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Unformatted text preview: The Nervous System 45.1 The nervous system consists of neurons and supporting cells. Neuron Organization Sensory neurons carry impulses into the central nervous system (CNS), and motor neurons carry impulses away from the CNS. (p. 940) Sensory and motor neurons together form the peripheral nervous system (PNS). (p. 940) Neurons are supported by neuroglia. Two of the most important neuroglia in vertebrates are Schwann cells and oligodendrocytes, which produce myelin sheaths. (p. 941) 45.2 Nerve impulses are produced on the axon membrane. The Resting Membrane Potential When a neuron is not being stimulated, it maintains a resting membrane potential. (p. 942) The inside of a cell is more negatively charged in relation to the outside of the cell, thus cations outside the cell are attracted to anions inside the cell. (p. 942) Due to combined activities of fixed anions, the sodium-potassium pump, and the membrane leak channels, the resting membrane potential for an average neuron remains stable at -70 mV. (p. 942-943) Graded Potentials Generate Action Potentials Graded potentials are caused by the activation of gated ion channels, and the ability of graded potentials to combine is referred to as summation, which results in the inward diffusion of Na + and the outward diffusion of K + . (p. 944) Threshold refers to the level of depolarization required to produce an action potential. (p. 944) Threshold refers to the level of depolarization required to produce an action potential....
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- Spring '08