Post on 21-Nov-2014
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Action Potential
Department of PhysiologySchool of Medicine
University of Sumatera Utara
Resting Membrane Potential
• Found in almost all cells.• In neurons, it is usually –70 mV.
Genesis of Membrane Potential
• The distribution of ions across cell membrane.
• K+ move out from cell by it’s concentration gradient via K+ channels.
• Na+-K+ATPases pumps 2 K+ back into the cell for every 3 Na+ out of the cell.
• K+ channels make the membrane more permeable to K+ Na+ influx does not compensate K+ efflux intracellular relatively Θ charged.
Ionic Basis ofResting Membrane Potential
• Actively, Na+ is transported out & K+ into cells.
• K+ moves out & Na+ moves in, but because of K+ channels, K+ permeability at rest is > Na+
permeability, K+ channels maintain the RMP.
• Catelectronic currents voltage-activated Na+ channels become active.
• When firing level reached voltage-activated Na+ channels overwhelm the K+ & other channels spike potentials results.
Ionic Basis ofResting Membrane Potential
Excitable Tissue
• Nerve and muscle cells can be stimulated electrically, chemically, mechanically action potential.
Nerve Cells
• Have a low threshold for excitation.• 2 types of psychochemical
disturbances:1. Local non propagated potentials:
Synaptic. Generator. Electrotonic.
2. Propagated disturbances action potential (nerve impulses)
Potential of Membrane
• Action potential; a characteristic series of potential changes if axon is stimulated and a conducted impulse occurs.
• Stimulus artifact; a brief irregular deflection of the baseline due to current leakage from the stimulating electrodes to the recording electrodes.
• Latent period; isopotential interval from stimulus artifact until the starts of the action potential impulse takes time to travel along the axon from the site of stimulation to the recording electrodes.
• If the stimulus is at or above threshold intensity, the action potential is therefore “all or none” in character.
• Strength duration curve; the relation between the strength & the duration of a threshold stimulus.
• Accommodation; a process that slowly raising currents fail to fire the nerve because the nerve adapts to the applied stimulus.
Potential of Membrane
• Electrotonic potentials;
Application of such currents with a cathode leads to a localized depolarizing potential change that rises sharply & decays exponentially with time.
The magnitude of this response drops off rapidly as the distance between the stimulating and recording electrodes is increased.
Conversely, an anodal current produces a hyperpolarizing potential change of similar duration. These potential changes are called electrotonic potentials.
Potential of Membrane
• Local responses;Effect on membrane potential due to an application of subthreshold stimuli but do not produce an action potential.
• Firing level;A threshold level that makes excitable membrane is triggered to undergo an action potential.
Potential of Membrane
Changes in Excitability During Electronic Potentials & the Action Potential
• Refractory Period;
– During the local response, the threshold is lowered, but during the rising & much of the falling phases of the spike potential, the neuron is refractory to the stimulation.
– Absolute refractory period; the period from the time the firing level is reached until repolarization is about ⅓ complete. not excitable.
– Relative refractory period; lasting from the repolarization is about ⅓ complete to the start of after hyper-depolarization. excitable by stronger than normal stimulus.
• During after-depolarization, the threshold again decreased.
• During after-hyperpolarization, the threshold is increased.
Changes in Excitability During
Electronic Potentials & the Action Potential
Electrogenesis of the Action Potential
• Nerve cell membrane is polarized at rest, ♁charges along the outside of the membrane and Θ charges along the inside.
• During the action potential, this polarity is abolished & for a brief period is actually reversed.
Saltatory Conduction
• Jumping of depolarization from node to node at myelinated nerve axon.