| _-_
+ | / | \
| / | \
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.