Lecture 8 - Membrane potentials Flashcards
Membrane potential (1)
Voltage (difference in electrical charge) across the plasma membrane.
Resting potential (3)
The membrane potential of a cell not sending signals.
Neuron = -70 mV = Polarised
Cardiac = -90 mV
Creating a resting potential (4)
Sodium ions actively transported OUT of the axon whereas potassium ions are transported INTO axon by sodium-potassium pump (3Na+ and 2K+).
More sodium OUTSIDE the membrane than INSIDE the AXON. So sodium ions DIFFUSE DOWN electrochemical gradient into axon.
Most ‘gated’ sodium channels are CLOSED, preventing the movement of sodium ions, potassium channels open DIFFUSES INTO AXON.
More positively charged ions OUTSIDE the AXON than INSIDE the cell so resting potential is -70mV.
Modelling the resting potential (5)
Modelled by artificial membrane that sepearted two chambers.
At equilibrium chemical = electrical gradient = Equilibrium potential (Eion).
e.g. To recreate a mammalian K+ concentration we place a 140mM KCl concentration inside the chamber, and a 5mM concentration outside the chamber. K+will diffuse down it’s concentration gradient into the outer chamber.
Cl- will remain inside the chamber as it has no means of crossing the membrane. Excess negative charge builds up inside the cell.
Nernst Equation = 62mV (log[ion]outside / log[ion]inside).
Permeable to a single ion.
Goldman-Hodgkin-Katz Equation = takes into account multiple ion permeabilities = RT/F ln(ionout/ionin)
R= 8.314 J/mol.K
F = 96500 C mol-1.
Hyperpolarisation/Depolarisation (3)
Takes into account different amount of channels.
K+ diffuses out = inside negative = hyperpolarisation.
Na+ diffuses in = inside positive = depolarisation.
Action potentials - basic info (2)
Difference in shape or neuron/cardiac cell graph due to different ions expressed in their respective membranes.
Refractory period - A short period of time when the axon cannot be excited again after the first action potential.
Action potential - Neuron (6)
On sheet
Conduction of action potentials - Neuron (2)
An AP is generated as Na+ flows inward across the membrane at one location. This depolarisation spreads to the neighbouring region and reinitiates the AP there.
Local currents of ions therefore propagate the AP along the length of axon.
Frequency of action potentials - Neuron (2)
Larger the stimulus = Greater frequency of AP.
Graded sensory stimulation e.g. quiet or loud noise.
Auditory neuron responds to sound, inaudible noise, slight depolarisation, might not reach threshold to fire.
Factors affecting speed of action potentials - Neuron (3)
Axon diameter - Bigger = faster transmission = less resistance to flow of ions.
Temperature - higher temp = faster transmission = ions diffuse at higher temps, only up to 40 degress otherwise Na/K pump will denature.
Myelin sheath.
Evolutionary adaptation of Axon structure - Neuron (5)
Oligodendrocytes in the CNS.
Schwann cells in PNS.
Myelinated neurons faster than non-myelinated, due to Nodes of Ranvier (depolarisation occurs here, sodium ions pass through protein channels).
Longer localised circuits, action potential jumps due to saltatory conduction = more energy efficient and faster than a wave of depolarisation along whole length of axon.
Corpus callosum is very white due to large amount of myelin present (communication between left/right sie of brain).
Action potential - Cardiac (6)
On sheet.
Frequency of action potentials - Cardiac (2)
Increase in AP frequency = Increase in heart rate.
Plateau phase shortens due to quicker inactivation of Ca2+ channels.
What does the action potential do? - Neuronal/Chemical synapse (1)
Arrives at presynaptic bouton, activates Ca2+ channels, signals vesicles to release neurotransmiters into synapse where they activate ionotropic/metabotropic receptors.
What does the action potential do? - Cardiac/Electrical synapse (1)
Does not release any neurotransmitters. Directly coupled to next cell with a gap junction. AP can flow from one cell to the other quickly.
