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y65 - Chapter 2 Brain and Behavior Table of Contents Exit...

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Chapter 2 Brain and Behavior Table of Contents Exit
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Neuron and Its Parts Neuron: Individual nerve cell; 100 billion in brain Dendrites: Receive messages from other neurons Soma: Cell body; body of the neuron. Receives messages and sends messages down axon Axon: Carries information away from the cell body Axon Terminals: Branches that link the dendrites and somas of other neurons Table of Contents Exit
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Fig. 2.1 An example of a neuron, or nerve cell, showing several of its important features. The right foreground shows a nerve cell fiber in cross section, and the upper left inset gives a more realistic picture of the shape of neurons. The nerve impulse usually travels from the dendrites and soma to the branching ends of the axon. The neuron shown here is a motor neuron. Motor neurons originate in the brain or spinal cord and send their axons to the muscles or glands of the body. Table of Contents Exit
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Fig. 2.2 Activity in an axon can be measured by placing electrical probes inside and outside the axon. (The scale is exaggerated here. Such measurements require ultra-small electrodes, as described later in this chapter.) At rest, the inside of an axon is about –60 to –70 millivolts, compared with the outside. Electrochemical changes in a nerve cell generate an action potential. When positively charged sodium ions (Na + ) rush into the cell, its interior briefly becomes positive. This is the action potential. After the action potential, an outward flow of positive potassium ions (K + ) restores the negative charge inside the axon. (See Figure 2.3 for further explanation.) Table of Contents Exit
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Fig. 2.5 A highly magnified view of the synapse shown in Fig. 2.1. Neurotransmitters are stored in tiny sacs called synaptic vesicles. When a nerve impulse arrives at an axon terminal, the vesicles move to the surface and release neurotransmitters. These transmitter molecules cross the synaptic gap to affect the next neuron. The size of the gap is exaggerated here; it is actually only about one millionth of an inch. Transmitter molecules vary in their effects: Some excite the next neuron and some inhibit its activity. Table of Contents Exit
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The Nerve Impulse Resting Potential: Electrical charge of an inactive neuron Threshold: Trigger point for a neuron’s firing Action Potential: Nerve impulse Ion Channels: Axon membrane has these tiny holes or tunnels Negative After-Potential: When a neuron is less willing to fire Table of Contents Exit
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Fig. 2.3 The inside of an axon normally has a negative electrical charge. The fluid surrounding an axon is normally positive. As an action potential passes along the axon, these charges reverse, so that the interior of the axon briefly becomes positive. Table of Contents Exit
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Fig. 2.4 Cross-sectional views of an axon. The right end of the top axon is at rest, with a negatively charged interior. An action potential begins when the ion channels open and sodium ions (Na + ) enter the axon. In this drawing the action potential would travel rapidly along the axon, from left to right. In the lower axon
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