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Nervous System Topic 6.5 What does the NS consist of? CNS [central nervous system] = BRAIN and SPINAL CORD PNS [peripheral nervous system] = NERVES Composed of cells called NEURONS that carry rapid electrical impulses. Diagram of a motor neuron Reflex arc Nerve impulses are conducted from receptors to the CNS by sensory neurons. Nerve impulses are conducted within the CNS by relay neurons. Nerve impulses are conducted from the CNS to effectors by motor neurons. Reflex arc diagram Resting potentials The resting potential is the electrical potential across the plasma membrane of a cell that is NOT conducting an impulse. It is equal to approx. -70mV The inside of the membrane is negative relative to the outside. Reasons why cytoplasm is negative compared to outside the membrane Higher concentration of anions (negatively charged ions) on the inside. Na+/K+ pump actively transports 3Na+ out for every 2K+ it pumps in. Membrane is more permeable to K+ than Na+ so diffusion more likely. Na+ channels often closed. Action potential An action potential is the reversal and restoration of the electrical potential across the plasma membrane of a cell as an electrical impulse passes along it. Action potentials Changes in membrane polarity during action potentials Depolarization and repolarization When a neuron is stimulated, some Na+ channels open and ions enter. Membrane potential rises (becomes less negative). If threshold potential of -50mV is reached, an action potential is triggered. DEPOLARIZATION occurs when most Na+ channels are open and the ions diffuse rapidly down their concentration gradient. Depolarization and repolarization At peak potential (+35mV), Na+ gates close and K+ channels open. K+ diffuse down their concentration gradient restoring the inside of the membrane to negative. K+ gates remain open making the membrane potential drop below resting temporarily. Closing of K+ gates and action of Na+/K+ pump restores resting potential to -70mV. Transmission along a neuron An action potential in one part of the neuron triggers an action potential in an adjacent area. Na+ ions diffuse from where the a.p. has just occurred to the area at rest, causing the impulse to travel along the axon. One-way movement is ensured by the ‘hyperpolarization’ or refractory period. Synaptic Transmission