Lecture 5: Action Potentials Flashcards
What are the 4 main properties of an action potential?
- They are initiated by a threshold
- The action potential is an all-or-nothing event
- The action potential is conducted without decrements (it propagates unchanged, undiminished down an axon – regeneratively).
- The action potential is follows by a refractory period
What is an action potential?
An action potential is a transient reversal in membrane potential
Describe how an action potential occurs.
When synaptic input or intrinsic cellular forces drive the membrane potential over a certain threshold, voltage gated sodium channels open up, causing sodium to flood into the neuron.
This causes rapid depolarization, as the membrane potential goes from resting state, closer to ENa
This constitutes a negative sodium current
As the membrane potential rises (becomes more positive), voltage gated potassium channels open, triggering an efflux of potassium
This constitutes a positive potassium current
Meanwhile, the inward sodium current has run out, and voltage gated sodium channels become inactivated.
Potassium channels take over and gradually will diminish
The kinetics of potassium channels are slower, accounting for the longer tail of the AP.
The relatively gradual descent of the membrane potential in the second stage is known as repolarization.
Hyperpolarization phase occurs after the membrane is repolarized, where membrane potential may undershoot the resting potential.
This happens because the voltage gated potassium channels remain open longer
This causes sustained permeability of potassium, causing the membrane potential to move closer to the EK.
How does an action potential propagate?
Depolarization beyond threshold that occurs locally spreads passively to other areas (down the axon).
This causes adjacent regions of the membrane to reach the threshold for generating action potentials.
Axons with longer length constants spread their currents further, thus leading to more rapid propagation of AP
