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Unformatted text preview: distance Occur only in the axon Graded action potential occurs at axon hillock AKA nerve impulse All-or-none Table 8-3 RMP Depolarization: Phases of the Action Potential
Voltage-gated Na+ channels Fast to open and close Voltage-gated K+ channels Slow to open and close Slow closing of K+ gates Repolarization: Hyperpolarization: RMP reestablished by Na+/K+ pump Figure 8-9 - Overview Absolute Refractory Period 2 msec before another AP can be initiated Time for resetting the Na+ channel gates Insures that AP's do not overlap (limits rate of AP's) and cannot travel backwards Relative Refractory Period Stronger than normal stimulus is required to reach threshold depolarization K+ channels are still open If all AP's are the same, how do we (the integrator) know that one stimulus is different from another stimulus? FREQUENCY of AP's Stronger stimulus will produce more AP's, not bigger ones More AP's = More NT Figure 8-13 - Overview Conduction Velocity Speed of AP (rate of impulse propagation) movement down the axon Variation between types of neurons Determined by: 1. Axon diameter the larger the diameter, the faster the impulse 2. Myelination Continuous vs. Saltatory conduction Continuous Conduction Unmyelinated axons Step-by-step depolarization of axon membrane Figure 8-14 Saltatory Conduction Myelinated axons
Myelin does NOT conduct electricity Acts as an insulator; prevents ion movement Electrical currents - only at the nodes of Ranvier High concentration of voltage-gated Na+ channels at nodes Action potentials JUMP from one node to the next Figure 8-18a Saltatory Conduction Allows humans to have small diameter axons with rapid AP's, rather than space consuming large diameter axons Other advantages: Smaller regions of membrane depolarize, etc. Less energy expended by Na+/K+ pump to reset RMP Increases speed of conduction What is the purpose of generating and propagating an AP down the axon to the axon terminal branches? Release of chemicals stored in vesicles! These chemicals are used to transmit information from the neurons to another cell (neuron, muscle cell, glandular cell, target cell) Chemical vs. Electrical Synapse Synapse: where neural communication occurs Electrical Synapse Electrical signal flows from the cytoplasm of one cell to the cytoplasm of the adjacent cell through gap junctions Advantage FAST! Able to stimulate many cells at the same time Chemical Synapse Chemical signal from a presynaptic neuron to a postsynaptic cell across a synaptic cleft (space in between) Chemicals Neurotransmitter (neuron, muscle, gland) Neurohormone (goes into the blood) Figure 8-2 Chemicals are stored in vesicles in the axon terminals Released by exocytosis into synaptic cleft (dependent on impulse dependent opening of Ca2+ channels) Diffusion across synaptic cleft Bind to receptors on the postsynaptic membrane (Note mitochondria in axon terminals) Figure 8-20 Figure 8-21 - Overview Table 8-4 (1 of 2) Table 8-4 (2 of 2)...
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- Fall '08