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Lecture 04 - Neural Synapses and Potentials

Lecture 04 - Neural Synapses and Potentials - Lecture 4...

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Lecture 4 – Neural Synapses and Potentials Action Potential Conduction In Unmyelinated Axon (Fig 8.19) 1. It is based on the flow of local currents produced by the influx of sodium. 2. The membrane in front of the action potential (still polarized) receives Na + that diffuses in from the membrane around the actual action potential, forcing that membrane’s Na + channels to open because it reaches the threshold of -55 mV. 3. The region behind the action potential goes through a refractory period (repolarization). 4. Even though some positive current flows backward from the action potential membrane, no depolarization occurs because the Na + channels are gated shut. There can be no action potential carried out via that membrane for a short period of time. Applying a stimulus to that area requires a larger-than-normal strength to cause an action potential (greater current). 5. This limits how fast information can be transferred across the neuron. Typically, for mammals, it is 200 stimuli per second. Refractory Periods (Fig 8.17) 1. The absolute refractory period is when all of the Na + channels are involved in the action potential (just opened or being closed off). During this period, no stimulus can generate an action potential. 2. The relative refractory period, just after the absolute refractory period, is when some of the Na + channels have recovered and are ready to go again. The K + channels are still open. This time is a period of increasing excitability. As a result, the later in the relative refractory period you are, the smaller the size of the stimulus required to generate an action potential. Refractory Period: Na + Inactivation & K + Activation (Fig 8.17) 1. You can see that during the absolute refractory period, most of the Na + channels are inactivated, and as time goes on and you enter the relative refractory period, more Na + channels are activated. 2. The same thing happens with K + channels. Myelin Sheath (Fig. 8.5) 1. Neurons are covered with layers of cells that serve as “insulation” called myelin sheathes. 2. There are breaks in the myelin called Nodes of Ranvier. 3. These nodes are the only place where the neuron is exposed, and thus, it is the only place where Na + gated ion channels exist and can enter the neuron. Saltatory Conduction (Fig 8.20) 1. This type of conduction occurs in myelinated neurons. Na+ ions flow into the axon through the Node of Ranvier, where all voltage-gated ion channels are located.
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