Session 3 Flashcards
What is a Membrane Potential
- All cells have an electrical potential (voltage) difference across their plasma membrane.
- Changes in this membrane potential underlie the basis of signal transmission in the nervous system as well as in many other types of cells (including muscle and heart)
How do you measure a Membrane Potential?
- Membrane potentials can be measured using a very fine micropipette - a microelectrode - that will penetrate the cell membrane. The other electrode is placed in the extracellular medium.
- When the microelectrode impales the cell membrane, the membrane potential goes negative.
- Membrane potentials are always expressed as the potential inside the cell relative to the extracellular solution
Give some examples of resting mmbrane potentials
- Animal cells have negative restin membrane potentials that range from -20mV to -90mV
- Cardiac and skeletal muscle cells have the largest resting membane potentials: -80 - -90mV
- Nerve cells have resting potentials in the range -50 to -70mV
- Smooth muscle cells have resting potentials in the range -50 to -75mV
- Standard nerve cells = -70mV
Describe the selective permeability of the phospholipid bilayer
- Hydrophobic interior so it is permeable to small uncharged molecules (O2, C02, H2O and ethanol) but very impermeable to charged molecules (ions)
- Ion channels are proteins that enable ions to cross the cell membrane as they have an aqueous pore though which ions flow by diffusion
How is a membrane potential set up?
- The membrane is selectively permeable to different ions (depending on what channels are open)
- The permeability occurs by way of channel proteins (membrane spanning transport proteins that allow ions to penetrate).
- Ion channels are characterised by:
- Selectivity: one (e.g. selective for Na+, K+, Ca2+, Cl-) or a few ion species (non-selective cation permeabiity)
- Gating: the channel can be open or closed by a conformational change in the protein molecule.
- A rapid high rate flow of ions that is always down the electrochemical gradient for the ion
- Passive
Describe the ionic concentrations for a typical mammalian cell
How is a Resting Membrane Potential Set Up?
- At rest the membrane has open K+ channels which dominate ionic permeability (for most cells) so is selectively permeable to K+. K+ wll begin to diffuse out of the cell down its concentration gradient.
- Since anions cannot follow, the cell will become negatively charged inside.
- The membrane potential will oppose the outward movement of K+ ions and the system will come into equilibrium when the chemical (diffusion) gradient for K+ and the electrical gradent for K+ are equal and opposite.
- There will be no net movement of K+ but there will be a negative membrae potential.
- Thus the resting membrane arises becaue the membrane is selectively permeable to K+ ions.
What is the Equilibrium Potential?
The membrane potential at which there is no net movement of the ion across the membrane (Concentration gradient = electrical gradient)
What is the Nernst Equation?
Can be used to calculate the Equilibrium Potential, given the extracellular and intracellular ion concentrations.
(at 37 oC)
Z = Valency so will be 1 for K+ but 2 for Ca2+
Is the real cell perfectly selective to K+?
No
The real cell is not perfectly selectively permeable to K+ alone so its membrane potential will not be at E(K) (-90mV)
The Na+ and Ca2+ are voltage gated ion channels, normally closed at rest but occasionally flicker and allow little influx of Na+ and Ca2+ ions raising the resting membrane potential to -70mV - less negative. This is because of IMPERFECT PROTEIN CONFORMATION
The dependence of resting potential on K+ permeability means that changing the E(K) wll change the membrane resting potential. Increasing extacellular [K+] makes E(K) more positive so changes the membrane potential in the same direction.
Describe the resting membrane potentials of cells with LOWER resting potentials
- Lower selectivity for K+
- Increased contribution from other channels
- Smooth muscle cells: ~-50mV
Describe the resting membane potential of skeletal muscle cells
- Many Cl- and K+ channels open in the resting membrane
- Resting potential = ~-90mV. Close to both E(Cl) and E(K)
What forms of signalling between and within cells do changes in membrane potentials underlie?
Examples include:
- Action potential in nerve and muscle cells.
- Triggering and control of muscle contraction
- Control of secretion of hormones and neurotransmitters
- Transduction of sensory information into electrical activity by receptors
- Postsynaptic actions of fast synaptic transmitters
What does depolarization mean?
A decrease in the size of the membrane potential from its normal value.
Cell interior beomes less negative (NOT NECESSARILY POSITIVE, not necessarily an action potential)
.E.g. a change from -70mV to -50mV
Opening Na+ or Ca2+ ion channels will cause depolarization
What does hyperpolarization mean?
- An increase in the size of the membrane potential from its normal value.
- Cell interior becomes more negative
- E.g. change from -70mV to -90mV
- Opening K+ or Cl- channels will cause Hyperpolarization
What is the Goldman-Hodgkin-Katz Equation?
Theoretical treatment that fits real membranes quite well
What is a Fast Synaptic Transmission
- The receptor protein is also an ion channel
- Transmitter binding causes the channel to open.
- The transmitter itself causes the channel to open
What happens in an Excitatory Transmission?
- Excitatory transmitters open ligand-gated ion channel that cause membrane depolarisation; depolarising transmitters open channels wth positive reversal potentials.
- Channels are selective for Na+, Ca2+ and some cations (nAChR)
- These transmitters lead to excitation of cells and the change in membrane potential they cause is called an Excitatory Postsynaptic Potental (EPSP
- An EPSP is not an action potential - it is a smaller event
- EPSP may reach different levels depending on amount of transmitter - Graded with the amount of transmitter
- Longer time cause than an AP
- Transmitters including ACh and Glutamate
Describe an Inhibitory Synapse
- Inhibitory transmitters open ligand-gated channels that cause hyperpolarization - open channels selective for K+ or Cl-
- These transmitters lead to inhibition and the change in membrane potential they cause is called an Inhibitory Postsynaptc Potential (IPSP)
- The cell is less excitable
- Transmitters include GABA (gamma-aminobutyric acid), Glycine
Describe Direct G-Protein Gating
- G-Potein diffuses through G-Protein Couled receptor (a 7 transmembrane domain protein) to channel
- It is small as it is one signal to one channel but quite rapid (tends to happen at synapses)
Describe Gating Via An Intracellular Messenger
- This transmission can take place throughout cell so can interact with distal channels as it allows diffusion throughout cell
- Amplification by cascade means the transmission lasts longer but it is slower
- Transmitter binding at the G Protein Coupled Receptor causes the G-protein to diffuse and bind to the enzyme which initiates a signalling cause resulting in an intracellular messenger or Protein Kinase changing the activity of the channel.
