Local Potential (“Passive” Depolarization)Local Potential (“Passive” Depolarization)Depolarization to ThresholdDepolarization to Threshold

•depolarization produced by the depolarization produced by the stimulusstimulus

chemical, electrical, mechanicalchemical, electrical, mechanical•depolarization due to what’s done to depolarization due to what’s done to (received by) this part of the membrane(received by) this part of the membrane

restrest

Caused by, for example Caused by, for example • an action potential upstreaman action potential upstream• a graded potential upstreama graded potential upstream

postsynaptic potentialspostsynaptic potentialsreceptor potentialsreceptor potentials

restrest

Fig. 12.13a

Action Potential Events #1

DepolarizationDepolarization““Active”/“Rapid” DepolarizationActive”/“Rapid” Depolarization

•depolarization produced in depolarization produced in response to the stimulusresponse to the stimulus•depolarization due to what this depolarization due to what this part of the membrane doespart of the membrane does

restrest

Acceleration indicates a Acceleration indicates a process that has positive process that has positive feedback. feedback. Depolarization continues Depolarization continues to about +35 mV.to about +35 mV.

restrest

Fig. 12.13a

Action Potential Events #2

RepolarizationRepolarization

restrest

Membrane potential returns Membrane potential returns to resting level.to resting level.

restrest

HyperpolarizationHyperpolarization(= after-hyperpolarization)(= after-hyperpolarization)

Membrane potential becomes Membrane potential becomes even more negative, falling a little even more negative, falling a little bit below resting membrane bit below resting membrane potential, and then gradually potential, and then gradually returns back to resting level.returns back to resting level.

Fig. 12.13a

Action Potential Events #3

Thresholdfiring level

The production of an action potential is an all-or-none response.

Moffett, Moffett and Schauf,Human Physiology

Fig. 12.13a

Electrical Changes in Excitable Membranes

Moffett, Moffett and Schauf,Human Physiology

squid giant axon

First sighting of a living giant squid: http://news.nationalgeographic.com/news/2005/09/0927_050927_giant_squid.html

ConductancesThe action potential is caused by

changes in gNa and gK.

• conductance = g = 1/resistance– measured in siemens

(formerly mhos)• (cf. resistance: ohms)

• “passive” depolarization– no significant changes

compared to resting state• “active” depolarization

– gNa rapidly increases• repolarization

– gNa rapidly decreases– gK rapidly increases

• after-hyperpolarization– gNa at resting level– gK slowly decreases to

resting levels

Guyton,Medical Physiology

after-hyperpolarization

• When studying action potentials you must always keep in mind the electrochemical gradients and the equilibrium potentials for Na+ and K+.– When studying the cardiac action potential, you must also

consider the Ca++ gradient.

• The action potential is caused by passive movements of Na+ and K+. – In an action potential the cell makes use of the electrochemical

gradients for Na+ and K+ to produce rapid changes in membrane potential. There is no direct use of energy from ATP.

Moffett, Moffett and Schauf,Human Physiology

Electrochemical Gradients

I = g x V

Electrical Views of the Membrane

from Mountcastle, Medical Physiology

Changes in conductances and currents during an action potential

Action Potentials

• Electrically speaking, the action potential is caused by changes in conductances of Na+ and K+.

• Molecularly speaking, changes in conductance are caused by the opening and closing of voltage-gated channels.

Channels

• Channels are transmembrane proteins.

• Channels are specific for a certain ion or ions, or for water.– the hourglass shape of a channel pore serves as a

selectivity filter

Fig. 3.8

Ion Channel

Alberts et al., Molecular Biology of the Cell

The Action Potentialthe importance of channels

Fig. 12.14(altered; channel cartoons from Katzung and Alberts)