Last Modified: November 1, 2001
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.
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.
|Type of Tumor||Cell of Origin||Function|
|Oligodendroglioma||Oligodendrocyte||Produces a substance called myelin, which covers the nerves and helps information to travel quickly between the brain and other parts of the body.|
|Ependymoma||Ependyma||Lines the ventricles and aids in the circulation of cerebrospinal fluid.|
|Meningioma||Meninges||Cover and protect the brain and spinal cord.|
|Lymphoma||Lymphocyte||Part of the immune system, the body's primary defense against infection and foreign substances.|
|Schwannoma||Schwann cell||Produce the myelin that protects the acoustic nerve, the nerve of hearing.|
|Medulloblastoma||Primitive neuroectodermal cell or Primitive nerve tumors (PNET)||These cells normally do not remain in the body after birth.|
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.
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.