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Sci2 Lect. 3- Action Potential © Dr Bill Phillips 2002, Dept of Physiology • • • • • • • Need for amplified signalling Depolarisation initiates neuronal signalling Voltage gated sodium channels Voltage gated potassium channels Ratio of permeabilities Na + Channel inactivation Information is encoded in by AP frequency Need for amplified signalling Muscle fibre Mot or neurone Nucleus Local circuit current spread depolarisation along membrane Inward current The membrane has electrical properties that affect the spread of changes in membrane potential Membrane has electrical capacitance ++++ -------- Ion channels in the membrane have resistance but allow leakage of local circuit currents out of the membrane The axoplasm is narrow and has resistance impedes the depolarising current down the membrane Electrical components of the resting membrane Axon Equivalent circuit + Vm Dist ance along dendrit e or axon Depolarisation initiates neuronal signalling • Membrane is depolarised when there is a net inward current • Depolarisation is usually initiated by opening of ligand gated cation channels at an excitatory synapse • Depolarisation can also be triggered artificially, say by applying an electrical stimulus (Prac class) Voltage gated sodium channels Vm Hodgkin Cycle PNa PK Voltage-gated sodium channels • • • • Closed when the membrane is polarised Begin to open as the membrane depolarises Selectively permeable just to Na+ Begin to inactivate as the membrane depolarises • Inactivation shuts off the Na+ current flow • ‘voltage gates’ and ‘inactivation gates’ Hodgkin Cycle • Stimulus depolarisation opens a small fraction of voltage-gated Na+ channels • Increase in inward current through these Na+ channels further depolarises the membrane • As membrane depolarises more, a greater fraction of the Na + channels open leading to more depolarisation etc Voltage-gated Na+ channels can exist in multiple states depending upon the recent history of depolarisation CLOSED depolarisation OPEN time repolarised INACTIVATED time depolarised Voltage gated potassium channels Vm PNa Slow opening of voltage-gated K+ channels PK Voltage gated potassium channels • Mostly closed when the membrane is polarised • Begin to open as the membrane depolarises • Do not inactivate • Fraction of channels open increases proportionately with depolarisation Ratio of permeabilities Vm PNa PK Ratio of permeabilitiesDuring the action potential: • Stimulus begins to open voltage-gated Na+ channels • If sufficient, the Hodgkin Cycle begins • As the membrane rapidly depolarises further, Na + channels begin to inactivate and K + channels begin to open • Shutting off of the inward Na + current and increase in the outward K + current repolarises the cell Limits to Action Potential Frequency • Voltage-gated Na+ channels stay inactivated for a fraction of a millisecond after the action potential preventing a second AP • After the action potential there is a brief hyperpolarisation (period of reduced excitability). Effect? Information is encoded by AP frequency • When they become depolarised most neurons fire a ‘train’ of several to hundreds of action potentials (spikes) • The frequency of spikes within a trains usually encodes the intensity of the sensation or instruction • Trains of spikes are usually interspersed by periods of silence