Membrane Potential Flashcards
What are the causes of the membrane potential?
Unequal distribution of ions
Selective ion channels
What is the equilibrium potential for an ion X?
Definition and Equation
Membrane potential at which X will be in equilibrium (no net movement), given the concentration of X outside and inside the cell
Ex=(RT/ZF)*ln(C2/C1)
If a membrane is selectively permeable to X only what is it’s membrane potential?
Membrane potential = Ex
Define: Depolarisation
Hyperpolarisation
Depolarisation
Depolarisation: Membrane Potential becomes less negative
Hyperpolarisation: Membrane Potential becomes more negative
Repolarisation: Membrane Potential becomes less positive
What are the characteristics of the Action Potential?
Only occur when the threshold potential is reached
Propagated without loss of amplitude
What happens to the membrane potential when the cell membrane becomes more permeable to an ion?
Membrane potential moves closer to the equilibrium potential of that ion
Define conductance
How permeable a membrane is to a particular ion (and how much that ions equilibrium potential contributes to the membrane potential)
Depends on the number of ion channels for that ion
What are the types of gated ion channels?
Ligand (binding of another molecule)
Voltage (change in membrane potential)
Mechanical (membrane deformation)
What is the differences between fast and slow synaptic transmission?
Fast: the receptor is the ion channel
Slow: the receptor and ion channel are separate
Both use ligand-gated ion channels to change selectivity of the membrane to certain ions
Describe the types of fast synaptic transmission
Excitatory: binding causes depolarisation
Inhibitory: binding causes hyperpolarisation
Fast synaptic transmission has a longer time course than action potential (based on the concentration of ligand)
Describe the types of slow synaptic transmission
GPCR: activates G-Protein which bind to receptor (quite rapid, localised)
Intracellular messenger: messengers formed from the action of enzymes bind to receptor (ubiquitous, can cause amplification cascade)
Describe the stages of the action potential
Change in the Membrane Potential (depolarisation)
Once Membrane Potential reaches threshold potential, Voltage-gated Na+ channels open
Causes an influx of Na+ into the cell and further depolarisation
When the membrane potential reaches its peak (Na+ channels become saturated) the Na+ channels inactivate and the K+ channels open slowly
K+ starts to leave the cell
This causes repolarisation and the membrane potential starts to return to its resting potential
When the membrane potential reaches its resting potential the K+ channels close slowly leading to hyperpolarisation
Describe the features of the Na+ channel:
1 alpha unit (4 subunits linked together)
6 transmembrane domains in each subunit
S4: positively charged voltage-sensing domain
S5-S6: pore forming domain (channel)
Between the 3rd and 4th subunit is the Inactivation gate
Describe the features of the K+ channel:
4 individual alpha units (4 subunits) 6 transmembrane domains in each subunit S4: positively charged voltage-sensing domain S5-S6: pore forming domain (channel) No Inactivation gate
Describe the states the Na+ channel can be in
Open (activated): cell permeable (voltage-dependent)
Activation gate: open
Inactivation gate: open
Inactivated: cell impermeable (time-dependent)
Activation gate: open
Inactivation gate: closed
Closed: cell permeable if big enough impulse
Activation gate: closed
Inactivation gate: open
Define Absolute Refractory Period (ARP)
Define Relative Refractory Period (RRP)
ARP: channels are the inactivated state
RRP: channels are in the closed state
Describe how local anaesthetics work
Reversible block Na+ channels from within cells
Lipophilic, uncharged bases (B) diffuse across cell membrane
In the cells they become protonated (BH+) bind with Na+ channels inactivating them
Describe the different pathways local anaesthetics can take
Hydrophobic pathway: aminoesters diffuse through membrane (not use-dependent)
Hydrophillic pathway: aminoamides diffuse from cytosol (use-dependent - only inactivate open channels)
(Aminoesters vs. Aminoamides: no. of ‘i’’s in the name)
Define length constant
Distance for the action potential to decrease to 1/e of its original value
Define capacitance
Define resistance
Capacitance: ability of the membrane to store charge
Resistance: difficulty of ion passing through the membrane (opposite of conductance)
How do myelinated axons conduct action potentials so well?
Provides insulation from negative extracellular charges
High resistance: prevents leakage of ions (decreases conductance)
Low capacitance: prevents slowing of ions (increases length constant)
What is saltatory conductance?
Node to node conductance of an action potential
Describe saltatory conductance
Action potential (Na+ influx) only appears at the nodes Myelin insulates the cell membrane (decreases conductance) The change in membrane potential is passed through the cytosol (local current flow)
What effect does demyelination have on the axon and action potential?
Decreases resistance
Increases capacitance
Therefore the action potential may not reach the next node or be weaker than the threshold potential
