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This PowerPoint shows circuit diagrams superimposed on the membrane in order to illustrate current flow in three of the tutorials: The Membrane Tutorial.

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Presentation on theme: "This PowerPoint shows circuit diagrams superimposed on the membrane in order to illustrate current flow in three of the tutorials: The Membrane Tutorial."— Presentation transcript:

1 This PowerPoint shows circuit diagrams superimposed on the membrane in order to illustrate current flow in three of the tutorials: The Membrane Tutorial (diagrams of current flow that accompany the steps of the tutorial) The Unmyelinated Axon Tutorial The Myelinated Axon Tutorial Ann E. Stuart

2 stimulating electrode inserted Patch Membrane Tutorial outside inside c Vm = 0 mV Vm Current may be injected through a microelectrode.

3 Patch Membrane Tutorial outside inside c Vm = 0 mV Vm Ach-gated channels Current may also be injected by the opening of a channel.

4 electrode injects positive charge (20nA current pulse) Vm = 200 mV at end of current pulse Patch Membrane Tutorial steps 1 & 2 Vm injected current pulse Vm

5 electrode injects positive charge (20nA current pulse) Vm = 200 mV at end of current pulse Patch Membrane Tutorial steps 1 & 2 Vm injected current pulse Vm

6 Patch Membrane Tutorial steps 1 & Vm = 200 mV at end of pulse Vm Q = CV dV/dT = 1/C [dQ/dt]; V = 1/C [Q] Icap = C [dV/dt] dV/dT = 1/C [Icap] dQ/dt = Icap

7 Patch Membrane Tutorial step 3: add leak channel r leak c Vm 12 t

8 Time constant Vm tau = Rm Cm tau = the time it takes for voltage to rise to 67% (1-1/e) or fall to 33% (1/e) of its final value t tau

9 Patch Membrane Tutorial step 4: add HH Na & K channels Vm Na channels (fast) K channels (slow) Depolarizing (positive) current is injected. The current first flows out through the capacitance C. As the voltage builds up across C, current flows out through leak channels. Na channels open, allowing the Na battery to drive positive current inward through these channels (outward Icap). K channels then open, allowing the K battery to drive positive current out through these channels (inward Icap).

10 Passive Axon Tutorial: add longitudinal resistances roro riri roro riri rmrm r m ( = r leak ) (usually negligible)

11 L = √ r m / r i distance x L L = the distance over which a voltage step decays to 1/e (33%) of its original value. If r m is large compared to r i, current will flow down the inside of the axon and L will be large. L tracks the (square root of the) r m /r i ratio: Length constant (L) mV

12 Unmyelinated Axon Tutorial: add HH Na & K channels the AP

13 Myelinated Axon Tutorial: add myelin the AP tau = RmCm Cm decreases because of many capactors in series Rm increases because of equivalent # of resistors in series tau does not change L = √ r m /r i r m becomes very much larger L becomes very much longer time constant: length constant:


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