Flashcards in Action Potential Deck (88):
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?
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?
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?
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?
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+?
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+?
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
What is happening in the rising phase of the action potential?
The sodium channels are opening letting sodium ions rush into the neuron and causing depolarisation
What happens in the overshoot that leads into the falling phase
The sodium channels close and the potassium channels have opened and let potassium flood out of the cell, reversing the effect caused by the inrush of sodium (repolarizing)
Why is there an undershoot?
The potassium channels stay up meaning the inside become even more negative (hyperpolarisation) but once they close it returns back to resting membrane potential
What did Hodgin and Huxley predit was the cause of the rising phase?
An increase in Na permability (pNa by the membrane)
What did Hodgin and Huxley predict was the cause of the falling phase?
pNa returning to normal (shutting of channels) with an increase in the K permability (pKa) my the membrane
What channels open in response to a voltage change near the channel?
Voltage gated ion channels
How many subunits are voltage gated ion channels made from?
What are 4 functional elements of a voltage gated ion channel?
1. Selectivity filter
2. Voltage sensor
3. Activation gates
4. Inactivation gates
Which of the functional elements of voltage gated ion channels apply to Na channels only?
What are the three states of the Na+ channel
2. Open allowing Na+ in to the neuron
3. Open but inactivated (blocked)
then back to closed again
What are the two different states of the K+ channel?
2. Open to allow K+ out of the neuron
No inactivation state, closes again
What is involved in a patch clamping recording?
A piece of membrane is suctioned up into a micropipette and the opening and closing of the channel can be seen
What does the patch clamping recording measure?
The current going going in and out of the cell
If the patch clamping recording shows the current directing inward what kind of channel must have opened?
A sodium channel because the sodium is positively charged and the going inward
In the falling phase of the action potential what is the state of the Na+ and K+ channels?
The Na+ channels are open but inactivated and the K+ channels are open causing the reverse of the depolairsation
During the undershoot what is the state of the NA+ and K+ channels?
The NA+ channels are closed and the K+ channels are open leading the the inside of the neuron becoming more negative leading to hyperpolarisation
What is the period of the falling phase known as when the Na+ channels are inactivated and K+ channels open?
The absolute refractory period
What is the period of the undershoot known as when they Na+ channels are closed and the K+ channels are just beginning to close?
The relative refractory period
What is the time between the Na+ channels opening and inactivating?
What is the time between the Na+ channels inactivating and closing completely?
What does "refractory" mean in the refractory period?
The membrane cannot generate another action potential
What is the minimum distance between two action potentials?
What is the time difference between the K+ ions being closed and them opening?
1ms which is why you get a rising phase because it takes the K+ channel longer to open
How is the Na+/K+ pump involved in the action potential?
Only to move Na and K ions back into theur original places - doesnt really play a big part
What is the flow of current along an axon from a single stimulus called?
Local current/electrotonic current
What happens longtitudnally down an axon when there is an action potential?
Since at overshoot in the AP the Na+ channels are open making the inside of the neuron positive at that part (depolarisation) there is an electrical gradient longitudinally due to one part being more positive than the other
What happens when an electrical gradient is set up between the two parts of the axon?
The part of the membrane ahead of the action potential become positive (depolarised) and once it reaches threshold an AP fires at point two
What does repetition of this depolarisation due to electrical gradient along the axon called?
Action potential propogation
What does the electrical gradient between one part of the axon and another cause to flow?
An electrotonic current
What direction does the AP propogation (electronic current) go when the axon is stimulated?
Electrotonic flows away from the site of stimulus in both directions
Why in a real axon would AP propogation only occur in one direction?
Due to the refractory period (thats why it only goes one way cause it cant jump back because that part of the membrane cannot send another AP)
What are the two factors the affect the speed at which the AP propogates down an axon
1. The diameter of the axon
2. The myelination of the axon
What happens to the speed of conduction along an axon when you increase the diameter?
It becomes faster
Why does increasing axon diameter increase the AP conduction velocity?
Because there is less internal resistance to flow
What animal has utilised the large diameter = better conduction factor?
The giant squid - axons have higher conduction velocities?
Is there higher condution velocities in myelinated or unmyelinated axons? why is this?
Myelinated because it increases the rate of spread of the electrotonic current
In a myelinated axon where do the APs occur?
At the gaps in the myelin known as the Nodes of Ranvier
What is it called when the APs form only at the nodes?
What are the myelinated parts of axon between the Nodes where APs occur called?
In myelinated axons what is the density of voltage gated ion channels at the nodes?
Very high density (10 x more than in unmyelinated)
Why would a high density of voltage gated ion channels at the node of ranvier increase conduction velocity?
Because the AP would come to threshold quicker
How many voltage gated ion channels are found along the internode?
Virtually none as there is no AP production here so no need for them
When would the fact the internode has no voltage gated ion channels be a problem?
In diseases such as multiple scelerosis where the myelin is destroyed and the action potentials will then move much slower
What would you called the patches of membrane at the node?
What would you call the patches of membrane at the internode?
Apart from at the nodes of Ranvier where else would you find a high density of voltage gated ion channels?
1. The axon hillock
2. The sensory ending of a mechano-sensory neurone
Why would you find a high density of channels in the axon hillock and sensory ending?
They are the places where the AP is initiated
The axon hillock and sensory ending in the mechano-sensory neurone can be called the spike initiation zones; what does this mean?
The area that decides whether or whether not an AP will occur based on input from EPSPs and IPSPs (summed by the dendritic membrane and the soma)
What the the dendrititic membrane and the soma in terms of electrical excitability?
If the dendritic membrane and the soma are both electrically inexcitable what is there role?
They sum the local currents because they reach the SIZ for the SIZ to then decide if an action potential should be fired
What makes the action potentials discrete events?
The fact that due to the refractory period a second AP cannot fire before the first has finished
What are the two functions of the refractory period in terms of AP propagation?
1. Ensuring they are discrete events
2. Ensuring they only propogate in one direction