Studying The Brain
At the end of the nineteenth century, Santiago Ramon y Cajal, a Spanish scientist, studied neurons using stain developed by Camillo Golgi. Cajal realized that the brain was made up of individual units and not a continuous net as was believed at the time. His studies uncovered a large variety of neurons that differed in size and shape. He explained that neurons received signals on dendrites and transmitted impulses on axons. Since his work, researchers have learned that neurons carry information in the form of brief electrical impulses called action potentials that result when positively charged sodium ions travel across the axon membrane from the fluid outside to the cytoplasm inside. When a nerve impulse reaches the end of an axon, neurotransmitters are released at junctions called synapses. The neurotransmitters are chemicals that bind to receptors on the receiving neurons, triggering the continuation of the impulse. Fifty different neurotransmitters have been discovered since the first one was identified in 1920. By studying the chemical effects of neurotransmitters in the brain, scientists have made advances in finding medicines for the treatment of mental disorders, and determining the actions of drugs on the brain.
Researchers today are able to trace various molecules that are transported along axons during action potentials. Microelectrodes are used to detect the currents that cross synapses. Using this information, wiring diagrams are created that model the patterns of information flow within the brain.
Considerable knowledge about the human brain has been obtained during brain surgery by stimulating specific areas with a mild electric current, and from the observation of patients with brain damage. In the 1920s, a Canadian neurosurgeon named Wilder Penfield electrically stimulated different parts of the brains of some of his patients. He found this caused them to remember specific events from the past. For example, one patient heard someone from the past singing a particular song. From this and other studies, scientists realized that specific functions are localized in specific parts of the brain. Recently, scientists observed the behavior of a woman whose amygdala (an almond-shaped group of cells in the cerebrum) was destroyed. The amygdala plays a role in emotions and social relationships. The researcher realized that without an amygdala, the patient could not read facial expressions. As a result, she couldn't judge the intentions of others, and often made poor social decisions.
Until recently, scientists believed that brain cells do not regenerate, thereby making brain injuries and brain diseases untreatable. Researchers are now trying to help such patients with neuron transplants, introducing nerve tissue into the brain. They are also studying substances, such as nerve growth factor (NGF), that someday may be used to help regrow nerve tissue.
Since the 1950s, scientists have begun to understand the process of sleep. They find that sleep occurs in different stages. One stage is called rapid eye movement (REM) sleep. We dream during REM sleep, a period when there is a lot of brain activity and eye movements and the body is inactive. The pons, an area of the brainstem, sets off REM sleep and dreaming. During REM sleep, the brain emits characteristic brain waves. Non-REM sleep usually comes first, takes up about 75% of our sleep, and is much quieter. Its stages get deeper and deeper. Non-REM sleep also has its own particular brain waves. The two types of sleep alternate during the night. Scientists are beginning to understand the factors that control sleep and wakefulness. These include a biological clock, a group of about 10,000 neurons in the hypothalamus that trigger off waking up; homeostasis, the body's tendency to maintain equilibrium in physiological systems; and changes in the level of norepinephrine and serotonin, neurotransmitters in the brain.
Where and how does memory occur? This is another question that has puzzled scientists for decades. Recent information suggests that memory is not stored in a single brain center, but instead is part of numerous processing systems in the cerebral cortex. Scientists believe that memory involves chemical and structural changes in neurons, as well as changes in the strength of synapses.
Technology provides useful tools for researching the brain and helping patients with brain disorders. An electroencephalogram (EEG) is a record of brain waves, electrical activity generated in the brain. An EEG is obtained by positioning electrodes on the head and amplifying the waves with an electroencephalograph, and is valuable in diagnosing brain diseases such as epilepsy and tumors.
Scientists use three different techniques that involve scans to study and understand the brain and diagnose disorders:
- Magnetic resonance imaging (MRI) depends on the use of a magnetic field to display the living brain at various depths as if in slices. Not blocked by bone, MRI allows the viewer to zoom in on any region and obtain reliable pictures of brain tissue.
- Positron emission tomography (PET) results in color images of the brain displayed on the screen of a monitor. During this test, a technician injects a small amount of a substance, such as glucose, that is marked with a radioactive tag. The marked substance shows where glucose is consumed in the brain. PET is used to study the chemistry and activity of the normal brain and to diagnose abnormalities such as tumors.
- Magnetoencephalography (MEG) measures the electromagnetic fields created between neurons as electrochemical information is passed along. When under the machine, if the subject is told, "wiggle your toes," the readout is an instant picture of the brain at work. Concentric colored rings appear on the computer screen that pinpoint the brain signals even before the toes are actually wiggled.
Using an MRI along with MEG, physicians and scientists can look into the brain without using surgery. They hope to use these techniques for the early diagnosis of disorders such as Alzheimer and Parkinson disease. They foresee that these techniques could help paralysis victims move by supplying information on how to stimulate their muscles, or indicating the signals needed to control an artificial limb. Furthermore, by understanding what areas of the brain are active while performing particular tasks, and by mapping information regarding increased or decreased metabolism in particular regions of the brain, a variety of questions regarding various disease states, as well as a variety of questions regarding the phenomenonal potential of the human brain, may be answered
Researchers are studying any number of issues regarding brain functioning. Fascinating research revealing the presence of tiny magnetic bits within the brain has suggested that humans (like some insects and birds) have the potential to navigate via interactions with the earth's magnetic field. Technological research into computers and artifical intelligence are furthered by a good understanding of the amazing, computer-like intelligence of the human brain; conversely, studies of the human brain have been furthered by efforts to create artifical intelligence. Research into the human brain is actually thought to be in its infancy; myriad topics of investigation must be explored in order to grasp the complexities and intricacies of the human brain.
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The Nature of the Nerve Impulse. Films for the Humanities and Sciences, 1994-95. Videocassette.
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