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.