Action Potential Generation
Action Potential Generation NSCI 3310
Popular in Cellular Neuroscience
Popular in Neuroscience
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This 9 page Class Notes was uploaded by Emma Notetaker on Tuesday September 29, 2015. The Class Notes belongs to NSCI 3310 at Tulane University taught by Jeffrey Tasker in Summer 2015. Since its upload, it has received 38 views. For similar materials see Cellular Neuroscience in Neuroscience at Tulane University.
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Date Created: 09/29/15
Action Potential Generation 09/11/2015 inward (negative direction), outward (positive direction) if you open Na channel, Na will flow IN (concentration gradient) K will flow OUT Cl will flow IN current clamp recordings: recording membrane VOLTAGE clamp current to measure voltage/potential (same thing) action potential generation – frequency coding stick electrode in membrane injection (with electrode) of current in cell - elicits changes in membrane potential (record this change) same electrode can both inject and record current positive current injection: depolarization o if reaches threshold (suprathreshold = above threshold), AP will occur frequency coding of NS (neural code): increased magnitude or duration of input increased frequency of AP (NOT increased magnitude of AP) voltage clamp recordings: recording membrane CURRENTS clamp of membrane voltage current is injected into cell to move membrane potential to defined potential (suprathreshold to create AP) o voltage clamp will inject current to COUNTERACT depolarization to bring it back down – CLAMPING it at that voltage and not allowing it to shift o negative feedback – inject current to exactly counteract moving membrane potential away from command potential o IF Vm DNE Vc (command) feedback current is injected to correct membrane voltage feedback current = membrane current action potential currents suprathreshold step in voltage causes Na current positive feedback K current negative feedback single channel recordings – patch clamp technique voltage-sensitive gating of Na and K channels absolute and relative refractory periods action potential: all or none (not graded) starts at resting potential rising phase: rapid depolarization overshoot – positive to 0mV (all of the graph that is over 0) o opening of selective channels and currents falling: repolarization (hyperpolarization) undershoot: “afterhyperpolarization” following the spike o under the resting potential depolarizes at end back to resting potential what are ionic currents responsible for action potential generation? action potential currents: suprathreshold step in voltage causes: o combined inward (negative) and outward (positive) current o inward turned on (positive charge moving in) then turned of o outward current (positive charge moving out) – DOES NOT turn of o inward current – voltage sensitive (evoked by depolarization) comes in first before potassium goes out mediated by voltage-gated Na+ channels turns on at beginning of depolarization, turns of BEFORE it ends transient currents depolarization then remains faster than K+ inactivating blocked by tetrodotoxin (TTX – blocks voltage gated Na+ channels) o outward current – voltage sensitive (evoked by depolarization) mediated by voltage-gated K+ channels persistent current – turns on at beginning of depolarization then stays much slower than Na+ non-inactivating blocked by tetraethylammonium (TEA – blocks voltage gated K+ channels) generating action potential: voltage-sensitive process positive Na feedback: 1. membrane depolarization 2. activate voltage-gated Na+ channels (sensitive to depolarization) 3. increased Na+ influx caused by Na+ channels opening 4. depolarizes membrane more o if subthreshold (not enough depolarization to open more Na+ channels), do not enter loop back to resting potential o if suprathreshold, ENTERS POSITIVE FEEDBACK LOOP of AP (depolarizes more, more Na+ channels, more depolarization, etc.) causes rising phase of AP negative K+ feedback (depolarization is opening voltage-gated potassium channels at almost same time as Na+ channels– a little delay) 1. activate voltage-gated K+ channels 2. increase K+ efflux - K+ fluxes OUT to reverse depolarization o causes Na+ influx to slow down o for split second, equal flux of K+ and Na+ o then K+ dominates 3. hyperpolarize membrane o exit feedback loop – back to resting potential voltage-gated channels: action potential generation o charged amino acid strings sense charges inside cell change configuration in response to voltage o voltage-gated channels are responsible for generating action potential o voltage-sensor in gate- sensitive to depolarization (more positive charge inside) o in neurons, threshold about -40mV o channel can open spontaneously (at any time) BUT much higher probability with depolarization due to sensing increased positive charge inside cell recording single channel currents with the patch clamp technique glass electrode adheres to membrane and creates high resistance seal (no ions can escape) if channel inside sealed patch, can monitor currents currents sum to form whole cell currents single channel currents = rectangular currents (graph looks rectangular) o on/of – all or none either in open (fluxing maximum current) or closed (no current) negative direction (bc inward current) – bringing POSITIVE charge into cell in response to depolarization diferent channels = diferent conductance diferent amplitude o current flow dependent on open time, opening frequency single voltage-gated channel currents: -40 mV – o Na+ will still flow into cell (still driving force on Na to drive it into cell – so negative current) o 20 mV driving force for Cl- to flow in (wants to hyperpolarize cell) – o K+ equilibrium of -80 to hyperpolarize membrane, will flow out09- voltage sensitivity – sensitive to depolarization o single current amplitude is a function of Vm o increased open probability with depolarization current amplitude changes with voltage o driving force is larger with higher voltage whole-cell currents (same as voltage clamp experiment) blow out patch – access to entire inside of the cell o record any current generated across whole cell membrane macroscopic currents – currents of whole cell o summation of single channel currents current direction and amplitude o functions of unitary currents action potential currents Na+ currents o inward (negative direction, positive change in voltage) o TRANSIENT current o single channel currents summate to depolarize o cause depolarization (AP rising phase) slows down at top o RAPID opening due to BIG driving force on Na+ o inactivating K+ current o outward currents (positive direction, takes + charge OUT of cell) o opens with a delay after Na+ o single channel current summate o causes hyperpolarization (AP falling) o SLOW K+ efflux continues to flow (afterhyperpolarization undershoot) because they are so slow to close o non-inactivating all voltage-gated channels closed after AP membrane goes back to resting potential because leak channels still open! (they stay open even while voltage-gated channels open) more Na+ will flux than potassium after the undershoot due to concentration gradients establish steady state due to leak channels Na+ channels: (FOUR separate states) 2 molecular gates: allows current to be transient or inactivating o activation gate (m gate) – voltage sensor domain opens in response to depolarization – channel activated starts closed rapid opening activation and deactivation = opening/closing of this gate o inactivation gate (h gate) – voltage-sensitive ball and chain charged protein ball tethered to inside of channel by protein when inside becomes positive (depolarization inside cell) – ball is unhappy and starts moving towards environment with less positive charge moves to wedge inside channel pore, which blocks ion flow INACTIVATION of current closes in response to depolarization starts open slow closing – causes current inactivation/deinactivation = opening and closing of this gate terminates current flow deinactivation: open channel but no ball and chain o BOTH are voltage sensitive and respond to depolarization BUT respond in opposite ways membrane cannot be re-activated no matter HOW MUCH depolarization if Na+ channels inactivated with the h gate – absolute refractory period K+ channel activation and deactivation single gate: activation gate (m gate) o voltage-sensitive – responds to depolarization (on only in depolarization) depolarized: gate opens, K+ flows out dissipates depolarization, causes hyperpolarization channel is now closed because NOT depolarized anymore o non inactivating (sustained current) o ONLY activation/deactivation Na and K conductances: Na conductance: o rapid onset (activation) o rapid ofset (inactivation – ball and chain) K conductance: (only m gate) o slow onset (activation) o slow ofset (deactivation) afterhyperpolarization (undershoot) refractory period: channel inactivation o absolute refractory: - Na+ channel inactivation CANNOT GENERATE AP o relative refractory period Na+ deinactivated – harder, need more depolarization afterhyperpolarization higher threshold active ion transport: Na/K pump o maintains Na and K concentration gradients (compensated for Na and K difusion down gradients) o requires energy ATP o 3 to 2 ratio (Na/K) – electrogenic o o o o ▯ driving force: ion equilibrium potential-membrane potential bigger diference = bigger amplitude ▯ ▯ ▯