3.3: The action Potential

when the neuron is at its resting state of -65mV it is…

Polarized!

Action Potentials

brief (short-lived) massive change in the membrane potential, that reoccurs down the axon. it starts in the original segment of axon and “propogates” rapidly down the axon (regenerated again and again)

it will overshoot temporarily crossing 0 and becoming very positive like +40 mV

What are the properties of an action potential (general things to know）

All or nothing: there are no half action potentials, its there or isnt there.

Frequency = Stimulis Strength
information is coded in the frequency of action potentials. if the stimulis is strong the frequency is going to be faster

After potentials: the change. in membrane potential after action potential

What is the role of ion currents in the action potential?

action potentials are triggered by movement of Na+ into the cell:
when the action potential is trigged, the voltage gated channels for Na ions are activated, and Na is momentarily permeable to the membrane (shifting from primarily K to Na (creating action potential) and then back to K)

Voltage gate channels

Na ion channels open in reaction to depolarization, Na is very attracted to negative charge inside, and floods in. when Na comes in more channels open leading to more Na. this continues until depolarization reaches +40mV. Na channels close

this process happens in a milisecond

afterwards K+ channels open in response and K+ moves out to increase neg of cell back to resting potential (-65mV)

how do voltage gate channels close

after action potential the cell returns to tis resting state, by blocking its channels with other proteins so Na cannot come in 

refractory periods

absolute refractory period

relative refractory period

absolute refractory period

a phase where no action potentials can occur in the membrane, and the Na channels are closed / inactive

relative refractory period

phase where its difficult to produce action potential (a stronger stimulus) is needed to trigger it, since K+ leaving the cell, hyperpolarizes the neuron, and it needs depolarizing stimulus to reach threshold

Propogation of aciotn potentials

action potentials are regenerated down the axon and cannot flow backwards, each ongoing section gets depolarized and has an action potential.
the action potential only moves forward 1 way because of the refractory period (the section before it cannot be depolarized again right away)

What are the conduction properties

conduction velocity
nodes of Ranvier
saltatory conduction

conduction velocity

the speed of propogation (regenration) of action potentials down the axon. the speed depends on the diameter of the axon

nodes of ranvier

spaces / gaps in myelin sheath

saltatory conduction

action potentials “jumping” from section/node to node down the axon
actually speading rapidly in nodes with myelin sheath (tracks in mc)

where does the aciton potential starts 

spike initation zone / integration zone (axon hlock) start of axon terminal where all local potentals are directed.

this helps integration zone reach threshold and cause first action potential

why don’t ation potentials start at dedrites or cell bodies in most neurons

because (bipolar, multipolar neurons) they lack Na+ channels, which are needed to cause action potential. (local potentials integrated open Na+ channels → action potential)

sensory neurons / unipolar neurons action potential start

initiation zone is close to nerve endings

Summarize the action potential

1. Accumulation of local potentials

2. Threshold reached

3. Depolarization

4. Repolarization

5. Hyperpolarization

6. Return to resting potential