Session 3 - Resting membrane potential Flashcards
How is resting membrane potential measured?
-Micropipette containig conducting solution (KCl) inserted into cell membrane and records PD
What are the typical resting potentials of
i) Nerve cells
ii) Smooth muscle cells
iii) Cardiac and skeletal cells
i) -70mV
ii) -50mV
iii) -90mV
What enables membranes to be selectively permeable to ions?
- Channel proteins within the membrane are permeable to one or a few species of ions
- Enable ions to cross the PM via diffusion when they are open
How do ion channels differ from transporters?
- When ion channels open there is rapid flow of ions down the electrochemical gradient
- Transporters ping-pong molecules one at a time
What are ion channels which are open permanently in cell membranes called?
-Voltage-insensitive channels
What determines a cells selective permeability to ions?
-The number, type and selectivity of ion channels open within a membrane
What ion is the main determinant in RMP in most cells?
-K+
How does permeability to K+ set up the resting membrane potential?
- Cells permeable to K+ at rest so it flows out of the cell down its concentration gradient
- Anions cannot follow (as the cell is not permeable) so the inside becomes more negative relative to the outside (membrane potential)
- This membrane potential opposes outward movement of K+ as a chemical gradient is established (negative inside attracts K+)
- When the chemical gradient is equal and opposite to the electrical gradient of K+ an equilibrium is reached where there is no net movement of K+ and a negative membrane potential is maintained
Are large quantities of K+ involved in setting up the resting membrane potential?
-No, the amount of K+ that moves is very small and a slight efflux can maintain the RMP for a long time
What is the equilibrium potential of an ion?
-The membrane potential at which there is no net movement of ions across the membrane as the chemical gradient is = and opposite to the electrical gradient for that ion
What is the nernst equation?
-calculates the membrane potential at which there will be no net movement of ions given the intra and extracellular concentrations ie, calculates the equilibrium potential of an ion
If a cell was solely permeable to K+, what would be the resting membrane potential?
-Ek (approx. -95mV)
Which ions have a positive Ep?
- Ena
- Eca
Which ions have a negative Ep?
- Ek
- Ecl
What happens to the membrane potential if a cell is permeable to more than just K+?
-Permeability to all the ions contributes to the membrane potential
What happens to the membrane potential if the permeability to a particular ion increases?
-The membrane potential moves towards the equilibrium potential of that ion
How is a cells permeability to a particular ion increased?
-Open more ion channels for that ion
Why does a cell, which is only permeable to K+ at rest, not have a resting potential of -95mV (Ek)?
- Cells are not perfectly selective
- Channels flicker open and closed
- This allows for ions to leak down their concentration gradients into the cell
- This leakage contributes to the RMP, deviating it from Ek
What happens to the resting membrane potential during depolarisation?
-It becomes less negative (moves closer to 0/+ve)
Does depolarisation=action potential?
-No, depolarisation can be a few mV as it doesnt always reach the threshold for an action potential
What happens to the resting membrane potential during hyperpolarisation?
-Membrane potential becomes more negative and falls below RMP
What happens to the membrane potential during repolarisation?
-Returns closer to the resting membrane potential
Give examples of membrane potential involvement in signalling roles
- Action potentials in nerve and muscle cells
- Control of secretion of hormones and neurotransmitters
- Transduction of sensory information to electrical activity by receptors
What types of gating are ion channels controlled by?
- Voltage-gated
- Ligand-gated
- Mechanical-gated (stretch receptors respond to membrane deformity)
Where can synaptic trasmission occur?
- Nerve:nerve
- Nerve:muscle
- Nerve:gland
- Sensory:nerve
What are the two types of synaptic transmission?
- Fast
- Slow
Describe fast synaptic transmission
- The receptor is also an ion channel and binding of a ligand causes it to open
- Eg NachR
What is the main difference in slow synaptic transmission from fast synaptic transmission?
-The receptor and ion channel are separate in slow transmission
What are the two types of slow synaptic transmission?
- Direct G-protein gating
- Intracellular messanging
How does direct G-protein gating slow synaptic transmission occur?
- Ligand binds to receptor
- G-protein dissociates from receptor and translocated to the ion channel where it binds to the channel causing it to open
How does intracellular messaging slow synaptic transmission occur?
-Ligand binding initiates an enzymatic cascade throughout the cell which results in the channel opening
Which is the fastest kind of slow-synaptic transmission?
-G-protein gated
What is hyperkalaemia?
-Raised serum K+ conc above 6mM
Does NaKATPase contribute to the resting membrane potential?
-NaKATPase is electrogenic as it pumps 3Na+ out for every 2K+ in, and thus it can contribute a few mV, however the contribution is minimal
What is an Excitatory Post-Synaptic Potential?
-A membrane depolarisation caused by excitatory transmitters opening ligand-gated channels, causing the cell to be more permeable to Na+/Ca2+
How does an EPSP differ from an AP?
- Has a longer time course than AP
- Is a smaller depolarisation
Give examples of excitatory transmitters
- Acetylchline
- Glutamate
What is an Inhibitory Post-Synaptic Potential?
-Hyperpolarisation caused by inhibitory transmitters opening ligand-gated channels, increasing permeability to K+ or Cl-
Give examples of inhibitory transmitters
- Glycine
- Gamma-aminobutyric acid (GABA- an a’a which acts as a ligand for Cl- channels)
What does hyperpolarisation do to a membrane in terms of excitability?
-Decreased the excitability of a membrane
What is a resting membrane potential?
-The electrical potential across a membrane generated by the distribution of ions in fluids inside and outside of the cell
