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Action Potential: Overview

Action Potential: Overview. The action potential ( AP ) is a series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state

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Action Potential: Overview

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  1. Action Potential: Overview • The action potential (AP) is a series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state • During the AP, voltage gatedNa+ and voltage gatedK+ channels open in response to changes in the membrane potential • Voltage gatedK+ channels take longer to open than voltage gatedNa+ channels • Na+ rushes in (depolarization), then K+ rushes out (repolarization) • Following the AP is a refractory period, during which another AP cannot occur, or can occur only with a larger stimulus

  2. Action Potential: Preview • GP’s are summed on the dendrites and soma • Sum of GP’s exceeds threshold • Na+ channels open, Na+ rushes in • Na+ channels close, K+ channels open • K+ rushes out • So much that membrane hyperpolarizes • Na+/K+/ATPase restores concentration and electricalgradients

  3. Action Potential: Resting State • In a resting membrane, inactivation gate of Na+is open & activation gate is closed (Na+cannot get in) • Voltage gatedK+channels are closed • K+ leakage channels are open • The electrogenicsodium-potassium pump maintains the concentration gradient and the electrical gradient • The resting membrane potential is at -70 mV

  4. Action Potential: Depolarization Phase • Chemical or mechanical stimulus causes a graded potential to reach threshold • Voltage-gated Na+ channels open & Na+ rushes into cell • When threshold (-55mV) is reached, Na+ activation gates open and Na+enters • Na+ inactivation gate closes again in few ten-thousandths of second • Only a total of ~20,000 Na+actually enter the cell, but they change the membrane potential considerably (up to +30mV) • Positive feedback process

  5. Action Potential:Repolarization Phase • When threshold potential of -55mV is reached, voltage-gated K+ channels also open • K+ channel opening is much slower than Na+ channel opening • The outflow of K+starts the repolarization of the membrane

  6. Action Potential: Repolarization Phase • When K+ channels open, the Na+ inactivation gates have already closed (Na+ inflow stops) • K+flows out and returns membrane potential to -70mV • So much K+leaves the cell that it reaches a -90mV membrane potential and enters the after-hyperpolarizing phase • K+ channels close and the membrane potential returns to the resting potential of -70mV

  7. Action Potential: Refractory Period • Period of time during whichneuron can not generateanother AP • Absolute refractory period • Even a very strong stimulus will not begin another AP • Inactivated Na+channels must return to the resting state before they can be reopened • Large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible • Relative refractory period • A suprathreshold stimulus will be able to start an AP • K+ channels are still open, but Na+ channels have closed

  8. The Action Potential: Summarized • Resting membrane potential is -70mV • Depolarization is the change from -70mV to +30 mV • Repolarization is the reversal from +30 mV back to -70 mV

  9. The Action Potential: Summarized

  10. The Action Potential: Propagation • The APpropagates along the axon • As the wave of depolarization moves along the axon, Na+ and K+ channels open in sequence • Eventually the AP reaches the synapse and neurotransmitters are released

  11. The Action Potential: Propagation

  12. Comparison of Graded & Action Potentials • Origin • GPs arise on dendrites and cell bodies • APs arise only at the trigger zone on the axon hillock • Types of Channels • AP is produced by voltage-gated ion channels • GP is produced by ligand or mechanically-gated channels • Conduction • GPs are localized (not propagated) • APs conduct (propagate) over the surface of the axon • Amplitude • amplitude of the AP is constant (all-or-none) • graded potentials vary depending upon stimulus strength • Duration • The AP is always the same • The duration of the GP is as long as the stimulus lasts • Refractory period • The AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none.

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