Membrane potentials Flashcards
How can glass capillaries achieve intracellular recordings
Microelectrode is filled with a conducting salt solution and a wire inserted into it to connect to an voltmeter that measures the difference in potential between the fine tip (<1um) and a ground
What does penetrating the neuronal membrane with the fine tip of an electrode reveal
A hyperpolarised resting membrane potential (-50 to -90mV, around -65mV)
Why is the plasma membrane called a lipid bilayer
It is made up of phospholipids with hydrophilic heads and hydrophobic tails that line up naturally to form a lipid bilayer when in an aqueous solution (Singer and Nicholson, 1972)
What does the fluid mosaic model describe
The structure of the plasma membrane as a mosaic of components, with cholesterol, proteins and carbohydrates floating in a sea of phospholipids
The phospholipids and embedded proteins can diffuse rapidly and laterally in the membrane
What are the 3 functions of the phospholipid bilayer
Impermeable, proteins allow communication and movement of molecules, insulator
3 functions of the phospholipid bilayer- impermeable
Impermeable to ions and organic molecules- can maintain the cell’s intracellular environment
3 functions of the phospholipid bilayer- proteins allow communicatino and movement of molecuels
The proteins that span from the intracellular to extracellular space allow the cell to communicate with its environment and allow selective movement of molecules across the membrane eg nutrients
3 functions of the phospholipid bilayer- insulator
The bilayer separates ionic charges in the intracellular and extracellular salt fluids, thus acting as an insulator between 2 conductors (capacitor), allowing charge to be stored on the neuronal membrane
What is the consequence of the asymmetric distribution of ions across the cell membrane with an impermeable membrane
Resting membrane potential of 0mV, as the negative and positive charges are balanced across the membrane (despite asymmetric distribution of ions)
Intracellular vs extracellular Na+ conc
Intracellular 18mM
Extracellular 145mM
Intracellular vs extracellular Cl- conc
Intracellular 5mM
Extracellular 115mM
Intracellular vs extracellular Ca2+ conc
Intracellular 100nM
Extracellular 2mM
Intracellular vs extracellular K+ conc
Intracellular 140mM
Extracellular 5mM
Intracellular vs extracelllar organic anions conc
Intracellular 75mM
Extracellular 15mM
What channels can mainly explain the hyperpolarised resting membrane potential
Leak channels in the membrane that are predominantly selective for K+ ions
What is the effect of K+ selective leak channels in the membrane on the movement of K+
K+ ions flow through these channels along their concentration gradient to from the inside to the outside of the cell, and accumulate on the outside surface of the membrane
What is the effect of K+ moving out of selective leak channels
As K+ is positively charged, there is a net accumulation of negative charges on the inner side of the membrane, and the membrane becomes hyperpolarised relative to the extracellular fluid
What is the effect on K+ of the membrane becoming hyperpolarised by movement of K+ out via leak channels
It generates an electrical force that attracts the K+ ions back into the cell
At first the diffusional gradient is stronger than the opposing electrical gradient and there continues to be a net efflux of K+
What happens as the membrane becomes progressively more hyperpolarised by the movement of K+ back into the cell
The system will eventually reach electrochemical equilibrium when the electrical gradient balances the diffusional gradient and there is no net flux of K+ across the membrane
What is the Nernst equation
Calculates the membrane potential at which equilibrium potential is reached by any single ion
How can we test the Nernst equation in real cells
By manipulating extracellular K+ concentration and examining the changes in membrane potential
The membrane potential at each conc should be predicted by the Nernst equation ie proportional to log ([K+]out (Bernstein, 1902)
What is the result of testing the Nernst equation in real cells by manupulating extracellular K+ concentration
Plotting extracellular K+ conc against RMP on a logarithmic scale reveals that it clearly deviates from the Nernst potential for K+ ions at low values of of [K+]out (Hodgkins and Horowicz, 1959)
Why is the RMP of cells not predicted by the Nernst equation for K+
The leak channels are weakly permeable to other ions, mainly Na+ ions moving into the cell (1/100 fold)
Other ions are moving across the membrane that are not taken into account by the Nernst equation for K+ alone
What can be used to calculate the correct electrochemical equilibrium potential for the leak channels (taking into account all ions) aka resting membrane potential
Goldman-Hodgkin-Katz equation- the average of Nernst potentials for each permeable ion weighted by their relative permeabilities
What happens to cations when they dissolve in water
The cations can dissociate as the partial negative charges of the water molecules surround the ion, creating a 3D sphere of water around the ion and allowing it to disperse in solution
What do ions have to shed in order to pass through the pore of the leak channels as single ions
Have to shed their hydration shells
How do ions shed their hydration shells at the pore of the leak channel
At the selectivity filter, the water molecules can be replaced by interactions with oxygen atoms from carbonyl or hydroxyl groups of the protein, literally puttling the K+ ion, allowing it to diffuse through the channel
How are the leak channels 100 fold more permeable to K+ than Na+
At the selectivity filter, the spatial configuration of the oxygen atoms from carbonyl or hydroxyl groups of the protein are better positioned for interactions with the K+ hydration shell, meaning it is harder for Na+ to shed its hydration shell to pass through the channel, enabling selectivity
What does setting up and maintaining ion gradients across the membrane depend on
Active transport to move ions up their electrochemical gradients
What does primary active transport use energy from
Hydrolysis of ATP
What are active transporters called
Pumps
What is the primary example of a membrane pump
The Na+ pump, which simultaneously pumps 3 Na+ ions out and 2 K+ ions in, using energy from hydrolysis of ATP
What does it mean to say the Na+/K+ pump is electrogenic
It directly causes a small hyperpolarisation of the membrane (few mV) due to imbalances in the movement of charge (net loss of one + ion)
What is secondary active trasport
When the energy stored in the electrochemical gradient is used to drive active transport aka movement of the ion up their concentration gradient is coupled with the movement of another ion down their concentration gradient
What are antiports
Ion exchangers- the driving ion and driven ion/molecule move in opposite directions
What is an example of an antiport
eg Na+/Ca2+ exchanger where the flow of Na+ down its electrochemical gradient back into the cell is used to expel Ca2+
What are symports
Cotransporters- the direction of transport is the same for the driving ion and the driven ion/molecule
What is example of a symport
K+/Cl cotransporter, where both K+ and Cl- move out of the cell
What are types of secondary active transport proteins
Symports and antiports
Evidence that the negative resting membrane potential of neurons is mainly due to leak channels not active transport
If we take a quiescent isolated neuron and block Na+/K+ pump using oubain, there is a small depolarisation due to the lost electrogenic activity of the pump, but the resting membrane potential does not collapse (Brisson et al, 2014)
How does oubain affect active neuronal networks differently to isolated neurons
In active neuronal networks, much higher demands for transporters to maintain intracelllar/extracellular environments so applying oubain can lead to rapid collapse of membrane potentials
eg glia use active transport to maintain ion gradients and uptake neurotransmitters, active transport used for neuronal signalling
What is the process of osmosis
If a membrane is semi-permeable to water, the water molecules will move down their concentration gradient from a region of low solute concentration to higher solute concentration
What is the effect on the water solution when a solid is placed into a chamber on one side of a semi-permeable membrane
The solid dissolves and becomes surrounded by hydration shells, leaving less free water molecules on this side
Water moves by osmosis down its conc gradient to this side, exerting hydrostatic pressure on the membrane that tries to push the water molecules back across the membrane
When does the system reach equilibrium in a chamber with a semi-permeable membrane where a solid is dissolved on one side
The system reaches equilibrium when the hydrostatic pressure balances the osmotic pressure
What impact can osmotic pressure on the membrane have on cells
Can cause them to shrivel or swell
What can cause neuron swelling
Accumulation of ions inside the neuron leads to the influx of water, and the corresponding increase in hydrostatic pressure will cause the neuron to swell
What can be the effect of cellular swelling
Reduces the volume of the extracellular space which affects the conc and diffusion of molecules in the extracellular fluid, which alters neuronal excitability
When may cellular swelling occur
May occur to some extent during periods of high electrical activity, but is more prononuced under pathological conditions eg ischemic stroke (loss of blood supply, leading to oxygen and glucose deprivation)
What normal long term processes involve changes in neuronal volume
Physiological processes like growth, endocytosis, endocytosis
What are aquaporins
Specialised water channels in the membrane that facilitate osmosis
Study showing the importance of aquaporins
Deleting the gene for Aquaporin4 in mice is associated with a sevenfold reduction in cell plasma membrane water permeability
Howevever, these mice still appear largely normal, so slow mechanisms of water transport appear sufficient to sustain normal brain function
Modelling the neuronal membrane as an electronic circuit- what are the leak channels represented as
A resistor, with the ability to conduct current specified in terms of resistance or conductance
If conductance is high, ions can move very rapidly through the pore and vv
Modelling the neuronal membrane as an electronic circuit- what are the ionic gradients across the membrane represented as
A battery (chemical driving force) in series with the resistor, which tries to clamp the membrane circuit at the Nernst potential of the channels
Modelling the neuronal membrane as an electronic circuit- what does the circuit give a transmembrane potential of
-60mV, with current continuously flowing through the leak conductance
Modelling the neuronal membrane as an electronic circuit- what can predict the change in transmembrane potential when current is injected into the circuit
The change in transmembrane potential can be predicted by Ohm’s law
Modelling the neuronal membrane as an electronic circuit- what property of the cells causes the time delay in reaching the membrane potential predicted by Ohm’s law
Due to the capacitance of the membrane (consists of an insulator separating 2 electrical conductive surfaces which can be modelled as a capacitor in parallel with the membrane conductance
Modelling the neuronal membrane as an electronic circuit- what happens to current injected into the circuit in terms of changing the voltage across the membrane circuit
To change the voltage across the membrane circuit, charge must be stored on the membrane capacitor- injected current is split between the battery/resisitor branch and the capacitor branch of the circuit
Modelling the neuronal membrane as an electronic circuit- why is there a time delay in reaching the membrane potential predicted by Ohm’s law due to membrane capacitance
At the onset, most current flows onto the capacitor but this decreases as the capacitor is charged up, until equilibrium is reached when all the current flows through the battery/resistor branch
Modelling the neuronal membrane as an electronic circuit- what happens when the current injection is turned off
The capacitor discharges back through the battery/resisto branch and the membrane potential decays back to rest
What determines the rate at which the transmembrane voltage changes
The membrane time constant (t=Rm*Cm)
Analogy with hydraulics- what is the resistance represented by
The thin bit of pipe in the circuit- as the pipe gets thinner, the force needed to applied in order to give the same flow rate would increase
Analogy with hydraulics- what is the capacitor modelled as
A pipe containing a sealed rubber diaphragm connected in parallel with the thin resistive pipe
Analogy with hydraulics- what happens when the constant current generator is turned on
Water flows down the capacitor pipe causing the diaphragm to bulge which pushes back against water flow
The diaphragm will continue to stretch until it reaches a stable state where the force generated by the stretched diaphragm= force generated by the pump
At this point, all water flow will be through the resistive pipe
What biological limits does the axon face in conducting electrical charge
Charge is carried by ions rather than free electrons, axon is poorly insulated and surrounded by electrically conductive extracellar fluid so the current could leak out
What overcomes the biologial limits of the axon
The action potential of the axonal membrane- ‘potential’ refers to separation of electrical charge across the membrane
How is information encoded by neurons
Encoded in the pattern of of electrical impulses aka frequency of action potentials of neurons, distribution and no of neurons firing action potentials in a nerve
What type of membrane do cells that can generate and conduct action potentials have
Excitable membranes
What is resting membrane potential
Refers to the difference in electrical charge across a membrane when a cell with an excitable membrane is at rest aka the cytosol along the inside of the membrane is negatively charged compared to the outside
When does an action potential occur
When this difference in charge across the membrane is reversed for 1/1000 second, meaning the inside of the membrane becomes positively charged compared to outside
What is the intracellular fluid of the neuron called
Cytosol
What is a divalent ion
Difference of 2 between an atom’s no of protons an electrons
What are hydrophilic substances
Have a net electrical charge and will dissolve in water eg ions
What are hydrophobic molecules
Substances with nonpolar covalent bonds no net electrical charge, won’t dissolve in water
What do all amino acids consist of
Central carbon atom covalently bonded to H, amino group, carboxyl group, and variable R group
What affects the chemical reactions different amino acids can participate in
Differences in size and nature of R groups
What property does the exposed surface of channel proteins have
Chemically heterogenous- consists of hydrophilic surfaces with exposed polar R groups exposed to watery environments either side of the membrane, and hydrophobic surfaces that associate with the bilayer
Describe the structure of ion channels generally
Typically consists of 4-6 similar proteins assembled to form a pore across the membrane- differing subunit composition affects the properties of different channels
What special properties of selectivity do many ion channels have
Ion selectivity- determined by the pore’s diameter and the nature of R groups lining it
Gating- can be opened/closed by changes in the membrane’s local microenvironment
What are concentrations of substances expressed as
moles (no of molecules) per l of solution
One mole is 6.02 x 10^23 molecules
How is concentration appreviated eg for an NaCl solution
[NaCl]
What is diffusion
The net movement of ions from regions of high conc to low conc down their concentration gradient
What is electrical current
The movement of electrical charge represented by I and measured in amps
What 2 factors affect how much current will flow
Electrical potential and electrical conductance
What is electrical potential
Voltage- the force exerted on a charged particle
Reflects the difference in charge between eiher side of the membrane
Measured in volts (V)
How does electrical potential affect current
As the difference in charge between the 2 sides of the membrane icnreases, more current will flow
What is electrical conductance
Relative ability of an electrical charge to migrate from one point to another (the inverse of electrical resistance, relative INABILITY)
Represented by g, measured in siemens
What does electrical conductance depend on
The no of ions electrons available to carry electric charge, and how easily they can travel through space
What is Ohms law (not formula)
The relationship between potential, conductance and the amount of current that will flow
What is the formula for Ohm’s law
I = gV I= current, g= conductance, V=potential
What does it mean for current is th conductance or potential difference is zero
No current can flow
What is required to drive an ion electrically across a membrane, given current depends on conductance and potential
Channels permeable to that ion to provide conductance, electrical potential difference ie voltage across the membrane
What is membrane potential
Voltage across the neuronal membrane at any time, Vm
What is ionic equilibrium potential
The electrical potential difference that exactly balances an ionic conc gradient if the membrane were selectively permeable to that ion alone
What can be the effect of a tiny change in ionic concentration either side of the membrane
Large changes in membrane potential- thus equilirium potential can be reached with a miniscule change in ionic conc across the membrane
What does the net difference in electrical charge occur at the inside and outside surfaces of the membrane
Ions either side of the thin phospholipid bilayer interact electrostatically
Thus, charges inside and outside the neuron are mutually attracted to the cell membrane, so net negative charge is localised at the inner membrane surface
What is the ionic driving force across the membrane
Proportional to the difference between the real membrane potential and the equilibrium potential for that particular ion
What information does the Nernst equation use
Ion’s charge, concentration difference of the ion across the membrane, and temperature
Effect of increased temp on Nernst equation
Increased diffusion, increased potential difference at equilibrium
Effect of increased electrical charge of particles on Nernst equation
Decreased potential difference needed to balance diffusion
What ions are more concentrated in vs outside the membrane
K+ is more conc inside the membrane, Na+ Cl- and and Ca2+ are more concentrated on the outside,
What is the membrane potential that would be reached if the membrane were selectively permable to only K+
-80mV
What is the membrane potential that would be reached if the membrane were selectively permable to only Na+
62mV
What is the membrane potential that would be reached if the membrane were selectively permable to only Ca2+
123mV
What proportion of the total ATP used by the brain does the Na+/K+ pump expend
Up to 70%
What creates the super low conc of Ca2+ inside the membrane
Calcium pump actively transports Ca2+ out the cytosol, intracellular calcium-binding proteins, organelles that sequester cytosolic Ca2+ eg mitochondria
Study that supports the selectivity of K+ channels
In 1987, researchers determined the amino acid sequences that line the pores of potassium channels in the fruit fly Drosophila melanogaster
A mutant strain called Shaker responded to ether vapours by shaking, caused by a defect in a type of K+ channel, and the gene was mapped
What is a pore loop
A critical part of the selectivity filter that makes leak channels selectively permeable to K+
Study in which the pore loop was discovered
Miller and MacKinnon (1988)- scorpions poison their victims via toxins that bind tightly to a site within K+ channel pores and block them
Allowed them to identify the exact region in the amino acid sequences that formed the selectivity filter
Study showing the importance of selectivity of K+ channels (mice)
Miller (1988)
A Weaver strain of mice have a mutated amino acid in their K+ channel pore loops, allowing Na+ to pass through and making the neuron’s membrane potential less negative, disrupting neuronal function and leading to cell death
The affected neurons are found in the cerebellum, important for motor coordination, explaining the difficulty of movement in the mice
Example of a neurological disorder in humans that can be explained by mutations of specific K+ channels
Epilepsy (Stoffel and Jan, 1998)
Example showing the sensitivity of the membrane to K+ concentration
A tenfold change in extracellular K would increase the membrane potential from -65 to -17mV, depolarising the neuron
What is depolarisation
When the membrane potential increases from the normal resting value to a less negative value
What has been the result of membrane sensitivity to K+ conc
Evolved mechanisms that regulate extracellular K+ conc in the brain eg blood-brain barrier and glia especially astrocytes
How does the blood-brain barrier regulate extracellular K+ conc in the brain
Brain capillary walls are specialised to limit the movement of potassium into the brain’s extracellular fluid
How do glia, especially astrocytes regulate extracellular K+ conc in the brain
Astrocytes take up extracellular K+ when periods of neural activity increase its concentration via membrane K+ pumps and channels
What is potassium spatial buffering
The taking up of K+ by astrocytes increases their internal potassium concentration, dissipated by the extensive network of astrocytic processes over a large area- a mechanism of regulating external K+ conc
Example of an excitable cell that is not protected from extracellular K+ increases
eg muscle cells don’t have equivalent protection mechanisms, blood K+ elevations can have serious consequences on body physiology
What type of ion channels are the ones involved in neuronal signalling
Gated ion channels- they actively open and close in reponse to various stimuli
What the 2 types of ion channel
Gated ion channels, resting/leak channels
What are resting/leak channels
Usually open, contribute to the resting potential
What types of gated ion channels are
Voltage-gated, ligand-gated (regulated by chemical transmitters), and mechanically- gated channels (regulated by pressure or stretch)
Most ion channels are selective for…
A particular ion
How many ions per second may pass through a single ion channel
Up to 100,000,000
How are sodium channels selective
Hille (1984)- an Na+ ion binds at an active site at the selectivity filter, where a -ve amino acid and water molucule lining the walls stabilise the Na+’s +ve charge
The K+ ion is too big, and is not effectively stabilised by the filter so can’t pass through
What is the effect of ions binding to the ion channel very weakly as they pass through
The bonds only last around <1 microsecond, allowing high conductance rates needed to rapidly change membrane potential during signalling
Evidence showing how gated ion channels open and close
Evidence from high-res electron microscopy and image analysis of the gap junction type of ion channel suggests closing/opening the channel involves a twisting and turning of the 6 subunits
What are ligand gated channels regulated by
The binding of chemical ligands eg neurotransmitters, hormones in the extracellular environment, or second messengers within the cell, releases energy which opens the channel
What 3 states do gated ion channels have
Closed and activatable (resting), open (active) or closed and nonactivatable (refractory)
What are voltage-gated channels
Opening and closing are associated with movement of a region of charge in the channel (caused by changes in membrane voltage) through the electric field of the membrane
What is the rate of transition between open and closed state of oltage-gated channels
Less than 10 microseconds, steeply dependent on membrane potential
How can transmitter-gated channels enter refractory states
Can go through desensitization through receiving prolonged exposure to the ligand
How can voltage-gated chanels go through inactivation
Following the change in membrane potential caused by activation
In Na+ and K+ channels it is controlled by a subunit of the channel that causes a conformational change
In Ca2+ channels, Ca2+ influx means Ca2+ may bind to a channel control site, or activate intracellular enzymes that inactivate the channel
How can each ion channel type differ
Each type has isoforms that differ in rate of opening/closing, conductance and sensitivity to different activators, adapting them to a different function
What 3 families can the genes encoding ion channels be grouped into
Genes that encode voltage-gated ion channels, genes for ligand-gated channels, and genes for gap-junction channels
What state are msot gated channels in when the membrane is at rest
Closed
What is hyperpolarisation
When the inside of the cell becomes more negatively charged and a more negative membrane potential is created
What happens to voltage gated ion channels at the threshold
Te cells respond actively by opening voltage-gated ion channels sufficient to produce an all or none action potential
Why don’t organic ions in the neuron leave
They are too large to permeate the cell membrane
Is the cell at equilibrium when its at its resting membrane potential and why?
No- metabolic energy must be used to maintain the ionic gradients across the membrane to maintain a steady state
Study showing how altering each external ion conc alters membrane potential (squid giant axon)
Hodgkin and Katz (1949)- systematically applied the Goldman equation to analyse how alternal external ion conc in squid giant axon changed the membrane potential
If membrane potential is measured after he conc change, before the internal ionic conc has altered, external K+ has strong effect on resting potential, Cl- has a moderate effect, Na+ has little effect
How do muscle cells/neurons differ from other types of cell in terms o their resting membrane potential
Most cells can’t cause a change in their permeability so can’t change their membrane potential, while muscle cells/neurons can
