Lecture 3 - Electrophysiology of the cell membrane Flashcards
Chemical gradient
Concentration gradient
the energy provided by the difference in concentration across the plasma membrane
Electrical gradient
Potential difference
the energy associated with moving charged molecules across the membrane - when a membrane potential exists
Resting membrane potential
Charge difference between the two sides of the membrane, seen as a membrane potential of about -70mV
Outside the cell
high Na+, low K+ and high Cl-
Inside the cell
Low Na+, high K+ and low Cl-
Potential difference across the cell membrane is principally generated by..
a sodium-potassium ATPase
which pumps 3 sodium out for every 2 potassium in (2 potassium in increases intracellular potassium and decreases extracellular potassium, 3 sodium out increases extracellular sodium and decreases intracellular sodium). The primary regulator is the Na+K+ ATPase but there are also other ways things can move across such as passive transport (Na+ channel, K+ channel etc)
Electrochemical gradients
Set up by the ionic distribution across the membrane
Only works if specific ion channels open and (a few) ions flow
Maintenance of resting membrane potential
K+ and Na+ are moved against their concentration gradient - energy from ATP
Nernst equation
Eion=60/Z log (Co/Ci)
Eion = equilibrium potential 60 = constant (that accounts for temp, R, F) Z= valence (charge) Log = base 10 log Co= concentration outside Ci=concentration inside
Equilibrium potential
The equilibrium potential is the energy (expressed in mV) of a concentration gradient of an ion OR The electrical potential (in mV) that exactly balances the concentration of an ion
It is the charge pushing the ion in one direction and balancing the concentration gradient pushing it in the other direction and then charge you need to do that is the equilibrium potential
Equilibrium potential of sodium
+65mV
This is the potential difference across the membrane for sodium that would exactly balance the concentration gradient trying to push sodium inside of the cell
Equilibrium potential of potassium
-89 mV
Resting membrane potential
Is largely an OUTWARD K+-mediated equilibrium potential modified by a much smaller INWARD equilibrium potential for Na+ ions
Flow direction for potassium is our and charge gradient for potassium is in
Flow direction and charge gradient are in the same direction for sodium
Vm = -70mV (negative inside)
What would happen if sodium channels were open in terms of the equilibrium potential for sodium?
the equilibrium potential for sodium was about +65mV in our generic neuron so if the sodium channels were open there would be a tendency for the cell to be polarised towards +65mV so you get an overshoot beyond 0mV
Afterhyperpolarisation reaches…
the equilibrium potential for potassium
Depolarisation
Becoming more positive
Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge. In response to this chemical stimulus, sodium channels open within the membrane.
Repolarisation
Becoming more negative but still above RMP
Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge). Following an influx of sodium, potassium channels open within the membrane. The efflux of potassium causes the membrane potential to return to a more negative internal differential.
Caused as a result of the stimulus being removed and excess sodium ions being transported out of the cytosol
Hyperpolarisation
More negative than RMP
Hyperpolarization is when the membrane potential becomes more negative. It is the opposite of depolarisation.
At this level the sodium channels begin to close and voltage gated potassium channels begin to open. After hyperpolarization the potassium channels close and the natural permeability of the neuron to sodium and potassium allows the return to resting membrane potential.
Overshoot
More positive than zero
Local potentials
Local potentials are graded, decremental, reversible, and can either excite or inhibit the membrane.
Small local ion fluxes (channel openings) across the membrane means that the membrane potential changes over time. These changes are called local potentials - because they are localised to a subsection of the cell membrane
Measuring membrane potential
Can be measured by sticking an electrode inside the cell and then having a reference electrode outside the cell and then having a voltmeter showing the potential difference between the cells
Current injected by experimenter and what this causes is different changes in the membrane of the neuron
Positive current injected - change in membrane potential is positive (become more positive) and the same occurs for negative current injected (chang in membrane potential is negative, membrane potential becomes more negative)
This causes local potential which are passive changes in membrane potential caused by the flow of current across the membrane - in this instance by current injected by the experimenter but in real life it is caused by the opening of channels when synapses are active
Action potential vs local potential
Local potentials are graded, decremental, reversible, and can either excite or inhibit the membrane. In contrast, action potentials are all-or-none, nondecremental, irreversible and always excitatory.
Local potential features
Graded in size
Decrease amplitude over distance
Summate in time (temporal summation) and area (spatial summation)
Can depolarise or hyper polarise
Influence the generation of an action potential
Passive potentials features
Since local potentials are not actively propagated they can also be called passive potentials
Length constant
As you move away in distance the voltage decays with a constant (length constant) and it is essentially the distance over which the voltage decays to 37% of its original size
Axial resistance
Resistance along its axis or the internal resistance
Bigger the pipe, the more easily it will flow therefore said to have lower axial resistance
Smaller diameter means that length constant is shorter so the voltage will decay much quicker away from the point of origin which is to do with the intrinsic physical properties of the axial or internal resistance
Passive conduction will not transport a signal from one end of an axon to the other unless the axon is very short (i.e., on the order of its length constant) because passively conducted signals decrease in size rapidly with distance from their origin.
Length constant
As you move away in distance the voltage decays with a constant (length constant) and it is essentially the distance over which the voltage decays to 37% of its original size
Axial resistance
Resistance along its axis or the internal resistance
Bigger the pipe, the more easily it will flow therefore said to have lower axial resistance
Smaller diameter means that length constant is shorter so the voltage will decay much quicker away from the point of origin which is to do with the intrinsic physical properties of the axial or internal resistance
Action potential features
Threshold - often approx 10mV depolarised from RMP (estimate, different for different neurons)
Overshoots beyond zero towards ENa (equilibrium potential for sodium)
All-or-nothing
Regenerative - propagated without decreasing amplitude
Afterhyperpolarisaiton define
Afterhyperpolarization, is the hyperpolarizing phase of a neuron’s action potential where the cell’s membrane potential falls below the normal resting potential. Hence, hyperpolarization persists until the membrane K+ permeability returns to its usual value.
Equilibrium potential calcium ***
equilibrium potential is positive which means that it is going into the cell (its concentration gradient is that way)
Equilibrium potential chlorin ***
equilibrium potential is negative which means that it is going out of the cell (its concentration gradient is that way)
If the RMP is less than the equilibrium potential of Na+ ….
then moving Na+ out of the cell will not occur via diffusion must be an active process to move it against electrochemical gradient therefore use the Na+-K+ ATPase
Membrane permeabilities to ions
Membrane is essentially impermeable to sodium but is somewhat permeable (leaky) to potassium at RMP when the Na+-K+ ATPase is active (3 sodium out to 2 potassium in)
If the RMP us greater than the equilibrium potential of Na+
If the Na+ equilibrium potential was +65mV and the membrane potential was more positive than this e.g. +100 mV then ions would flow down the electrochemical gradient without active transport
If the RMP is -60mV it means
that the inside of the cell is -60mV if the outside of the cell is 0mV
