Author: Bittersweet Biology
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Action Potential Explained - The Neuron. An Action potential is the neurons way of transporting electrical signals from one cell to the next. This is a picture of a neuron, where you have dendrites in one end, and the axon terminal in the other end. In the middle of the neuron you will find the axon, which is where the electrical signal will travel from the dendrites to the axon terminal to the synapse. This is a picture of a myelinated neuron, which is covered with a myelin sheath that allows the electrical signal to travel faster through the axon. So instead of activating every ion channel down the axon, only channels in the small spaces between each myelin sheath called the Node of Ranvier is activated and generates an action potential. So lets take a closer look at the axon. On the membrane of the neuron, you will find small ion channels that are closed when the cell is in its resting state. The neuron is able to create an action potential because of the concentration difference of ions between the intracellular space and the extracellular space. There is a higher concentration of sodium ions outside the neuron and a higher concentration of potassium inside the neuron. the extracellular space is more positive than inside the neuron. This creates a voltage difference of -- 70 mV, which is created with the help of leaky ion channels that are more permeable to potassium ions than sodium ions, which will cause potassium to leave the cell and only small amount of sodium to enter. The sodium potassium pump also regulates the intracellular environment, by pumping out 3 sodium ions in exchange for 2 potassium ions. The -- 70 mV is the neurons resting potential, but when the neuron is stimulated by a presynaptic neuron, it causes sodium channels to open, letting in positive ions. This will change the electrical environment and make it more positive on the inside and less positive on the outside. This is called Depolarization, and which causes a chain reaction where the next sodium channels will open letting in positive ions, all the way down the axon. Shortly after a channels have opened, they will close again and potassium channels will then open letting out positive potassium ions, to recreate the negative environment inside the neuron and the positive environment outside. This happens at around + 40 mV, and causes a Repolarization where the intracellular space is becoming negative again. The neuron then reaches a state of Hyperpolarization where the cell has let out too many ions and has now become more negative than the cells resting potential. This is quickly corrected by the leaky channels and the sodium potassium pump, and the neuron stabilize at the resting potential at a -- 70 mV. The action potential that has traveled through the axon, reaches the axon terminal, where vesicles with neurotransmitters are released out into the synaptic cleft to transport the signal to another neuron.