BIOLOGY 2C03 Lecture Notes - Voltage Clamp, Axon Hillock, Spacetime

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28 Jan 2013

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Outline of Lecture 05 (02-19 A; Linden)
Membrane Potential
0. Fundamental Concepts
1) Resting potential is –70 mV, close to EK due to open K+ channels
2) Ion distribution gives rise to different equilibrium potentials via Nernst eqn
- High Na+ outside (ENa = 64 mV) , high K+ inside (EK = -86 mV)
- Very low Ca2+ inside (ECa = 116 mV), high Cl- outside (ECl = -78 mV)
3) Due to very low capacitance of membrane (5 pF), very few ions most move to achieve
equilibrium (Q = CV)
4) Current flow through ion channels can be approximated with Ohm’s Law
I. Passive current flow (aka cable property of dendrites and axons)
A. Basic observations
- At steady state, a point depolarization V is graded: it tails off exponentially according to length
scale λ, a measure of the relative resistance of the membrane and axoplasm
- Due to resistance (ion channel) and capacitor (neuronal membrane) in parallel, upon current
pulse voltage asymptotically increases to steady state value according to time constant τ,
the product of resistance and capacitance of the membrane
B. Consequences
- Since λ goes as r, resistance decreases with diameter of dendrite/axon
- Spatio-temporal summation: depolarizations nearer axon hillock are more likely to cause
action potential, input signal summation requires appropriate timing
II. Active current flow (aka action potentials)
- Small depolarizations do not result in action potentials due to outward K flow
- Threshold depolarizations open voltage gated Na channels and initiate action potential
- Na channels inactivate quickly, helping stop rise in membrane potential and give refractory
- Slower voltage gated K channels help repolarize cell
- Experimental verification by Hodgkin and Huxley, using voltage clamp on squid giant axon,
along with expts with selective ion channel blockage
- Due to depolarization of nearby Na channels, action potential is propagated in all-or-none
fashion; one-way due to refraction
- Conduction velocity depends on passive current spread; myelination raises conduction velocity
by increasing λ (bigger r) and decreasing τ (low membrane capacitance trumps higher resistance)
Summary of major points:
See end of lecture notes
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