Action Potential Generation Initiation of AP? threshold? Rising - PowerPoint PPT Presentation

Action Potential Generation � Initiation of AP? threshold? Rising phase? Falling phase? All happening in 1 msec!

Current across membrane I m = I Rm + I Cm Examples of passive voltages Initiating AP?? AP spread ahead Receptor potentials Synaptic potentials

Experimental measurement of space constant Typical values 0.01 to 1.0 mm.

Evolution of fast signal conduction Increase membrane resistance to leak current flow Reduce internal resistance to current flow

Lambda and tau interact to determine speed and amplitude of voltage change ahead of AP Tau 1-20 ms

Initiation Na vs K Na wins Threshold- dynamic Again Na vs K Fall- K

Traveling AP- unmyelinated nerve Local current flowing ahead of AP initiates next AP What happens first?- Spreading charge depolarizes C m . 1 ms!

Comparison of traveling AP Traveling AP- myelinated nerve Also: Cs in series add reciprocally

Traveling AP- myelinated nerve Repolarization at Nodes of Ranvier by fast tau for repolarization unmyelinated nerve resting g= 0.2-1 mS/cm 2 , tau 1-20 ms myelinated nodes resting g= 40 mS/cm, tau= 100 � S!

AP conduction velocity: Squid giant axon vs. myelinated nerve 19 m/s = 43 mph 500 micron Same as 10 micron Frog myelinated fiber!! Alpha motor neuron: 120 m/s = 268 mph 8.6 microns Nerves Squid Mammal Unmelyinated C pain fibers: 2m/s = 4 mph 1.5 micron

Refractory periods

Why doesn’t the AP go backwards?

I/V curve explanation for AP V r stable V th unstable V peak could be stable but conductances are time and V sensitive

Refractory periods D- g K >>>g Na C- g Na recovering, but no AP B- smaller AP A- full AP A B C,D Notice threshold change

Calcium APs Add Ca to the equation

Examples of calcium APs

Na vs. Ca AP

Calcium current contributing to APs

Calcium current inactivation

Ca-activated K current

Na AP vs. Heart AP

Cardiac AP currents