Membrane Potentials Flashcards
What is diffusion?
the number of molecules that cross a unit area per unit of time.
What is the zero-volt level?
a reference electrode is placed outside the cell.
What is electrochemical equilibrium?
- where electrical forces balance diffusion forces
- a stable transmembrane potential is achieved.
- occurs when electrical forces balance diffusional forces
What is equilibrium potential?
- the potential at which electrochemical equilibrium has been reaches
- calculated using the Nernst equation
Why do membrane potentials not rest at E(k) or E(Na)?
Because membrane have mixed K and Na permeability (but at rest K»_space;> Na)
What is the Goldman-Hodgkin-Katz GHK equation?
- estimating membrane potentials in more complex systems
- takes into account the relative permeabilities of all the ions at one time
i. e the size of each ion’s contribution is proportional to how permeable the membrane is to the ion - describes the resting membrane potential E(m)
- P is permeability so when P=0, 100% channels closed, if P=0.5 = open 50% of time.
What is overshoot in terms of membrane potential?
-when the membrane potential becomes positive (than 0)
What is hyper polarisation?
-when the membrane potential becomes more negative than the resting potential
What is decremental spread of graded potentials?
- charge ‘leaks’ from axon as the impulse propagates
- the size of depolarisation will gradually fade in terms of its size along the axon (since they do not reach the threshold to produce an AP)
- ‘graded potential’ because they depend on the size/nature of the stimulus
Where do graded potentials tend to occur?
- at synapses and sensory receptors
- can contribute to action potentials or prevent them
Tell me about action potentials.
- occur in excitable cells (mainly neurones, muscles cells and some endocrine tissues)
- allow transmission of information reliably and quickly over long distances.
- can be used to activate intracellular processes
- ‘all or nothing’ principle
- they’re regenerative
Outline the 5 phases of the action potential.
P1) RMP
- permeability for K>Na
- membrane potential near equilibrium potential for K (-90mV) than that for Na (+72mV)
P2) Depolarising stimulus
- stimulus depolarises the membrane potential
- moves it in the positive direction towards the threshold
P3) Upstroke
- VGSCs open, Na enters
- VGKCs open slowly, K leaves
- membrane potential moves towards the Na equilibrium position
P4) Repolarisation
- VGSCs close
- K leaves the cell
- membrane potential moves toward the K equilibrium potential
- new AP cannot be triggered
- absolute refractory period
- at first, activation gate open and inactivation gate closed
- later, activation and inactivation gates closed
P5) Hyperpolarisation
- VGKCs open, K leaves
- membrane potential moves closer to the K equilibrium
- some VGKCs close to the membrane potential returns to the resting potential
- relative refractory period
- inactivation gate is open
- stronger than normal stimulus required to trigger an AP
What is passive propagation?
- results from a local change in ionic conductance (e.g synaptic or sensory that produces a local current that spreads and becomes exponentially smaller)
- only resting K channels open
- internal (or axial) & membrane resistance alters propagation distance and velocity.
- larger-diameter neurones have lower resistance so the potential decay happens more slowly
Where are the voltage-gated channels mostly located?
-at the nodes of Ranvier
What are the 3 main factors that influence the movement of ions across the membrane?
- the concentration of the ion on both sides of the membrane
- the charge on the ion
- the voltage across the membrane
