Chapter 4: The Action Potential

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9 Apr 2012
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Chapter 4 - The Action Potential
Introduction
o Action potential: rapid reverse of the negatively charged cytosol such that it becomes positively
charged with respect to the outside; aka spike, nerve impulse, discharge
All have same size and duration; frequency and pattern emit information
Properties of the Action Potential
The Ups and Downs of an Action Potential
o Resting membrane potential: -65mV
o Oscilloscope: used to study action potentials; measures voltage changes over time
o Rising phase: rapid depolarization of membrane; Vm peak 40mV
o Overshoot: part of action potential where the inside is + compared to -
o Falling phase: rapid repolarization until membrane is actually more negative than resting potential
o Undershoot/after-hyperpolarization: last part of falling phase
o Then there is a gradual restoration of the resting potential
The Generation of an Action Potential
o Depolarization/generator potential: the depolarization due to incoming Na+ from opening of Na
channels and electrochemical gradient
Can be due to channels sensitive to membrane stretching, neurotransmitters , injecting
electrical current
o Threshold: the critic level of depolarization that must be crossed to generate an action potential
The Generation of Multiple Action Potentials
o Firing frequency: Rate of action potential generation depends on the magnitude of the continuous
depolarizing current
One way stimulus intensity is encoded
Maximum firing frequency 1000 Hz
Absolute refractory period: 1ms; the period of time after an initial action potential is
generated during which it is impossible to generate another one
Relative refractory period: several ms after end of absolute refractory period where amount
of current required to depolarize the neuron to action potential threshold is above normal
Methods of Recording Action Potentials
o Intracellular: required impaling of the neuron or axon with a microelectrode filled with salt
solution (KCl) connected to amplifier that find difference in potential and displayed on an
oscilloscope
o Extracellular: electrode (fine glass capillary with salt solution or thin insulated metal wired) is
placed near the membrane and measured compared to the ground; changes can be seen with
oscilloscope or connecting amplifier to loudspeaker which shows impulses as pop noises
The Action Potential in Theory
Membrane Currents and Conductance
o K+ concentrated 20x inside; Na+ concentrated 10x outside
o Ek= -80mV; Ena= 62mV
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