Module 3: Lecture 3 Flashcards
what causes a graded potential?
- some triggering event allows the response to change the cells membrane permeability
what is the action potential?
everything that occurs following the threshold potential being reached
- the depolarization and the repolarization phases back down to rest
- occurs very rapidly
because the action potential has so much amplitude, what does it reach during depolarization?
0mV and even positive mV membrane potential
- note: membrane potential at rest is -70mV
how fast is repolarization after a depolarization of action potential?
just as rapid as the depolarization
what is an overshoot?
the reversion period (‘reversing’ the potential above 0 mV)
how long is an action potential?
always the same duration
- 1 msec in a nerve cell
what is another name for an action potential?
a spike
when an excitable membrane is triggered to undergo an action potential, we say it?
‘fires’
what is the key to an action potential?
the change in membrane permeability to ions
which gated ion channel uses change of voltage/membrane potential as a mechanism of opening and closing triggering event?
voltage-gated channels
what is the triggering event of a mechanically gated ion channel?
- open in response to mechanical deformation
what is the triggering event of a chemically gated ion channel?
- open in response to some sort of molecule binding to the ion channel specifically neurotransmitters
which gated ion channel opens to initiate action potential?
voltage gated ion channels
what are the two voltage gated ion channels?
- voltage gated K+ (potassium) channels
- voltage gated Na+ (sodium) channels
how does a voltage gated K+ (potassium) channel operate?
simple with one one activation gate that opens once the threshold potential has reached - senses voltage difference and it opens
what are the two differences between Na+ and K+ voltage-gated channels?
- Na+ channels are much faster to respond to change in the membrane voltage / potential
- Na+ channels have in their cytosolic region an activation and an inactivation gates
what happens when threshold potential is reached at the channel level?
- potassium and sodium gates open
- sodium inactivation gate begins to close
- all occurs at the same event once the threshold is reached
- only thing that differs is how quickly these events occur
how fast do the gates of the sodium voltage gated channels open?
activation gate - very rapidly
inactivation gate - quite slow
how fast is the potassium’s gate to open?
quite slow
what has huge implications for the shape and physiology of the action potential?
the rate at which the voltage gated ion channels are opening
what is the state of the voltage gated channels at rest (-70mV)?
- all voltage-gated channels are CLOSED
- Na+ channels are in the configuration with the inactivation gate open but the activation gate closed
the electrical and concentration gradients favour sodium movement into or out of the cell?
into the cell
what leads to greater depolarization of the membrane?
inward movement of Na+ ions
what leads to the positive feedback cycle?
opening of more voltage-gated Na+ channels because of the sodium ions being entered from the already opened voltage gated sodium ion channels
when we have the initial depolarization, are all the voltage gated sodium channels opening?
no, only some
what does the positive feedback cycle esnure?
that once the threshold potential is reached, all the voltage gated sodium channels will open
- this is what is responsible for the explosive increase in Na+ permeability
there is so many voltage gated sodium ion channels that membrane permeability is how much more permeable to sodium than potassium when they are open?
membrane is 600 times more permeable to Na+ than to K+ when their is an active potential
- this is why there is the explosive depolarization event
why does depolarization stop very abruptly at +30mV in action potential?
because of the sodium voltage channel inactivation gate
what allows the very rapid depolarization event to occur in action potential?
the rate at which the sodium channels open and then inactive
how long is the ball and chain inactivation gate of a sodium channel in a closed and incapable of opening again state for?
until the membrane potential is back to its resting value
what resets your sodium ion channels?
bringing the membrane potential to rest
what happens if you want to generate multiple action potentials?
you need to first depolarize and repolarize the cell to ensure the sodium voltage gated ion channels are capable of opening again
what does the potassium gated ions respond to?
threshold potential being reached
- starts opening at the exact same time as your voltage gated sodium ion channels just occurs more slowly
what repolarizes after a depolarization of action potential?
the potassium voltage gated ion channels
- this makes the inside of the cell more negative because the potassium ions are leaving
summary of what happens/gets activated when threshold potential is reached
- rapid opening of voltage-gated Na+ channels –> Na+ influx until inactivation gate closes
- slow closing of the Na+ channel inactivation gates –> Peak; prevents the action potential from increasing further and exceeding +30mV
- slow opening of voltage-gated K+ channels (reverse polarity, returns membrane potential towards resting)
when do the potassium voltage gates begin to fully open?
at peak during action potential
- permeability increased to about 300x because the outward K+ movement is down its concentration AND electrical gradient at peak
what is responsible for the rapid repolarization of the action potential?
the electrical and chemical gradient pushing potassium out of the cell using all the K+ gated ion channels
how come there is a hyperpolarization after repolarization?
because the potassium ion gated channels take awhile to close again
when does an action potential only occur?
ONLY when the triggering stimulus AND the current that is generated via the opening of Na+ channels (graded potentials) are sufficient to reach the threshold potential
what is threshold stimuli?
if the strength of your graded potentials are sufficient to reach threshold
what is the threshold of most excitable membranes be?
15 mV less negative than the resting potential
- ex. resting of a neural cell is -70mV so its threshold is -55mV
what happens if the initial depolarization is lower than the threshold?
- action potential will not be activated - ONLY graded potential
- the positive feedback cycle does not start
what is weak depolarization that does not activate an action potential called?
sub-threshold potential
what is the stimulus that does not active an action potential called?
sub-threshold stimulus
what happens if the initial depolarization reaches the threshold?
- an action potential is generated
- all action potentials have similar amplitude regardless of how long it took to reach threshold (same magnitude and duration every time)
- once the threshold is reached –> All-or-None law
what is used for communication?
action potentials
if all action potentials have the same magnitude, how do we integrate the magnitude of the stimulus? (how do we feel the different levels of touch/pain)
the frequency of action potentials
- the rate at which they are firing
