Action Potential (AP) NEUROEXCITABILITY II-III. Na + and K + Voltage-Gated Channels. Voltage-Gated Channels. Voltage-Gated Channels
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1 NEUROEXCITABILITY IIIII Action Potential (AP) enables longdistance signaling woohoo! shows threshold activation allornone in amplitude conducted without decrement caused by increase in conductance PNS Ch. 8, 9 and K + Once upon a time, two famous scientists figured out that changes in and K + permeability occur during action potentials Since they were so smart, they realized that voltagegated ion must be responsible for the observed currents, dubbed I Na and I K However, these overlap in time and alter V m and K + So, they simply invented the voltage clamp: forces membrane potential to stay put at any command V m via I clamp = I ion (measurable) still react normally to an imposed V m ions still go in and out, but they don t affect V m conductance (g ion ) can be calculated via g = I / V and K + Circuit form: V m = E I Na / g Na membrane potential is determined by battery force and the voltage drop across resistors Ohm s law form: I Na = g Na (V m E Na ) current is determined by conductance (= # open x single channel conductance) and the driving force for flow (no driving force = no flow) Voltage clamp form: g Na = I Na / (V m E Na ) calculated conductance is proportional to measured current (constant driving force set by voltage clamp) and K + Then, they used specific toxins to paralyze one type of channel and analyze the other: tetraethylammonium (TEA) blocks voltagegated K + tetrodotoxin (TTX) blocks voltagegated 1
2 and K + Here is what they discovered: depolarization increases the probability that any single voltagegated channel will open the greater the depolarization, the more voltagegated open the greater the depolarization, the faster they open and K + Furthermore: voltagegated K + are slow and lazy, so they open slowly in response to depolarization, and they stay open as long as the depolarization continues Whereas: voltagegated are quick, so they open as soon as there is depolarization, but they also close right after they open seems fishy VoltageGated Channels It turns out that have an extra gate, causing them to cycle through 3 different states: VoltageGated Channels Resting state (closed) depolarization 2. Activated (open) prolonged depolarization 1. Resting (closed) time 3. Inactivated (closed) activation gate (normal) inactivation gate (extra) VoltageGated Channels Activated state (open) VoltageGated Channels Inactivated state (closed) activation gate (normal) inactivation gate (extra) activation gate (normal) inactivation gate (extra) 2
3 AP Generation Nernst equilibrium potential for : E Na = +55 mv Nernst equilibrium potential for K + : E K = 75 mv Resting membrane potential: V m = 60 mv AP Generation At rest, the only open are membrane leakage, which are permeable to K + : g K >> g Na K + likes to flow out (down its gradient) V m is closer to E K than E Na because the membrane is more permeable to K +, therefore K + dominates at rest AP Generation Suddenly, a depolarization comes along! are the quickest to respond likes to flow in (down its gradient) This inward positive flow starts to counter the outward leaking AP Generation Threshold = potential at which enough are open to dominate the net current: g Na > g K Net inward pos current = further depolarization, causing even more voltagegated to open: g Na >> g K So V m is driven closer to E Na than E K because the membrane is more permeable to, for now Fast Positive Feedback Depolarization Slow Negative Feedback Depolarization time Inward I Na increases Cycle Generates Action Potential open open Inward I Na increases time inactivate Inward I Na ceases + + K + open late Outward I K increases Action Potential Repolarization 3
4 Action Potential Membrane potential (mv) E Na peak 0 thres hold V m E K I Na (in) depolarization Time (ms) I K (out) repolarization recovery hyperpolarization area of depolarization Na Na + Na + + currently how you doin? Local circuit flow of current direction of propagation Na Na + Na + + Na + died out because currently area of depolarization Local circuit flow of current direction of propagation Passive spread of local depolarization is a ratelimiting factor in AP propagation Active regeneration of AP is required to maintain fidelity of signal, i.e. in order to prevent depolarization from dying out Contribute to allornothing AP propagated without decrement (= constant amplitude) To inc conduction time, inc current (I) by dec resistance (R) or dec capacitance (C): I = V/ t and I = V/R and V = Q/C So membrane conduction time ( V/ t) determined by axial resistance (r a ) and membrane capacitance per unit length (c a ) Membrane length constant (lambda, λ) measures efficiency of passive spread: defined as distance at which an injected signal has decayed to 37% of its original amplitude λ = (r m / r a ) Membrane resistance determined by leakage, which is generally constant Inc diameter = dec axial resistance (more influx) Causes inc length constant = inc efficiency 4
5 Membrane time constant (tau, τ) measures rate of passive spread: τ = (r a * c m ) Inc diameter = dec axial resistance Causes dec time constant = inc velocity Inc conduction velocity via inc axon diameter: r a 1 / diameter 2 c m diameter inc diameter = dec axial resistance and inc capacitance, but resistance dominates Net effect is dec τ = (r a * c m ), faster velocity Also inc λ = (r m / r a ), greater efficiency Inc conduction velocity via myelination: inc effective membrane thickness by 100x c m 1 / thickness of plates inc thickness = dec capacitance (by a lot) insulation = inc effective membrane resistance (less opposing leakage out) Net effect is dec τ = (r a * c m ), faster velocity Also inc λ = (r m / r a ), greater efficiency smaller diameter, but myelinated > bigger diameter, but nonmyelinated Myelination causes a proportionally greater increase in conduction velocity than does an equivalent increase in diameter alone In a myelinated neuron, an action potential is initiated upon threshold summation of all synaptic inputs (excitatory and inhibitory) converging on the trigger zone of the axon hillock, which is not myelinated and has a very high density of extra sensitive voltagegated. The resultant inward flow is sufficient to start passive spread down the axon, but not strong enough to depolarize the entire length of the axon (fully discharge the capacitor plates). The signal spreading under the myelin sheath is like being trapped under ice; it needs to come up for air periodically by gaining reinforcement or it will decay exponentially. Hence, the nodes of Ranvier. 5
6 The membrane at the nodes (vs. under sheath) is very rich in voltagegated, causing an intense inward depolarizing current in response to passive depolarization. These jolts provide regular regeneration of the allornone AP, preventing signal decay. Because conduction velocity is so fast beneath the myelin sheath (due to its low c m ), but slows significantly at the nodes of Ranvier (high c m ), the propagated signal appears to be jumping from node to node. This is called saltatory conduction. Channelopathies Concepts not details to review: Diversity of ion (styles of gating, firing) General structures of gene superfamilies Roles of intracellular Ca 2+ Neuron variability in terms of inputs (dendrites) and signaling (ion channel types/distribution) Mutation types that can cause channelopathies Diseases reflecting phenotypic or genotypic variability (myotonia vs. paralysis) Diseases caused by regional specificity of ion (myotonia vs. epilepsy) Heterozygotes in Cl channel dimer mutations GOOD LUCK!!! Ben Kennedy (bck2104@columbia.edu) Jessica Sims (jas2033@columbia.edu) 6
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