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readerL6 - Propagation and Integration of Electrical...

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Propagation and Integration of Electrical Signals (1) At the point along the axon where the action potential is at peak depolarization , there is a very high in fl ux of sodium ions . Sodium ions will fl ow down the axon in both directions from this point. (2) In the forward direction (the direction the action potential is traveling down the axon) this fl ow of sodium ions will depolarize the membrane until it reaches action potential fi ring threshold . This will cause rapid opening of many voltage gated sodium channels, causing the peak of the depolarization to now be further down the axon. In this way the action potential peak will continue to move down the axon. (3) The action potential will not double back on itself, because in the reverse direction (the direction the action potential has come from) voltage gated sodium channels are inactivated , and voltage gated potassium channels are open so no new action potential can be initiated (this part of the membrane is experiencing the absolute refractory period). Propagation of action potential The main purpose of action potentials is to carry information swiftly and reliably along an axon. How does the action potential propagate? Page 6-1
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(4) However, there is nothing intrinsically directional about the axon. We generally think of action potentials as starting near the cell body in the spike initiating zone (axon hillock) and traveling down towards the axon terminal. However, in some neurons, like sensory receptors in the skin, action potentials are actually generated in the axon terminals and travel back towards the cell body. Page 6-2
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Saltatory ” (jumping) conductance occurs in myelinated axon In a myelinated axon, voltage gated channels are found almost exclusively at the nodes of Ranvier. This, along with the fact that the myelin sheath does not allow ions to fl ow out of the membrane through what few channels may be in the internode regions, means that action potentials are only generated at the nodes of Ranvier and “jump” between nodes. Page 6-3
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Passive electrical properties of neurons (1) How fast an electrical potential can travel down an axon (or a dendrite) depends on a number of factors, including the diameter of the axon, the cytoplasmic resistance of the axons, the permeability and capacitance of the membrane, and whether the axon is myelinated or not. (2) In order to understand why this is, and also to understand how neurons integrate synaptic inputs, it is useful to study the passive electrical properties of a neuron. (3) Passive spread of current ( i.e., spread of current in the absence of action potentials) occurs in most dendrites because most dendrites do not have voltage gated sodium channels (therefore cannot fi re action potentials). Passive spread of current also occurs in axons (in addition to action potentials ).
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