the person may have difficulty maintaining their balance, known as ataxia, loss of coordination, headaches, and vomiting. Tumors in the hypothalamus may cause emotional changes, and changes in the perception of hot and cold. In addition, hypothalamic tumors may affect growth and nutrition in children. With the exception of the cerebellum, a tumor on one side of the brain causes symptoms and impairment on the opposite side of the body. For example, a tumor on the left side of the brain may cause numbness in the right arm.
As a brain tumor grows, it invades the healthy tissue in the brain, often causing further deterioration. Because of the limited space within the skull, the tumor may place pressure on the brain. There may also be a buildup of fluid around the tumor, a condition known as edema. Both of these may cause frequent headaches that are often unrelieved by over-the-counter medications. Headaches are the most common presenting symptom for patients with brain tumors.
Since all of these symptoms can be caused by other problems, you must be seen by a physician to have your symptoms properly evaluated. Your physician may refer you to a neurologist, a doctor who specializes in diagnosing and treating disorders of the brain and central nervous system, or to an oncologist, a doctor who specializes in diagnosing and treating cancer.