Action Potential Flashcards
What does the migration rate of an ion depend on?
The size of the hydrated molecule (i.e. how big the ion is plus the water molecules that surround it in solution)
What other term is used to describe the migration rate of an ion?
The mobility of an ion
Why is the mobility of the Cl- ion bigger than that of Na+
Because due to Na+ having an extra electron shell the atomic radius is larger so the mobility smaller
Do small or large ions migrate faster?
Small
What does the difference in the migration rates of ion generate?
A diffusion potential due to the charge seperation
What influences the size of the diffusion potential?
The difference in the mobilities of the ions because a bigger charge seperation = a larger diffusion potential
What is a semi permeable membrane?
A membrane that is selective to some ions but not others
What is the mobility of an impermeant ion?
Zero as it cant go through the semi permeable membrane
In a semi permeable membrane what balances out the concentration gradient?
The electrical gradient
What is the difference between a diffusion potential and a membrane potential?
The membrane potential remains indefinitely
What causes the membrane potential?
The positive K+ ions moving into the cell produces a concentration gradient which an electrical gradient must oppose until it is equal to the concentration gradient and the system is at equillibium
Why would the membrane potential remain indefinitely?
Because the uneven distrubutuon of ions remains so the concntration gradient remains so the electrical gradient must remain to oppose it
Describe the differences in the concentrations of ions inside a squid giant axon?
Inside: lots of K+ and protein but little Na+, Ca2+ and Cl- compared to outside
What is the overall voltage (mV) of the inside of the squid giant axon?
-73mV
What is the reason for the big difference in voltage between the inside and the outside of the squid giant axon?
The inside contains a lot of protein carrying a negative charge as compared to the outside which carries none.
Why are there no proteins on the outside of the cell but lots on the inside?
The proteins are impermeant anions
At rest, which ions are the most permeable through the membrane?
K+ ions
Why are K+ ions the most permeant through the membrane at rest?
Because there are cation channels selective for K+ that are open when the neurone is at rest
What are the K+ selective ion channels open at rest otherwise known as?
Leak channels
Why cant Na+ ions get through these leak channels?
Their radius is too large
If there is more K+ on the inside of the neuron what does this mean for the concentration gradient?
There concentration gradient is going toward the outside of the neurone
What does the concentration gradient of K+ mean the electrical gradient will be?
IT will be opposing it in the opposite direction until they are at equillibrium
Why is the equillibrium potential for K+ ions slightly more negative than the resting membrane potential?
Because the electrical gradient must increase slightly to match the force on the concentration fradient
What is the equllibrium potential for K+?
-73mV
Since there is more Na+ on the outside of the neuron than the inside what will the concentration gradient be?
Directed toward the inside
Why does the electrical gradient follow the same direction as the concentration gradient of na+?
Cause it is a positive force directed to the inside until it is as positive as the outside to counteract the concentration gradient
What is the equillibrium potential for Na+?
+54mV
How do Na+ ions move out of the neurone and how do K+ ions move in (against their concentration gradients)
Using the Na+/K+ pump
Since the movememnt of the ion is against their concentration gradients in a Na+/K+ pump what is required to move them?
The movement of the ions is coupled with the splitting of ATP to ADP + Pi
How does the Na+/K+ pump contribute a small amount to the negative resting membrane potential (negative inside of the neurone)
Because 3Na+ ions are being pumped out but only exchanged with two K+ ions (electrogenic)
What is an intracellular recording?
Recording changes in voltage across a membrane using two microelectrodes, one inside and one outside the cell
What is an action potential?
When a neuron sends a signal down an axon away from the cell body
What causes an action potential?
The opening of sodium channels, Na+ flooding into the cell causes depolarisation and builds the voltage up towards zero, once it reaches the threshold an action potential will fire
What is the threshold?
About -55mV at which point the action potential WILL fire
What does it mean when you say the action potential is an all or nothing event?
When the membrane potential reaches threshold an action potential of a fixed size is always fired - there is no big or small action potentials