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Name ____________________________________ Date __________ Class ___________________ SECTION 15-1 SECTION SUMMARY How the Nervous System Works Guide for Reading ◆ What are the functions of the nervous system? ◆ What are the three types of neurons and how do they interact? T Science Explorer Grade 7 © Prentice-Hall, Inc. 142 Unit 3 Resources he nervous system receives information about what is happening both inside and outside your body. It also directs the way in which your body responds to this information. In addition, the nervous system helps in maintaining stable internal conditions. A stimulus is any change or signal in the environment that can make an organism react. Stimuli can come from the outside environment or inside your body. After your nervous system analyzes a stimulus, it causes a response. A response is what your body does in reaction to a stimulus. The cells that carry information through your nervous system are called neurons, or nerve cells. The message that a neuron carries is called a nerve impulse. A neuron has a large cell body that contains the nucleus. The cell body has threadlike extensions. One kind of extension, a dendrite, carries impulses toward the cell body of the neuron. An axon carries impulses away from the cell body. Axons and dendrites are sometimes called nerve fibers. A bundle of nerve fibers is called a nerve. Different kinds of neurons perform different functions. Three kinds of neurons are found in the body—sensory neurons, interneurons, and motor neurons. Together they make up the chain of nerve cells that carry an impulse through the nervous system. A sensory neuron picks up stimuli from the internal or external environment and converts each stimulus into a nerve impulse. An interneuron is a neuron that carries nerve impulses from one neuron to another. A motor neuron sends an impulse to a muscle, and the muscle then contracts. Nerve impulses begin in a dendrite, move toward the cell body, and then move down the axon. A nerve impulse travels along the neuron in the form of electrical and chemical signals. The axon tip ends at a synapse. A synapse is the tiny space between the axon of one neuron and the dendrite of another neuron. Chemicals released by the axon tip carry the nerve impulse across the synapse to the next structure. The next structure can be a dendrite, or it can be a cell in another organ such as a muscle.