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An action potential is an electrical impulse that begins at
the axon hillock (trigger zone), propagates away from the soma, and moves down
the axon, synaptic knob, and synapse. The action potential is triggered by a
depolarization of the neuron from -70mV to -55mV. This depolarization occurs
when the neuron receives either repeated stimuli or collective stimuli (local
potentials) that open up sodium channels and allow Na+ to flow into the
neuron until the neuron reaches a threshold of -55mV. 


When the sodium channels open, Na+ flows into the neuron as
a result of its concentration gradient (Na+ is more concentrated in the
extracellular fluid so it flows inside the neuron) and electrical gradient
(inside of neuron is more negative so the positive sodium ions flow in). When
the sodium diffuses inside of the neuron, this triggers potassium channels to
open and allow K+ to flow out (since K+ is more concentrated inside and the
inside is beginning to get more and more positive.) When the cell reaches -50mV,
the voltage change opens up voltage-gated sodium channels at the trigger zone. This
causes more Na+ to flow in and further depolarizes the cell, which ends up
opening the neighboring voltage-gated sodium channels along the axon and away
from the cell body. Voltage-gated potassium channels also open up during the voltage
change from -70mV to -50mV. K+ channels here are slower to respond, but they
eventually allow enough potassium to flow out of the cell (due to K+
concentration gradient and electrical gradient) that it repolarizes the cell.

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When the action potential reaches 0mV, the voltage-gated
sodium channels begin to close and the potassium channels come closer to all
being open. This causes the voltage to shift back to the value of the resting
membrane after a slight hyperpolarization (when the K+ leaving the cell makes
the cell slightly more negative than the resting membrane value.) Resting
membrane potential is reached when the voltage is restored to -70mV due to the
K+ and Na+ leaving or entering the cell based on their respective electrical
gradients. When the shifting of K+ and Na+ finally result in an intracellular
voltage of -70mV, both the voltage-gated K+ and Na+ channels close. Although
the charge is restored to -70mV, the ions aren’t restored (Na+ needs to be more
concentrated in the extracellular fluid and K+ in the intracellular fluid.) This
is where the Na+/K+ pump comes in. The Na+/K+ pump essentially binds Na+ in the
intracellular fluid and exchanges it for a K+ from the extracellular fluid. It
eventually restores the ions to their original concentrations at resting


The absolute refractory period is a period where the cell
does not permit a stimuli, regardless of its strength, to trigger another
action potential. This period is dependent on the voltage-gated sodium channels
and lasts from the start of the action potential to when all voltage-gated
sodium channels are closed.


The relative refractory period is the period where a cell
can produce another action potential if the stimulus reaches the threshold. The
relative refractory period encompasses the hyperpolarization phase until all
voltage-gated potassium channels close. Since the hyperpolarized membrane is at
a slightly lower voltage than the resting membrane potential, the distance from
the hyperpolarized state to the threshold of -50mV is greater and, thus,
requires a stimulus that is greater than the normal stimulus strength that
would have triggered an action potential in the previous resting state.


An action potential can move more quickly down a neuron if the
neuron is myelinated. When segments of the axon are myelinated, it creates a
barrier that makes it hard for Na+ to flow into the axon and down its
concentration & electrical gradient. Because of this, the Na+ ions become
concentrated in the nodes of Ranvier (unmyelinated segments) when they diffuse
into the axon. The Na+ from the trigger zone is able to diffuse freely because
the Na+ that came into the axon have already attracted themselves to the
negative ions present. When the Na+ from the trigger zone approaches a Na+ ion
in the axon, it repels it and that repelled Na+ ion goes on to repel the next
sodium ion, which creates this chain of events called saltatory propagation
where an action potential leaps from one node of Ranvier to the next. 

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