Characteristics of Action Potentials

-depolarization (influx of Na+)

-repolarization (efflux of K+)

In's and out's of Action Potential

The net movement of K+ across the membrane is the current.

What is the current net movement of K+ across the membrane?

The number of potassium channels is proportional to electrical conductance.

What is the relationship between potassium channel number and electrical conductance?

Membrane potassium current is influenced by flow and driving force.

What factors contribute to membrane potassium current?

Law of premeability

the membrane potential is always driven toward the Eion of the ion to which the membrane is most permeable

Hondgkin and Huxley

hypothesized that sodium "gates" are activated by depolarization above threshold and inactivated when the membrane acquires a positive membrane potential

Voltage clamp

"clamp" membrane potential at any chosen value

Voltage-gated sodium channel

A membrane protein forming a pore that is permeable to Na+ ions and gated by depolarization of the membrane.

Require negative Vm reset

What is required for a Voltage gated Na+ channel to close and ractivate?

-open with little delay

-stay open for about 1 msec

-cannot be opened again immediately by depolarization

Functional properties of sodium channels

Absolute refractory period

channels are inactivated which require minimal amount of time

Channelopathies

Diseases caused by dysfunctional ion channels

generalized epilepsy and febrile seizures

What can Channelopathies cause?

-puffer fish tetrodotoxin (TTX): clogs Na+ permeable pore

-red tide saxitoxin: Na+ channel-blocking toxin

-Batrachotoxin (frog) blocks inactivation so channels remain open

-similar blocking of inactivation from veratridine (lilies) and aconitine (buttercups)

toxins that target voltage gated sodium channels:

-both open in response to depolarization

-potassium gates open later than sodium gates (~1ms delay)

Potassium vs. sodium gates

delayed rectifier

potassium conductance serves to rectify or reset membrane potential

4 separate polypeptide subunits join to form a pore

structure of Voltage gated potassium channels

membrane Vm changes by K+ efflux through voltage gated potassium channel which closes itself

What allows for a voltage gated potassium channel to close?

rising phase

At what time point indicated on the action potential is sodium current the greatest?

undershoot

At what time point indicated on the given action potential is the driving force on sodium the greatest?

threshold for initiation, a critical number of voltage gated Na+ channels open

T1 of the production of Action Potential?

the voltage gated Na+ channels INACTIVATE and gated K+ channels open

T2 of the production of Action Potential?

the voltage0gated K+ channels are closing, stops K+ efflux

T3 of the production of Action Potential?