Membrane potentials and action potentials (2) Flashcards
What is equilibrium potential?
- the potential at which electrochemical equilibrium has been reached
- electrical force prevents diffusion of the ion down its concentration gradient
What does the Nernst equation calculate?
equilibrium potential, E, for each ion if you know the conc. of ions on either side of membrane
What is the extracellular concentration of Na+?
150mM
What is the extracellular concentration of K+?
5
What is the intracellular concentration of Na+?
10
What is the intracellular concentration of K+?
150
Why do membrane potentials not rest at Ek or Ena?
because membranes have mixed K+ and Na+ permeability but at rest K>Na
What is the Goldman-Hodgkin-Katz (GHK) equation?
- takes into account relative permeabilities of all ions at one time
- describes the resting membrane potential (Em)
What is depolarisation?
membrane potential increases from negative (-70mV) towards 0
What is repolarisation?
membrane potential decreases towards resting potential (becomes more -ve)
What is the overshoot?
membrane potential becomes more positive than 0
What is hyperpolarisation?
membrane potential decreases (becomes more -ve) beyond resting potential
What are graded potentials?
- decay down the length of axon
- changes in membrane potential in response to stimulation
- occur at synapses/ sensory receptors
- contribute to initiating or preventing APs
How do ion channels change their permeability depending on their conformational state?
- opened by depolarisation
- inactivated by sustained depolarisation
- closed by membrane hyperpolarisation/repolarisation
How does resting membrane potential occur? (Phase 1 of action potential)
- set up by K+ flowing out of cell, leaving -ve charge in cell
- Pk>Pna (permeability)
- membrane potential nearer to equilibrium potential for K+ than Na+
How is an action potential initiated? (phase 2: depolarising stimulus)
- activation of a receptor to produce a graded potential
- stimulus depolarises membrane potential to threshold value–> more +ve
What happens after the threshold potential is reached? (phase 3: upstroke)
- voltage-gated sodium channels open, so inc. Pna–> Na+ enters cell down electrochemical gradient
- membrane potential moves towards the Na+ equilibrium potential
How does repolarisation occur (Phase 4)?
- voltage-gated sodium channels close–> dec. Pna–> Na+ entry stops
- more voltage-gated K+ channels open–> inc. Pk–> K+ leaves cell down its electrochemical gradient, REPOLARISING cell…membrane potential moves towards K+ equilibrium potential
What is the refractory period?
- inactivation plug stops functioning of Na+ channel for a small period of time
- cannot restimulate nerve even w/ v. strong stimulus
What occurs during phase 5, after-hyperpolarisation?
- K+ continues to leave cell down electrochemical gradient
- sodium channels still inactivated
- membrane potential moves closer to K+ equilibrium
- returns to resting potential
How do voltage-gated sodium channels inactivate?
- part of protein plugs membrane rapidly, stopping sodium moving through
- cannot reactivate channel until you get rid of the plug
How does active propagation of action potentials occur?
- adjacent area to peak of action potential becomes more depolarised until it reaches threshold
- then sodium influx activated and moves down axon a bit further etc…
What is the purpose of myelination?
- insulation–> conducts more effectively bc sheath is interrupted by nodes of Ranvier, so AP jumps
- ^ SALTATORY CONDUCTION
- speeds up transmission
What factors affect conduction velocity?
- diameter of axon bc larger= less resistance to ions moving along (bc more space)
- myelination
- small diameter, non-myelinated axons 1m/s, whereas large diameter, myelinated axons 120m/s
What diseases reduce myelination?
multiple sclerosis and diphtheria
