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At rest, the membrane potential of a typical neuron is about -70 millivolts. This is maintained by the action of the sodium-potassium pump, which constantly moves sodium ions (Na+) out of the cell and potassium ions (K+) into the cell, against their electrochemical gradients.

Activity in cells that synapse onto a neuron can result in their release of various neurotransmitters. These substances can bind to sites on the neuron, directly or indirectly resulting in the opening of ion channels in the cell membrane, allowing for the movement of certain ions (depending on the type of ion channel) according to their electrochemical gradient. If this ionic movement results in a depolarization of the membrane to -50 millivolts, then an action potential will occur.

The depolarization of the membrane to -50 millivolts causes voltage-gated sodium channels to open, momentarily allowing Na+ to flow freely into the neuron, depolarizing the cell further and causing more voltage-gated sodium channels to open. As a result, the membrane potential rapidly reaches peak depolarization, at about +40 millivolts. The depolarization also opens voltage gated potassium channels, and K+ flows out of the cell, which repolarizes the membrane. The depolarization resulting from an action potential is often followed by a brief hyperpolarization of the membrane as a result of the delayed closure of potassium channels.

The action potential is propagated down unmyelinated axons because the initial depolarization perpetuates itself by causing nearby sodium channels to open. In axons with myelin, the depolarization results in a flow of current through the axon that is boosted by the opening of sodium channels at the nodes of ranvier.

  |             _-_
+ |            / | \
  |           /  |  \
c |          |   |   \
h |         /    |    \
a |        |     |     --
r |       /      |       \
g |   __-'|      |        ----
e |--'    |      |            '--__---
  |  |    |      |               |   |
- \--a----b------c---------------d---e
             time -->

A graph of electrical charge at any given point on the axon of a neuron during an action potential.

a: graded potentials (EPSPs), or action potentials earlier in the axon, trigger the gradual depolarization of the plasma membrane from -70 mV

b: the charge at typically -55 mV reaches threshold, where voltage-gated Na+ gates open into the active state, allowing large amounts of Na+ ions to flow into the axon, rapidly depolarizing the axon

c: Na+ gates close into inactive state at approximately +40 mV. K+ voltage gates open, allowing K+ ions to flow from the cell, repolarizing the cell.

d: Hyperpolarization occurs, and K+ gates close. Na+ gates have already entered ready state. The axon can drift down to -90 mV at this point

e: the axon stabilizes at -70 mV

  |                         _-_
  |                        / | \
  |                       /  |  \
  |             _-_      /   |   \
+ |            / | \    /    |    \
  |           /  |  \  /     |     \
c |          |   |   \/      |      \
h |         /    |   |       |       \
a |        |     |   |       |        --
r |       /      |   |       |          \
g |   __-'|      |   |       |           ----
e |--'    |      |   |       |               '--__---
  |  |    |      |   |       |                  |   |
- \--a----b------c---c1------c2-----------------d---e
             time -->

More action potentials can be transmitted on top of each other, called wave summation. The second potential is stronger than the first because it builds on the first.

c1: second potential causes Na+ gates to open again

c2: second peak: higher than first

Many waves can be added to each other, each increasing the amount of depolarization, but, as always, the law of decreasing returns kicks in, and the charge is proportional to a function of the square root of the number of potentials.

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