The Whole Brain Atlas
The nervous system is composed of trillions of nerve cells, as well as other cells which maintain and support the nerve cells. The scientific name for a nerve cell is a neuron. Neurons function electrically to create thought and action in our bodies. Without nerve cells, human life would not, and could not be. Most of the cells which maintain and support neurons are glial cells. The word "glial" means glue. Glial cells not only maintain and support neurons, they serve as glue to keep them in place.
Neurons have a cell body, which like all cell bodies, is called a soma. Protruding from the soma are many nerve fibers. There are two kinds of nerve fibers, axons and dendrites. Axons are very long nerve fibers which send electro-chemical messages to other neurons or to glands and muscles to create action in them. Dendrites are short nerve fibers which receive electro-chemical information from the axon of other nerve cells. A typical nerve cell has several dendrites, but only one axon. When neurons are linked and bundled together to form and electrical line of conduction, they are called nerves. When neurons have specialized in detecting environmental events, they are called sensory receptors. The rods and cones of the eye and taste buds, for example, are specialized neurons.
Since neurons and their nerve fibers communicate with each other electrically, many of the fibers require insulation to function properly. Those nerve fibers which are insulated are insulated with fatty substances called myelin sheath and neurolemma, and are white in appearance. Those which are not insulated are gray in appearance. Insulation permits the nervous system to exercise fine control over muscles. The reason that babies cannot smile or move precisely at birth is that the insulation for their nerve fibers is not completely developed. The reason that a baby horse can run well shortly after birth is that their insulation is more complete at birth. As the insulation does develop in a child, they can smile and move with greater coordination and precision.
Multiple Sclerosis is a disease in which certain white blood cells think that the myelin sheath does not belong in the body. These white blood cells attack the myelin and destroy it. This causes the nerve to short-circuit, affecting muscle control. Such diseases in which the immune system attacks a body part are called auto-immune diseases.
Where the axon of one neuron meets the dendrite of an adjacent neuron, and where nerve fibers connect to muscles and glands, these connections occur at special junctions called a synapse. The synapse is a tiny gap which must be bridged by a chemical called a neurotransmitter. Some well known neurotransmitters are GABA, histamine, norepinephrine, serotonin, dopamine and acetylcholine. Without neurotransmitters neurons could not communicate with each other or with muscles and glands.
Many diseases are caused by an imbalance of neurotransmitters. Some of these are:
a. Major Depression (Clinical Depression) is a mood or emotional disorder often caused by a deficiency of serotonin and or norepinephrine. Antidepressant drugs such as Effexor, Prozac, Sinequan and Zoloft make more of these neurotransmitters available to the brain. Electrical Convulsive Shock Therapy (ECT) also makes more of these neurotransmitters available to the brain.
b. Schizophrenia is a thought disorder that may represent too much, or an over-sensitivity to the transmitter dopamine. Although schizophrenia is incurable, drug treatment often allows the disease to be controlled. Drugs used to treat schizophrenia, such as Thorazine, Haldol and Clozaril make less dopamine available to the brain.
c. Myasthenia Gravis--occurs due to a deficiency of the transmitter acetylcholine at the neuro-muscular junction. Without this transmitter nerves cannot make muscles contract and do work. Myasthenia gravis produces muscular weakness.
d. Parkinson's Disease--occurs due to a lack of dopamine in specific parts of the brain which control movement. This results in a tremor (shaking), and Parkinson's can also affect behavior. Parkinson's is treated with the drug which adds dopamine to the brain. Too much L-dopa can result in schizophrenic-like symptoms.
The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS) is composed of all the nerves which exit from the brain or spinal cord to go out into the body to bring information into the central nervous system, or to create action in the body.
The spinal cord is an electrical conduction line which serves to connect the brain to the body and the body to the brain. For protection, it is housed in a bony canal composed of many small bones called vertebra. The spinal cord is about as thick as a pencil. At the top of the spinal cord is the brain stem. The brain stem is that part of the spinal cord which is actually inside of the head.
The brain stem is composed of several structures:
a. the medulla oblongata--called the vital center because it regulates heart rate and breathing. For this reason, an injury to the medulla often causes death. In the medulla is the vomiting center, and the medulla also controls such reflexes as yawning, hiccuping.
b. the hypothalamus--recognizes hunger and thirst. It contains the "start eat center" and the "stop eat center." The hypothalamus also controls the pituitary gland, and is involved in emotion.
c. the thalamus--is called the "great relay station" of the brain because most incoming and outgoing electro-chemical messages--to and from the brain--go to the thalamus before they are relayed to their appropriate destination in the body or the brain.
d. the reticular activating system (RAS)--the inner-most core of the brain stem. It is an alertness switch which turns on or off the brain. The RAS must deactivate the brain for sleep, and turn the brain on for us to awaken. Damage to the RAS often results in coma. Some attention deficit disorders (ADD) are believed to be caused by an under-active RAS. In ADD the brain is not alert enough to focus attention, so one with ADD will often be hyperactive in an attempt to "turn on" their brain. Stimulant drugs, such as Ritalin, activate the RAS, which can then turn on the brain in a proper manner. This is why stimulant drugs, such as Ritalin are used to treat ADD and hyperactivity.
Deep within the brain mass and surrounding the brain stem are a group of structures called the limbic system. The limbic system is the primary emotional center of the brain. Circuits of the limbic system create feelings of pleasure, pain, anger, and fear. Drugs which people become addicted to have their effect on the pleasure circuits of the limbic system. A class of drugs called antianxiety agents (which are sometimes called minor tranquilizers) have their calming effect by chemically altering the limbic system. Librium, Valium and Xanax are brand name examples of antianxiety agents.
Above the limbic system is the cerebrum. The word "cerebrum" is the Latin word for brain. The cerebrum is divided into two hemispheres which are connected by a pathway of nerve fibers called the corpus callosum. So in reality we have two brains. The left brain specializes in speech, language, words, writing and logic. The right brain specializes in spatial perception, the recognition of faces, art, music, mathematics and emotion. The outer surface of the cerebrum is about three inches thick and is called the cerebral cortex. The word cerebral means thought or intelligence. It is in the cerebral cortex which thinking occurs. The cerebral cortex is folded like an accordion so that a lot of cortex can be crammed into the skull. The more cerebral cortex an animal has, the greater is their capacity for thought. Humans have more cerebral cortex than any other animal. This is why we named ourselves homo Sapiens, which means wise or thinking man.
The cerebral cortex (often called just the cortex) also receives and processes information from the senses. The cortex is divided into areas by lines and creases which run on its surface. These areas are called lobes of the cerebral cortex. In the front of the brain are the frontal lobes. The frontal lobes control speech and movement. Much of our personality is contained in the frontal lobes. On top are the parietal lobes which receive information from the body senses. To the sides are the temporal lobes which receive information from the ears. To the back are the occipital lobes which receive information from the eyes.
Almost all nerve fibers which enter or leave the brain cross over on the brain stem. This means that the left brain controls the right side of the body, and the right brain controls the left side of the body. For most people the movement centers of the left brain are better developed than those of the right brain. For this reason most people are right-handed.
To the back of the skull, below the cerebrum is the cerebellum. The word cerebellum is a Latin word which means little brain. The cerebellum does many things; however, it appears that its primary function is to coordinate movements in the body. Damage to the cerebellum will often produce a spastic person.
The inner-most core of the CNS is hollow and filled with a fluid called cerebrospinal fluid. The primary function of cerebrospinal fluid is to cushion the brain to protect it from the impact of a collision. Cerebrospinal fluid is extracted in a procedure called a spinal tap or a lumbar puncture. This is done by physicians to diagnose certain illness. Sometimes in the unborn child, the drainage canal for cerebrospinal fluid becomes stopped up. The fluid builds up and the pressure causes the brain to expand like a balloon. This condition is called hydrocephalus (water head) and causes the child to have a very large head and to be mentally retarded.
The central nervous system is encased by three layers of tissue called the meninges. The three layers are the dura, pia, and arachnoid. If the meninges become infected, it is called meningitis.
The brain has many arteries and capillaries to supply neurons with oxygen and nutrition. If an artery or capillary bursts, or gets clogged, the neurons which depended on that vessel for blood die. This is called a stroke. Other names for a stroke are a cerebral hemorrhage, and a cerebral vascular accident (CVA). The effect of a stroke depends on what function the dead chunk of brain served. If it controlled speech, speech may be abolished. If it controlled movement, the person may be partially paralyzed. Because most nerve fibers cross-over, the damage usually appears on the opposite side of the body. Physical therapy may help undamaged parts of the brain to take over the lost functions which resulted from a stroke.
The brain has no neurons which allow it to recognize pain. Therefore, some neurosurgery is conducted while the patient remains wide awake. In neurosurgery, brain tissue can be probed and cut with no perception of pain. The scientific name for a headache is neuro-phalgia. Most headaches are caused by blood vessels of the brain over-expanding. When they over-expand they leak blood chemicals into the brain. This irritates neurons and causes a headache. Headaches can also be caused by pressure on brain tissue.
Any infection of the brain is called encephalitis. Encephalitis can be bacterial, viral or the result of heavy metal poisoning. Horses and birds often incubate a virus which is transmitted to humans through mosquito bites. This virus is a common cause of encephalitis.
Epilepsy is a convulsive disorder of the brain which produces what is commonly
called a seizure. It occurs due to burst of violent electrical activity which
spreads across the brain. Causes for epilepsy include brain injury, encephalitis,
meningitis, chemical poisoning and genetics causes. Epilepsy can usually be
controlled with drugs, such as Dilantin, Tegretol or phenobarbital.