Action Potentials Notes

Action potentials are changes in membrane potential, similar to graded potentials, but with key differences.
Crucially, compare and contrast action potentials and graded potentials to understand their similarities and differences.
Graded potentials occur on the receiving end, while action potentials are the result of an adequate graded potential.
Action potentials start at the axon hillock and propagate down the axon to axon terminals, triggering neurotransmitter release.

An action potential lasts about 4 milliseconds.
A neuron can fire approximately 250 action potentials per second.
Membrane potential is measured in millivolts (mV).
Threshold is the "point of no return," a specific voltage that must be reached to trigger an action potential.

Resting membrane potential is typically around -70 mV in a neuron.
At rest, sodium $(\text{Na}^+)$ is concentrated outside the cell, and potassium $(\text{K}^+)$ is concentrated inside.
The inside of the cell is negatively charged relative to the outside.
Ions flow down their concentration gradients through leakage channels, which are balanced by the sodium-potassium pump.
The sodium-potassium pump maintains resting conditions by counteracting leakage channels.

A graded potential can cause a positive deflection in membrane potential.
If the change in membrane potential reaches threshold, depolarization occurs.
Depolarization is a large positive change in membrane potential, shifting from negative to positive values.
Repolarization follows, involving a rapid drop in membrane potential back to negative values.
Hyperpolarization can occur temporarily, where the membrane potential becomes more negative than -70 mV.
These events (depolarization, repolarization, hyperpolarization) are driven by ion movement across the membrane, mainly sodium and potassium.

Threshold is crucial for initiating the opening of ion channels, which allows ions to cross the membrane, leading to depolarization, repolarization, and hyperpolarization.

Action potentials are caused by the movement of ions across the membrane, leading to depolarization, repolarization, and hyperpolarization.

Resting membrane potential (-70 mV) is a state of equilibrium, with no net change.
Sodium ions rush in, and potassium ions diffuse out through leaky channels.
The sodium-potassium pump restores this equilibrium.
At rest, the neuron is not in the middle of an action potential; it is in a state of balance.

Sodium voltage-gated channels are integral proteins in the cell membrane that regulate ion flow.
These channels change shape based on membrane potential.
In state 1, the channel is closed at resting membrane potential.
A stimulus can cause the membrane potential to rise.
When the membrane potential reaches threshold (-55 mV), the sodium voltage-gated channel opens (state 2).
Sodium ions rush into the cell.
The channel closes again at +30 mV (state 3).
The channel opens only when the membrane potential is moving in a positive direction.
The protein restores to its original shape (state 1) only when the membrane potential falls below threshold.

Potassium voltage-gated channels also respond to membrane potential.
In state 1, the potassium channel is closed at resting membrane potential.
When the membrane potential reaches +30 mV, the potassium channel opens, allowing potassium to flow out of the cell.
The potassium channel opens at +30 mV and closes when the membrane potential is hyperpolarized.
The sodium-potassium pump restores ion concentrations.

Depolarization: Sodium voltage-gated channels open, sodium flows in, and membrane potential rises.
At +30 mV, sodium voltage-gated channels close, and potassium channels open.
Potassium flows out, causing repolarization and a drop in membrane potential.
Hyperpolarization occurs, leading to the closing of potassium voltage-gated channels.
The sodium-potassium pump restores ion balance, returning the cell to resting membrane potential.
Action potentials are solely related to the movement of sodium and potassium across the membrane.
They directly involve voltage-gated channels and the sodium-potassium pump.

Threshold is critical because action potentials are all-or-none phenomena; they either happen or they don't.
If the membrane potential doesn't reach threshold, no sodium voltage-gated channels open, and no action potential occurs.
Opening the voltage-gated sodium channel is necessary for an action potential to occur.
This is a positive feedback loop: reaching the threshold triggers sodium influx, leading to depolarization and the opening of voltage-gated potassium channels.

If the stimulus is too low, the membrane potential does not reach threshold, and no action potentials occur.
If the stimulus is adequate and reaches threshold, action potentials are generated.
Action potentials are all or none, meaning they either happen or they don't.
The threshold must be reached to open sodium voltage-gated channels and trigger an action potential.

All action potentials are the same size and magnitude.
Neurons distinguish between weak and strong stimuli through rate coding.
Rate Coding: Neurons code the intensity of a stimulus based on the number of action potentials generated.
A weak stimulus may not generate any action potentials.
A moderate stimulus generates a few action potentials.
A strong stimulus generates many action potentials.
The number of action potentials reflects the magnitude of the stimulus.

Absolute and relative refractory periods describe phases within an action potential.

Threshold is the membrane potential needed to open voltage-gated channels.
When sodium voltage-gated channels open, they stay open until +30 mV, when potassium channels open.

From the start of depolarization until the cell repolarizes below resting membrane potential, another action potential cannot be generated.
The sodium voltage-gated channels are either open or cannot reopen.
The channels must reset to state 1 to allow another action potential.

During hyperpolarization, the membrane potential is below normal.
A second action potential can occur, but it requires a larger stimulus to reach threshold.
The stimulus must raise the membrane potential from the hyperpolarized state back to -70 mV before reaching threshold.

An action potential occurs when the membrane reaches threshold, opening sodium voltage-gated channels.
Intensity is coded by the number of action potentials (rate coding).
The absolute refractory period prevents another action potential during depolarization and repolarization.
The relative refractory period needs a larger stimulus to generate another action potential during hyperpolarization.