Membrane Potential and Action Potential Flashcards
What is diffusion and flux and where is this method of transmission efficient / useful?
Movement of molecules down a concentration gradient until an equilibrium is achieved
Over short distances and when no energy input required
What is a flux of ions?
The number of molecules that cross a unit area per unit of time e.g. molecules.m^-2.s^-1
What are the properties of ions?
Like charged ions repel
Oppositely charged ions attract
What is voltage VS current VS Resistance?
Voltage - Same as potential difference. measured in Volts, generated by ions to produce a charge gradient
Current - measured / describes in Amps, it is the movement of ions due to the potential difference
Resistance - measured in Ohms, barrier that prevents movement of ions
In the picture: Volts doing the pushing of the current (Amps) against the resistance (Ohms)
Do all cells have membrane potentials (MPs)?
How can you measure membrane potential and what is the resting membrane potential of a typical nerve cell?
Yes
By placing one electrode inside the cell and the other in saline solution of zero-volts outside of the cell. The electrodes are both connected to a voltmetre, which shows the p.d. and so the MP
-70 mV
How do cells get across ions across their membranes (a hydrophobic layer) and what are their features?
Ion channels / pumps - they give the membrane permeability to the ions
Ions channels can be selective i.e. selective to the type of ion Ca2+ or Na+ etc.
Movement occurs when the channel is open
Can open and close in response to ligands / various MPs
How is the MP produced?
If the membrane is impermeable, how is MP affected?
Due to the diffusion of ions across a selectively permeable membrane
If there is no way for ions to get across, difference in ions inside and outside makes no difference to the MP, as the MP always equals 0
Explain what will happen in this picture? (Compartment 1 has Na+ ions, compartment 2 has K+ ions, and there are K+ channels that allow for the diffusion of K+)
What is the electrochemical equilibrium?
When K+ channels are introduced, K+ diffuses into compartment 1 down the concentration gradient, making compartment 1 more positively charged and compartment 2 more negatively charged
Eventually the difference in charges prevents the net movement of K+ ions into compartment 1 to stop
So an electrochemical equilibrium is reached - electrical forces exactly balance the diffusion forces
This forms a stable (trans)membrane potential (MP)
What is the equilibrium potential?
The potential at which the electrochemical equilibrium is reached - so the potential that prevents the diffusion of ions down the concentration gradient (no net movement of ions)
What is the Nerst equation and what is it used to calculate?
Equilibrium potential (E) - states E in mV
How can the Nerst equation be simplified?
This is a picture of a simplified Nerst Equation calculated:
By assuming T = 37 degrees C (like in our bodies) = 310 Kelvins
Convert natural log to log base 10
Typical concentrations of the ion inside and outside of the cell
What is the composition of the main fluid compartments in the intra and extra cellular fluids?
[At least learn the highlighted ones]
Using these typical intracellular and extracellular concentrations, calculate the theoretical E values (in mV)?
The theoretical E values for K+ and Na+ come out to be:
What is the typical value of E and why do membrane potentials not rest at the equilibrium of Na+ or K+?
The membrane is not fully specific in the ion it transports at any one time, so there is still some leakage of ions across the membrane
At rest, although K+ channels are the only channels fully open, there is still a small entry of Na+, so the E value instead rests at -70mV (rather than -90mV)
What is GHK equation and why is it better?
Takes into account the the relative permeability of each ion
More physiologically relevant
More specific / accurate / exact
P is the relative permeability of each ion, so when P=1 channel is open, P = 0.5 channel is open half the time, P=0 channel is closed
Takes into account the small permeability during RMP - e.g. Na+ channels only open for like 5% of the time
What do both these equations show during depolarisation?
The inside of the cell becomes more and more positive as the Na+ channels open, so the membrane potential increase
Some worked examples using the GHK equation to show how the membrane at resting potential is slightly permeable to Na+, despite supposedly being only permeable to K+, changes the RMP from -90mV to around -70mV?
.
What is depolarisation?
MP increases from negative and goes towards 0mV
What is repolarisation?
MP decreases (becomes more negative) towards the RMP
What is overshoot?
MP increases (becomes more positive) above 0mV
What is hyperpolarisation?
MP decreases (becomes more negative) beyond the RMP
A diagram to explain / label the terms depolarisation, repolarisation, overshoot and hyperpolarisation?
.
What are graded potentials?
Change in MP in response to stimuli e.g. light
They are dependent on the nature and size of the stimulus, and decay along the axon
What can different stimuli lead to in MPs, and what specific property causes these differences?
Some stimuli lead to hyperpolarisation, some lead to depolarisation
Some stimuli are stronger, and some stimuli are weaker and so lead to a larger ro smaller change in MP
Depending on whether the threshold value is reached, an AP is generated
These differences in MP are in response to different stimuli and are due to different ion permeabilities being activated according to the NT being released in the synapses between receptors, nerves etc.
Why do small stimuli that cause a small depolarisation in MP fade away along the axon, over a distance?
Because as the deploarisation travels along the axon, some of the charges ‘leak’ out of the axon, til the charge in the membrane is insufficient to cause an AP
What are graded potentials dependent on?
Length of axon
Size of stimulation
Nature of the stimulation
Where are graded potentials likely to occur?
Sensory receptors
Synapses
What can graded potentials do / why are they useful?
Can contribute to or prevent APs being formed
Filter out background stimuli
What are action potentials (APs) and where do they occur?
Known as nerve impulses in neurons - allow for excitation to be carried down an axon
Allow transmission over long distances e.g. all they way to the effectors
Occur in all excitable cells - neurons, muscle cells and some endocrine tissue
What is the diff between GPs and APs
idk
Why are APs important / useful?
Reliable and quick in neurons
How do action potentials work on an ionic basis? i.e. What does permeability of the ions depend on?
Conformational state of the ion channels
Opened by depolarisation e.g. Na+ channels open
Inactivated by sustained depolarisation - electrochemical gradient reached
Closed by repolarisation / hyperpolarisation
What 2 things happen when the membrane permeability of the ion increase?
The ion moved down the electrochemical gradient
MP moves towards equilibrium potential (E) of the ion
Why are Na/K pumps important?
Allow for the concentration gradient of the ions to be maintained - i.e. during RMP
Which is the basis or nerve impulses, as the opening of different ion channels can then activate an AP
However, pumps do not contribute directly to the AP (i.e. do not cause depolarisation, repolarisation etc.)
What is meant by an ‘all or none’ event?
Either it happens or it doesn’t- If the threshold value is reached, an AP is generated
What are the 5 phases of an AP?
RMP
Depolarisation
Upstroke / Overshoot
Repolarisation
Hyperpolarisation
All happen rapidly between 2-3 ms
Describe the different phases of the AP?
RMP - maintained as the membrane is more permeable to K+ than Na+, MP lies close to E of K+ than Na+ at -70mV
Depolarisation - receptor activated by a stimulus, which moves the MP towards 0 (positive direction) to the threshold value
Upstroke - threshold value reached, voltage gated Na+ channels open quickly at this mV, voltage gated K+ channels also open at this mV but more slowly so fewer K+ diffuse in, MP moves towards E of Na+
Repolarisation - Na+ channels are inactivated, Na+ stops diffusing into the cell, K+ channels are now fully open so K+ diffuses out of the cell down the electrochemical gradient, MP moves towards the E of K+
Hyperpolarisation - VG K+ channels remain open so K+ continues to leave down the electrochemical gradient. the VG channels close when the MP is close to the E of K+, eventually the MP returns to the RMP, resting potential K+ pumps stay open
How are the Na+ channels inactivated, what does it cause and why is it important?
The channel is a large protein that has 2 mechanisms at the threshold value, first it opens up to allow for the flow of Na+
After, a part of its protein structure blocks the channel to stop the flow of Na+
This causes a refractory period
New stimulus cannot be evoked as the Na+ channels are taken out of action, and the plug is removed during repolarisation
What is active propagation?
How the AP is conveyed down the axon
After an AP, Na+ diffuses along the axon, increasing the next part of the axon’s MP to reach the threshold value, triggering another AP, and so on
Where are VG channels located?
At the nodes of Ranvier
What are nodes of Ranvier and what is saltatory conduction?
Myelinated axons have gaps called nodes of Ranvier
APs are formed at these nodes, and in the myelinated (insulated) areas, the ions diffuse across so the next AP can be generated at the next node
What affects conduction velocity?
Diameter / axon size (conduction velocity is proportional to the square root of the axon diametre)
Myelinated or not (insulation)
What are some examples of diseases that reduce myelination of axons and how does this affect the person?
Multiple sclerosis and diptheria
Reduces the speed at which signals can be transmitted
What are the 3 main factors that influence the movement of ions across the membrane?
Charge 1+/- or 2+/-
The concentration of the ion on both sides of the membrane
Voltage across the membrane - influences drive across the membrane
Why is the K+ equilibirum potential (E) negative at -90mV, but the Na+ E positive at +72mV, when both these ions are positive?
Due to the relative concentrations of Na+ and K+ inside and outside the cell
There is a higher [K+] inside than outside the cell, so K+ diffuses out creating a negative E
Opposite for Na+. there is a higher [Na+] outside than inside the cell, so Na+ diffuses in creating a positive E
A negative E for K+ is required to attract the K+ to stop net outwards flow, a positive E for Na+ is required to repel the Na+ to stop Na+ from entering the cell
Which ion is important for the upstroke of and which for the repolarisation, and in which direction do these ions move?
Upstroke - Na+ moving down their concentration gradient into the cell
Repolarisation - K+ ions going down their concentration gradient out of the cell
What factors influence the speed of propagation of an AP along an axon?
Larger axons have a lower resistance, so ions move faster (conduction velocity is proportional to the square root of the axon diametre)
Linear relationship between conduction velocity and myelin thickness
