Membranes and Cell Excitation Flashcards
An electrical signal involves…
.a change in the balance of positive and negative charges
Name 5 important charged particles
Na+, K+, Cl-, Ca2+ and protein side chains
What controls ion movement inside and outside of a cell?
The membrane
What generates an electrical signal?
The transfer of ions through ion channels
What maintains gradients?
Ion pumps
Describe lipid bilayer permeability
Impermeable to most molecules, including ions
The inside of a cell contains…
an excess of anions and therefore there is a negative membrane potential (Em)
What is the voltage inside the cell called?
Membrane potential (Em)
What is the membrane potential of a neurone?
-65mV
What determines membrane potential?
The balance of charges
What type of process is ion movement?
Passive
Which ion channels are permeant?
Na+, K+, Ca2+, Cl-
What controls membrane potential?
Membrane proteins
Name the three types of gated channels
Non-gated, voltage, ligand
What are non-gated channels also known as?
Leak channels
What do voltage-gated channels do?
Generate action potentials
What do ligand-gated channels do?
Generate membrane potential changes at synapses
What type of cells are neurones?
Excitable cells
Neurones have a resting membrane potential of…
-60mV to -70mV
What factors influence ion movement across the cell membrane?
Unequal ion distribution of ions creating a chemical gradient
Electrical force when ions are attracted or repelled by the voltage inside the cell.
What determines membrane permeability?
The number of ion channels for a particular ion. Therefore ions which move more easily through the membrane, have a higher number of ion channels within the membrane.
What do efflux and influx mean?
Efflux = movement out Influx = movement in
Define equilibrium
An ion is in equilibrium when chemical gradient and electrical force are in balance
What is equilibrium potential abbreviated to?
Eion
What does the Nernst equation do?
Defines the voltage at which the ion is at equilibrium
What is happening at equilibrium potential?
The amount of efflux = influx. This is the voltage the cell needs to be at for there to be no net movement of the ions.
What is different between Eion and Em
Eion is a constant, Em can vary
What is ionic driving force?
the net force resulting from chemical and electrical influences.
Em - Eion
Explain ionic driving force
The force that drives the ion across the membrane when Em is not equal to Eion
In a neurone, what influences Em?
Na+ and K+
What ion equilibriums does Em rest between?
EK and ENa
How do we calculate Em?
The Goldman equation
What is Em dependent on?
Relative permeabilities (Pion)
Compare the relative permeabilities of Na and K
PK is 40x bigger than PNa
Why is Em closer to EK?
Because of the difference in permeabilities
What is happening at -65mV in a neurone?
Driving force acting on both Na+ and K+
Na+ influx
K+ efflux
What is making Na+ move in?
Chemical gradient and electrical force
What is making K+ move out?
Chemical gradient, but electrical force is making K+ move in. K+ doesnt move in however since the chemical gradient is greater than the electrical force moving in therefore the driving force is effluxing.
What ensures Em of -65mV is at rest?
Na+ influx = K+ efflux
Compare the driving force of Na+ and K+
Na+ = large K+ = small
Compare the permeability to Na+ and K+
Na+ = small K+ = large
Do ion pumps control Em?
No, they maintain concentration in the long term
What does the sodium-potassium pump do?
Exchanges internal Na+ for external K+
against concentration gradient
Is K+ higher conc on intracellular or extracellular?
intracellular
Is Na+ conc higher intracellular or extracellular?
Extracellular
What is the value for Ek?
-80mV
What is the value for ENa?
+62mV
When is the driving force for an ion >0?
whenever Em is different than the equilibrium potential of that ion
How is the resting membrane potential maintained?
By a large driving force of Na+ coupled with a low permeability
-small driving force of K+ with a large permeability
How does the sodium-potassium pump work?
- catalyses ATP breakdown
- exchanges internal Na+ for external K+
- against conc gradients (active transport)
- operate in background constantly
- not for controlling Em
How is the Resting membrane potential generated?
- K+ leak channels open and therefore since [K] greater inside the cell, this leads to K efflux
- [Na] lower inside cell and fewer Na channels in membrane therefore K efflux > Na Influx
- Em becomes negatively charged
- Voltage < 0 and therefore an electrical force is generated so the negative Em causes a drive for K and Na ions to move into the cell
- K efflux > K influx so K ions still move out and thus the electrical force gets stronger.
- Results in Em being closer to Ek
- Eventually Em has no net movement as K efflux = Na influx so voltage is set at -65mV
What causes Em at rest to be closer to Ek than ENa?
Ek = -80mV ENa = +62mV
Since K efflux > Na influx as membrane is more permeable to K ions.
K efflux is trying to bring Em to -80mV whereas Na influx tries to bring Em to +62mV
At rest Pk = 40 x PNa so Em is closer to Ek.
Why can the chemical gradient of resting membrane potential be considered as a constant?
Over short time scales, Changes in concentration are extremely small and hence the chemical gradient is almost constant.
