Membrane Potential Flashcards
What is a P-type ATPase?
An ion channel that hydrolysed ATP to move the ion, forming an phosphoenzyme intermediate
What is approximately the resting membrane potential of a cell?
-70mV
What is mainly responsible for maintaining a resting membrane potential of -70mV?
The diffusion of K+ ions out of the cell
How would you define a symporter ion channel?
An ion channel that transports 2 different types of ion in the same direction
By which mechanism does a secondary active transporter function?
A secondary active transporter transports its ion(s) using the energy of an electrochemical gradient, as opposed to the hydrolysation of ATP in primary active transport
What 2 factors does the Na+ H+ Exchanger regulate in a cell? Explain how.
The Na+ H+ Exchanger regulates pH, via the removal of H+ ions from within the cell.
It’s also regulates cell volume, by pumping Na+ ions into the cell - water follows these ions into the cell down the concentration gradient that the Na+ ion movement creates.
Although indirectly, what ion transporter plays a major role in the maintenance of a cells pH?
The Na+ K+ ATPase pump
What 3 osmotically ‘active’ ions play a key role in the movement of water through a cell? How does water react to their movement?
Na+, K+, and Cl-
In general,Mayer follows these ions in/out of the cell
Do animal cells have a positive or negative resting membrane potential?
Negative
Define the membrane potential.
The magnitude of electrical charge across a plasma membrane
State the 2 main factors that are crucial in maintaining a membrane potential
Asymmetric distribution of ions across a membrane
Selective permeability
Are K+ ion concentrations higher inside or outside the cell?
Inside
Are cations positively or negatively charged?
Positively charged ions
What type of molecules is the lipid bilayer of the plasma membrane permeable to?
Small uncharged molecules
What equation is used to work out the chemical,gradient of an ion?
RTln( (X)out / (X)in )
R = gas constant T = temperature in Kelvin
At equilibrium, what are the chemical and electrical gradient in regards to one another?
In balance
What is the Nernst Equation used to calculate?
The resting membrane potential at which a specific ion will be in equilibrium (an ions concentration and electrical gradient will be in equilibrium)
Which ion in particular plays the most prominent role in maintaining the resting membrane potential?
K+ ions
Will an ion prefer to move down its chemical or electrical gradient first?
An ion will move down its chemical gradient before its electrical gradient
What are the equilibrium potentials for Na, K, Cl, and Ca?
Na = 70mv K = -95mv Cl = -96mv Ca = 122mv
What is depolarisation?
Depolarisation is where the membrane potential of the cell decreases, causing the interior of the cell to become more positive
What is the Golman-Hodgkin-Katz equation used to work out?
It is used to work out the membrane potential of a cell based on its permeability to Na, K, and Cl
What is hyperpolarisation?
An increase in the size of the membrane potential, where the interior of the cell becomes more negative
How many acetylcholine molecules must bind a nicotinic acetylcholine receptor for it to open? What will this then allow through?
2 acetylcholine molecules must bind the receptor for it to open - it will let through cations (namely Na and K)
Concerning ion channels, how many ways of ‘gating’ are there? What are they?
3 ways
- ligand gated
- voltage gated
- mechanical gated
List 2 inhibitory neurotransmitters. How do they act?
Glycine and GABA - they act by opening anion ligand-gated channels which allow K and Cl to enter the cell - this causes hyperpolarisation, meaning the cell will not be able to fire an action potential
In the Goldman-Hodgkin-Katz equation, which ion has an opposite orientation to the other?
Cl (in/out)
How does opening a nicotinic acetylcholine receptor affect the membrane potential of a cell?
It lets in cations, particularly Na and K, moving the membrane potential towards 0mv (an intermediate between the membrane potentials of Na and K)
List 3 excitatory neurotransmitters. What do they cause?
Acetylcholine, glutamate, and dopamine - they cause depolarisation of the cell via influx of ions such as Na or Ca
Where might synaptic transmission be slow? Where might it be fast?
Synaptic transmission may be slow when the initial receptor and eventual ion channel are seperate proteins - synaptic transmission will obviously quicker if the receptor protein is also an ion channel
What is the resting membrane potential of a cardiomyocyte?
-90mv
Explain the mechanism of an action potential in cardiomyocytes.
- initially, fast-acting Na channels begin opening, causing the cell to depolarise
- as membrane potential reaches -70mv there is a surge of Na influx
- this rapid influx causes depolarisation to 0mv
- L-type Ca channels open when membrane potential reaches -40mv
- K channels open, causing these cations to leave the cell which leads to repolarisation (as positive charges are leaving)
- delayed rectifier K channels remain open and propagate the expulsion of further K from the cell, hyperpolarising it
What is the name of the equation given to calculate the membrane potential of a single ion?
The Nernst Equation
What is responsible for the unequal distribution of ions between the intracellular and extracellular fluid?
Ion transporters
How can you measure membrane potential?
Using a microelectrode (which is really thin as to not cause the cell to burst) attached to a voltmeter - the microelectrode will be filled with a conducting solution (KCl)
What happens to the membrane potential when K ions leave?
It hyperpolarises
What is the Nernst equation (with constants at room temperature)?
E = 61/Z log10(ion out / ion in)
Z = valency
In which direction does Ca flow in a myocytes when it undergoes an action potential?
It flows intracellularly
What happens to the membrane potential when Na ions enter?
The membrane potential depolarises
Do action potentials propagate with or without a decrease in amplitude?
They propagate without a decrease in amplitude
If a cells permeability to a specific ion is increased, how will this ion act in the cells membrane potential?
The ion will act to reach its own equilibrium potential, and will act on the membrane potential in whatever manner to get closer to its own equilibrium potential
How can you show experimentally that Na+ is responsible for the initiation of an action potential?
The peak of an action potential changes in a manner parallel to the changes seen when Na+ attempts to reach is equilibrium potential - therefore the upstroke of an action potential is due to a large increase in a permeability to Na+ ions
Do you need a large influx of ions to generate an action potential?
No
How do K+ channels act in comparison to Na+ channels?
K+ channels open more slowly (causing repolarisation) and also close more slowing (leading to hyperpolarisation) in comparison to Na+ channels
Does the Na K ATPase pump influence repolarisation of a cell?
No
Are Na+ channels inactivated or closed in the absolute refractory period?
They are inactivated, and as such cannot generate an action potential
Describe that state of Na+ channels in the relative refractory period.
They are recovering from inactivation and beginning to close - therefore if a stimulus is great enough some channels may be able to propagate an action potential
What region of the Na+ and K+ channels contain positive amino acid residues? What is their function?
The S4 region - these positive residues contribute to voltage sensitivity
How does an action potential induce a conformational change in a voltage-gated channel?
The voltage-gated channel is surrounded by a voltage field - an action potential will cause a change in this field, and have a different effect on the charges of the voltage-gated channel - this different force on the amino acids causes them to undergo a conformational change
Concerning myelinated axons, which order do local anaesthetics block them?
1 - small myelinated axons
2 - unmyelinated axons
3 - large myelinated axons
Generally, what is the mechanism of local anaesthetics? Give an example.
Generally, local anaesthetics block Na+ channels - an example is procaine
What is membrane resistance dependent on? Therefore, what would produce a low membrane resistance?
Membrane resistance is dependent on the number of open ion channels - if there is a low membrane resistance, there will be many open ion channels
Define capacitance.
The ability to store charge
In terms of resistance and capacitance, what can increase the spread of local membrane resistance?
High membrane resistance, and a low membrane capacitance
How does increasing resistance increase local spread?
More channels are closed, so the charge is kept inside and can propagate further
How does a myelin sheath work?
Myelin sheaths act as good insulators, therefore local currents can depolarise adjacent Nodes of Ranvier much more easily
How does a myelin sheath increase conductivity?
Increases membrane resistance
Decreases membrane capacitance
What is the most common demyelinating disorder?
Multiple sclerosis
How does demyelination result in poor transmission of an action potential?
There are no Na+ channels directly under a myelin sheath - as this has been broken down, it can no longer insulate local currents, and an action potential cannot propagate as there are no voltage-gated Na+ channels
State the extracellular concentrations of Na+, K+, Ca+, and Cl-.
- Na+ = 145 mM
- K+ = 4 mM
- Cl- = 123 mM
- Ca2+ = 1-2 mM
State the intracellular concentrations of Na+, K+, Ca+, and Cl-.
- Na+ = 12 mM
- K+ = 155 mM
- Cl- = 4.2 mM
- Ca2+ = 10^-7 M
What percent of body weight can be attributed to water? How much water is therefore in a 70Kg female?
Water is responsible for 60% of an individual’s body weight
- 60% of 70 = 70 X 0.6 = 42 litres
What proportion of body fluid is intracellular and extracellular?
1/3 is extracellular - 2/3 is intracellular
What compartments comprise the extracellular fluids? What is their volumes?
The interstitial fluid (11 litres) and blood plasma (3 litres)
What ion is most greatly associated with the movement of water?
Na+ ions
What would be the consequence of vessels being permeable to plasma proteins?
Plasma proteins would move out of the blood vessels into the interstitial spaces - water would follow these plasma proteins down a concentration gradient, leading to oedema on a mass scale
How do intracellular concentrations of K+ compare to extracellular concentrations? Use appropriate figures.
Intracellular concentrations of K+ (155 mM) are much greater than extracellular concentrations (4 mM)
How do intracellular concentrations of Na+ compare to extracellular concentrations? Use appropriate figures.
Intracellular concentrations of Na+ (12 mM) are much lower than extracellular concentrations (145 mM)
How do intracellular concentrations of Ca2+ compare to extracellular concentrations? Use appropriate figures.
Intracellular concentrations of Ca2+ (10^-7 M) are much lower than extracellular concentrations (1 mM)
How do intracellular concentrations of Cl- compare to extracellular concentrations? Use appropriate figures.
Intracellular concentrations of Cl- (4.2 mM) are much lower than extracellular concentrations (123 mM)
What does all-or-nothing mean?
A threshold is needed to be met before an action potential is induced - if this threshold is not met, no action potential can form
What is meant by the terms activation and inactivation, in regards to voltage-gated ion channels?
Activation relates to when the threshold of a voltage-gated ion channel is met - inactivation relates to the period after activation, where the voltage-gated ion channels are in recovery
What is the absolute refractory period?
This is the period directly after an action potential where no action potential can be generated
What is the relative refractory period?
The period after the absolute refractory period where some Na+ receptors are just becoming active again, and so can be stimulated again to reach threshold and instigate another action potential
How do local anaesthetics act to block action potentials at peripheral nerves?
They block action potentials by blocking the entry of Na+ into the cell by blocking Na+ channels
Describe the mechanism behind a drug that acts as a depolarising blocker.
These type of drugs ensure the membrane remains depolarised, so another action cannot form
How does acetylcholine release result in an action potential at a skeletal muscle fibre?
Acetylcholine binds to a nicotinic muscle receptor at a neuromuscular junction - this instigates HAVE A LOOK
What is responsible for the unequal distribution of inorganic ions between the intracellular and extracellular fluid?
Ion channels and transporters
Given the concentration gradient that is set up, in which direction would you expect K+ ions to move?
To the extracellular space, outside the cell
Why is the membrane potential not parallel to any one ion acting on its membrane?
Due to the electrical influence of all the other ions that are having an influence on the cellular membrane
What equation is used to measure an ions resting membrane potential? Write it out.
The Nernst equation - E[ion] = RT/ZF Log([ion out / ion in])
What is the Nernst equation measured in?
Millivolts (mV)
How is glucose transferred from the gut lumen into the bloodstream?
Glucose is taken into the cell via SGLT1, a symporter than facilitates the transfer of Na+ and glucose into the cell - from here, glucose is transferred into the bloodstream by a GLUT2 uniporter
What type of transport transfers glucose from the gut lumen into the cells of the gut wall? Does this require energy?
Symport, with a Na+ molecule - this is secondary active transport as the Na+, K+, ATPase uses ATP to move Na+’out of the cell and set up a Na+ concentration gradient
How does insulin stimulate the uptake of glucose in adipose and skeletal muscle cells?
Insulin recruits GLUT4 glucose transporters from internal vesicles to the plasma membrane, increasing the transport capacity of the membrane
Other than glucose, what other macromolecules use the Na+ gradient set up by the Na+, K+, ATPase pump to move from the gut lumen into surrounding cells?
Amino acids
What prevents glucose moving back down its concentration gradient from cells when there is little glucose in the gut lumen?
Glucose is rapidly converted to glucose-6-phosphate upon entering the cell by the actions of glucokinase (cells) and hexokinase (liver) - glucose levels therefore never rise to high enough levels to move down its concentration gradient
How does uptake of glucose differ in different cell types? Why?
Glucose uptake differs depending on the type of glucose carrier the tissue expresses (GLUT1-7) - these transporters are also not permanently expressed - the level to which they are will depend on the tissue, and it’s priority in needing glucose
