The Nervous System
The nervous system is the most complex part of the body, as they govern our thoughts, feelings, and bodily functions. It is an important factor in science because it can lead to new discoveries for cures or diseases. The studies of the nervous system helped lower death rates from heart disease, stroke, accidents, etc. The nervous system is a network of neurons (nerve cells) that that sends information to the brain to be analyzed. Neurons live both in and outside the central nervous system. Understanding how the neurons work is vital to understanding the nervous system. Neurons The neuron has two important structures called the dendrite and axon, also called nerve fibers. The dendrites are like tentacles that sprout from the cell and the axon is one long extension of the cell. The dendrites receive signals from other neurons, while the axon sends impulses to other neurons. Axons can extend to more than a meter long. Average sized neurons have hundreds of dendrites; therefore it can receive thousands of signals simultaneously from other neurons. The neuron sends impulses by connection the axon to the dendrites of another nerve cell. The synapse is a gap between the axon and the adjacent neuron, which is where data is transmitted from one neuron to another. The neuron is negatively charged and it bathes in fluids that contain positively charged potassium and sodium ions. The membrane of the neuron holds negatively charged protein molecules. The neuron has pores called ion channels to allow sodium ions to pass into the membrane, but prevent the protein molecules from escaping (potassium ions can freely pass through the membrane since the ion channels mostly restrict sodium ions). When a neuron is stimulated (not at rest), the pores open and the sodium ions rush in because of its attraction to the negatively charged protein molecules, which makes the cell positively charged. As a result, potential energy is released and the neurons send electrical impulses through the axon until the impulse reaches the synapse of any neurons near it. Once the signal is sent, the ion balances out and becomes at rest. The electrical impulse that runs down the axon releases a chemical called acetylcholine, only one of many chemicals that transmits signals across the synapse. These substances are called neurotransmitters because they transmit data from one neuron to another. Once the chemical binds to the dendrites of another neuron, it is converted back to an electrical impulse, which is brought to the cell body. The impulse is then sent to another neuron, and the process repeats until the nerves are at rest. The effect of the signals depends on what the target is. If the target of the signal is a muscle cell, the effect might be a muscle contraction. The speed of the electrical impulse depends on the size of the nerve fiber. In small nerves, the rate it transmits impulses is from a half to two meters a second. The larger the diameter of the nerve fiber, the higher rate of conducting impulses. There is less electrical resistance in thick fibers. When nerve impulse jumps from one node (gaps in nerve fibers) to the next, it is called saltatory conduction. Saltatory conduction conducts faster because it contains an insulator that prevents leakage of currents. The rate of conduction is 2 to 120 meters a second. Not all nerves conduct impulse electrochemically. Some impulses jump from nerve to nerve, bypassing the synapse. Unlike other cells, once neurons are lost, they can't be regenerated. Fortunately, there are about 10 billion neurons and they have other cells to aid them in carrying messages to other nerves. But if nerves are severed, the nerve fibers can regenerate if the two ends are reattached precisely. However, restored functions may produce different actions because the nerves might not be connected to the right channel. There are three main parts of the nervous system: the central, peripheral, and autonomic nervous...
Bibliography: 1. Clayman, Charles; The Brain and Nervous System; © 1991 Dorling Kindersley, Ltd., and American Medical Association; published by Reader 's Digest Association, Pleasantville, New York. 2. Nathan, Peter; The Nervous System; © 1969 by Peter Nathan; published by Penguin Books, England. 3. The Human Brain: Controlling the Body; © 1998 Knowledge Adventure, Inc. ; http://www.letsfindout.com/subjects/body/rfibrain.html 4. The Nervous System; © 1992, 94, 97, 98, MJ Farabee; http://gened.emc.maricopa.edu/bio/bio181/biobk/biobooknerv.html 5. The Spinal Cord: Sensation and Movement; © 1998 Knowledge Adventure, Inc.; http://www.letsfindout.com/subjects/body/rfispinl.html
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