The Action Potential (Prof. Pedarzani) Flashcards
How is information encoded in the Nervous System ?
As AP frequency (often codes for simulus intensity).
What are the 4 phases of the AP ?
The rising phase = raid depolarization
Overshoot = part of the AP where the inside is positive w/ respect to the outside
Falling phase = rapid depolarization of the membrane until is is actually
Undershoot = hyperpolarisation of the membrane
What is the consequence of Na+ influx (e.g. if stretch-activated Na+ permeable channels open) ?
The membrane is depolarized, which results in a generator potential (or graded potential).
Depolarization alone NEVER generated APs.
Applying increasing depolarization to a neuron only has a graded, local effect until it crosses the threshold.
How do APs work ?
They are “all or none” : if the generator potential achieves a critical level, the threshold, the membrane will generate an action potential.
What does the firing frequency of APs reflect ?
The magnitude of depolarization, and thus often the intensity of the stimuli.
What will a neuron do if it is continuously stimulated beyond threshold ?
It will fire many APs in succession.
What is the limit of AP fequency and why ?
About 1000Hz.
After each AP there is a refractory period during which the threshold is higher than normal. It is thus harder to generate another AP during this phase.
What are the 2 phases of the refractory period ?
Absolute RP = aprox 1ms, threshold infinite
Relative RP = several ms, AP can be generated if stimulus is sufficiently strong
How do the conductances for the different ions vary during the AP ?
During the rising phase, gNa+»gK+
During the falling phase, gK+»kNa+
During the undershoot gK+»Na+ but a little more than usual to restore the negative interior of the call. The increase in K+ conductance falls more slowly than the increase in Na+ conductance.
How long do Na+ channels stay open before they inactivate ?
1 ms
What is the accepted molecular model for the inactivation for the inactivation of the Na+ channel ?
The hinge and lid model (NOT the ball and chain model).
Basically, the “lid” is made of Ile, Phe and Met on one of the intracellular loops that swing on the movable “hinge”, the Gly, Gly pair.
Why does the AP occur so quickly ?
Because of the rapid opening of Na+ channels in response to depolarization and their rapid inactivation.
What is TTX (Tetrodotoxin)?
Why is it useful ?
TTX is a toxin that was isolated from the ovaries of the puffer fish that selectively block Na+ channels by clogging the pore upon tight binding to a specific site outside of the channel.
Ingested TTX = fatal
It is however useful to understand the function of specific Na+ ions channels, deduce their structure and discover different sites.
Why does the AP not turn back on itself ?
Because of the refractory period (rapid inactivation of Na+ channels).
What factors influence conduction velocity ?
Axon width and membrane resistance (if it has lots of leak channels or not).