![]() The overshoot is the peak of the action potential where the membrane potential is positive. The depolarizing rising phase moves the membrane potential from threshold to above 0 mV. EPSPs that summate to reach threshold initiate the action potential. ![]() Finally, the membrane potential will return to the resting membrane potential. The falling phase is a rapid repolarization followed by the undershoot, when the membrane potential hyperpolarizes past rest. The rising phase is a rapid depolarization followed by the overshoot, when the membrane potential becomes positive. It will run through all the phases to completion. Once initiated in a healthy, unmanipulated neuron, the action potential has a consistent structure and is an all-or-nothing event. The action potential begins when the cell’s membrane potential reaches threshold, caused by the summation of EPSPs ( Chapter 5). The actions of the voltage-gated channels cause the different phases of the action potential ‘Voltage-Gated Channel’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC-BY-NC-SA) 4.0 International License. The dotted, blue channels represent voltage-gated sodium channels the striped, green channels represent voltage-gated potassium channels the solid yellow channels represent chloride channels. In the animation, sodium ions flow inward. Once the channels are open, ions will move toward equilibrium. Reaching threshold causes voltage-gated ion channels to open. As EPSPs summate, a result of ion movement not shown in the animation, the cell’s membrane potential will depolarize. The neuron reaches threshold after enough EPSPs summate together.Īnimation 6.2. Voltage-gated channels open when the cell’s membrane potential reaches a specific value, called threshold. The main difference between voltage-gated channels and leak channels are how they are opened or “gated”. Voltage-gated channels allow ions to cross the membrane using the same ion movement principles covered in previous lessons. ![]() ‘Voltage-Gated Channel Location’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC-BY-NC-SA) 4.0 International License. Voltage-gated channels critical for the propagation of the action potential are located at the axon hillock, down the axon at the Nodes of Ranvier, and in the presynaptic terminal. They are necessary for the propagation of the action potential. For our purposes, these channels are located primarily at the axon hillock, along the axon and at the terminal. In this chapter, we will examine a different type of ion channel: voltage-gated ion channels. In the previous lessons, we have learned about the principles of ion movement and have discussed non-gated (leak) channels at rest, as well as ion channels involved in the generation of postsynaptic potentials. The change in membrane potential during the action potential is a function of ion channels in the membrane. ‘Action Potential Propagation’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC-BY-NC-SA) 4.0 International License. When the action potential reaches the synaptic terminal, it causes the release of chemical neurotransmitter. This saltatory conduction leads to faster propagation speeds than when no myelin in present. The action potential moving down a myelinated axon will jump from one Node of Ranvier to the next. The action potential moves down the axon beginning at the axon hillock. The propagation of the action potential from the axon hillock down the axon and to the presynaptic terminal results in release of chemical neurotransmitters that communicate with a postsynaptic neuron.Īnimation 6.1. ‘Action Potential’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC-BY-NC-SA) 4.0 International License. The membrane potential will begin at a negative resting membrane potential, will rapidly become positive, and then rapidly return to rest during an action potential. The action potential is a brief but significant change in electrical potential across the membrane. During the action potential, the electrical potential across the membrane moves from a negative resting value to a positive value and back. As covered in Chapter 1, the action potential is a very brief change in the electrical potential, which is the difference in charge between the inside and outside of the cell.
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