Action Potential Flashcards
What is the action potential?
- Change in voltage across membrane
- Depends on ionic gradients and relative permeability of the membrane
- Only occurs if a threshold level is reached
- All or nothing
- Propagated without loss of amplitude
What initiates an action potential and where is it initiated?
Depolarisation to threshold initiates an action potential at the axon hillock
What happens if the conductance to an ion is increased?
If the conductance (g) to any ion is increased the membrane potential (Vm) will move closer to the equilibrium potential for that ion.
How is conductance increased?
Opening more ion channels - The conductance of the membrane to a particular ion is dependent on the number of channels for the ion that are open.
What is the resting membrane potential?
Neurons have a negative concentration gradient most of the time, meaning there are more positively charged ions outside than inside the cell. This regular state of a negative concentration gradient is called resting membrane potential.
Are there more sodium/potassium ions inside or outside the cell during the resting potential?
More sodium ions outside
More potassium ions inside
The concentration of ions isn’t static though! Ions are flowing in and out of the neuron constantly as the ions try to equalize their concentrations. The cell however maintains a fairly consistent negative concentration gradient (between -40 to -90 millivolts). How?
Name 3 ways
The neuron cell membrane is super permeable to potassium ions, and so lots of potassium leaks out of the neuron through potassium leakage channels (holes in the cell wall).
The neuron cell membrane is partially permeable to sodium ions, so sodium atoms slowly leak into the neuron through sodium leakage channels.
The cell wants to maintain a negative resting membrane potential, so it has a pump that pumps potassium back into the cell and pumps sodium out of the cell at the same time.
Which channels are closed during the resting potential?
During the resting state (before an action potential occurs) all of the gated sodium and potassium channels are closed. These gated channels are different from the leakage channels, and only open once an action potential has been triggered. We say these channels are “voltage-gated” because they are open and closed depends on the voltage difference across the cell membrane.
What are the 3 states a voltage gated sodium channel can exist in?
Deactivated (closed) - at rest, channels are deactivated. The m gate is closed, and does not let sodium ions through.
Activated (open) - when a current passes through and changes the voltage difference across a membrane, the channel will activate and the m gate will open.
Inactivated (closed) - as the neuron depolarizes, the h gate swings shut and blocks sodium ions from entering the cell.
Describe depolarisation
Depolarization - makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients) . Voltage-gated sodium channels at the part of the axon closest to the cell body activate, thanks to the recently depolarized cell body. This lets positively charged sodium ions flow into the negatively charged axon, and depolarize the surrounding axon. We can think of the channels opening like dominoes falling down - once one channel opens and lets positive ions in, it sets the stage for the channels down the axon to do the same thing. Though this stage is known as depolarization, the neuron actually swings past equilibrium and becomes positively charged as the action potential passes through!
How do we know which direction ions will move?
Depolarisation - Na+ electric and chemical gradients inwards - Na+ moves in moving Vm closer to ENa - generally will not reach the Na+ equilibrium potential bc other ion channels are open
As membrane potential rises above 0mV, electrical gradient changes in direction
When gradients @ equilibrium =, no net movement of Na+
K+ then has to move out to make membrane potential more -ve (the chemical conc gradient is slightly more then the electrical gradient here)
Describe repolarisation
brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.
Describe hyperpolarisation
makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.
What is the all or nothing principle?
Action potentials work on an all-or-none basis. This means that an action potential is either triggered, or it isn’t – like flipping a switch. A neuron will always send the same size action potential
What determines the strength of a signal?
When the brain gets really excited, it fires off a lot of signals. How quickly these signals fire tells us how strong the original stimulus is - the stronger the signal, the higher the frequency of action potentials. There is a maximum frequency at which a single neuron can send action potentials, and this is determined by its refractory periods.
What is absolute refractory period?
Absolute refractory period: during this time it is absolutely impossible to send another action potential. Nearly all sodium channels inactivated, and make it so no sodium will pass through. No sodium means no depolarization, which means no action potential. Absolute refractory periods help direct the action potential down the axon, because only channels further downstream can open and let in depolarizing ions.
What is relative refractory period?
Relative refractory period: during this time, it is really hard to send an action potential. Sodium channels recovering from inactivation.This is the period after the absolute refractory period, when the h gates are open again. However, the cell is still hyperpolarized after sending an action potential. It would take even more positive ions than usual to reach the appropriate depolarization potential than usual. This means that the initial triggering event would have to be bigger than normal in order to send more action potentials along. Relative refractory periods can help us figure how intense a stimulus is - cells in your retina will send signals faster in bright light than in dim light, because the trigger is stronger.
What is a purpose of refractory periods?
Refractory periods also give the neuron some time to replenish the packets of neurotransmitter found at the axon terminal, so that it can keep passing the message along. While it is still possible to completely exhaust the neuron’s supply of neurotransmitter by continuous firing, the refractory periods help the cell last a little longer.
How can we show experimentally that Na+ is responsible for AP depolarisation?
• The straight line is the predicted change in ENa as external [Na+] is
reduced.
• The peak of the action potential changes in a manner parallel to the changes in ENa.
• This supports the idea that the upstroke of the action potential is due to a large increase in permeability to Na+ ions.
As Na+ decrease on outside
Peak becomes more -ve
Slopes parallel to sodium equilibrium potential
Changing sodium on outside, changing value of quilibrium potential
Sodium influx key to generating action potential
How much does each AP increase the sodium in the axon?
So each action potential increases [Na+] in the axon by only 40µM. If the resting [Na+] is 10mM this represents an increase of 0.4%. For a larger diameter axon the change in concentration will be even smaller.
What does a voltage clamp do?
Enables membrane currents to be measured at a set potential
At 0mv, K+ moves out of cell
Open more slowly than Na+ channels
Do not inactivate
When return memb potential back to -70, k+ channels take a while to close
-70 all Na+ closed
At 0mv, influx of sodium
Maintain depolarisation but current decreases to 0
Na+ channels undergo inactivation
Inactivate quickly after opening
Describe how the time course of conductance changes during an action potential
Conductance - no of channels open
At rest closed, memb potential negative enough so they are closed
When open, up stroke, then number of open sodium channels decreased due to inactivation
Number of k+ channels increases after a delay as membrane potential comes back K+ channels close slowly
Membrane potential comes back down to a velue a bit more negative than resting potential
Vg k+ channels stay open or a while - drives memb potential towards -ve as vg+channels close, memb potential go back to resting
Describe the channel activity during an axonal AP
Raise to threshold, Vg Na+ channels open Na+ influx Depolarisation - initiates more Na+ channels to open Positive feedback needed to generate AP All or nothing Need to open enough Not an ap for every depolarisation Vg k+ open K+ efflux Na+ inactivate Ap back down Not many ions have to move Na+/k+ pump is not involved in repolarisation Role of pump maintains gradient in background
Describe recovery after an action potential
Cant open more channels after ap as Na+ channels inactivated so no 2 aps after each other
In rrp sodium channels recovering from inactivation
When closed they can open
From open state they then become inactivated
Part of channel plugs the pore to inactivate
They need to recover before passing current
When memb hyperpolarises sodium channels recover back to closed state
