200011.vtt
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WEBVTT
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The nervous system is made up of two parts.
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Each part contains billions of neurons.
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The first part is the central nervous system.
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It contains the brain and spinal cord, which is a fibrous,
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ropelike structure that runs through the spinal column down the middle of the back.
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The other part is the peripheral nervous system.
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It consists of thousands of nerves that connect
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the spinal cord to muscles and sensory receptors.
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The peripheral nervous system is responsible for reflexes,
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which help the body avoid serious injury.
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It's also responsible for the fight or flight response
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that helps protect you when you feel stress or danger.
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Let's examine an individual neuron up close.
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Here is a peripheral nerve.
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Each one of the nerve bundles, or fascicles,
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contains hundreds of individual nerve fibers.
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Here's an individual neuron, with its dendrites, axon, and cell body.
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The dendrites are tree-like structures.
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Their job is to receive signals from other neurons
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and from special sensory cells that tell us about our surroundings.
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The cell body is the headquarters of the neuron. It contains the cell's DNA.
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The axon transmits signals away from the cell body to other neurons.
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Many neurons are insulated like pieces of electrical wire.
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The insulation protects them and allows their signals to move faster along the axon.
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Without it, signals from the brain might never reach muscle groups in the limbs.
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Motor neurons are responsible for voluntary control of the muscles all over the body.
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The operation of the nervous system depends on how well neurons communicate.
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For an electrical signal to travel between two neurons,
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it must first be converted to a chemical signal.
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Then it crosses a space about
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a millionth of an inch wide.
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The space is called a synapse.
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The chemical signal is called a neurotransmitter.
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Neurotransmitters allow the billions of
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neurons in the nervous system
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to communicate with one another.
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That's what makes the nervous system the body's master communicator.