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
What influences the conduction velocity?
-the axon diameter and myelination
(-conduction velocity is proportional to the square root of the axon diameter
- linear relationship between conduction velocity and myelin thickness)
Which diseases causes reduced myelination?
-multiple sclerosis & diphtheria
What decreases the conduction velocity?
-reduced axon diameter
-reduced myelination
(-cold, anoxia, compression, drugs-some anaesthetics)
Why is the K+ equilibrium potential negative (e.g. -70mV) and the Na+ equilibrium potential positive(e.g. +40mV) when both are positive ions?
- due to the relative concentration of Na & K across the cell membrane
- more K+ inside the cell than outside so tend to flow out of the cell, while more Na+ outside the cell than in, therefore tend to flow into the cell.
- a potential of -70mV is needed to attract K+ and stop net outward flow, while a positive charge of +40mV is needed to repel Na+ from entering the cell.
What is the Nernst equation used for?
-to calculate the electrochemical equilibria for single ion in idea (control) system.
lecture slides
https://imperiallondon-my.sharepoint.com/personal/jmoss1_ic_ac_uk/Documents/Bioregulatory%20Systems/01.%20Neurology%20%26%20neuroscience/Student%20resources/Neuro_LE03_Membrane_potentials_and_action_potentials.pdf?&originalPath=aHR0cHM6Ly9pbXBlcmlhbGxvbmRvbi1teS5zaGFyZXBvaW50LmNvbS86YjovZy9wZXJzb25hbC9qbW9zczFfaWNfYWNfdWsvRVMwSmlIdXFaMDVFazI2bVBHTENuX3dCSFNaTG5BS2ZYczl4U29nUEVvbnVCZz9ydGltZT1MYmd1RzdHTTEwZw
