Section 4 Flashcards
How are action potentials caused?
By a triggering event or stimulus that results in a localized depolarization.
What is a major difference between action and graded potentials?
When an action potential is triggered, it will conduct or propagate throughout the entire membrane and does not diminish in strength like a graded potential would.
For this reason, action potentials can be used for long distance signalling
What happens when a trigger event is not strong enough to initiate an action potential?
The membrane potential will quickly repolarize, and the action potential does not occur.
*actions potentials are “all or none”
What is the threshold?
The certain potential that the magnitude of the triggering event has to reach in order for an action potential to occur.
What happens when the threshold is reached?
There is a rapid depolarization as the membrane potential becomes positive. Just as rapidly, the membrane potential repolarizes to the resting membrane potential. Sometimes this repolarization may go more negative than RMP and it is called an after hyperpolarization before returning to the RMP.
What is the main difference between voltage gated K+ channels and Na+ channels (structure)?
Voltage-gated Na+ channels are one large transmembrane protein with a single activation gate and a single inactivation gate.
Voltage-gated K+ channels have four subunits, each of which contribute to activation and inactivation of the channel as a whole.
Describe the process of action potentials and how ion channels contribute to it.
At rest: Channels are mostly closed
Threshold and rising phase: When a triggering event reaches the threshold, voltage-gated Na+ channels open. Membrane potential rapidly approaches the Na+ equilibrium potential (Goldman equation). K+ channels are also open but their contribution is minimal
The falling phase: almost right after opening, the v.g. Na+ channels “inactivate” as the inactivation gate swings to block the pore. V.g. K+ channels are still open and membrane potential now moves towards the K+ equilibrium potential. RMP is restored.
Can a neuron have multiple action potentials at once?
Not at once, but possibly immediately after…depending on the state of the voltage-gated Na+ channels (may take some time to reset)
What is the refractory period?
A brief period in time following an action potential in which another action potential cannot be generated.
This occurs when a second triggering potential arrives, but the majority of the Na+ channels are inactive and they may not generate another action potential, even if the threshold is reached.
What is the absolute refractory period vs the relative refractory period?
During the absolute refractory period essentially all sodium channels become inactive, preventing a second depolarization. However during the relative refractory period a second action potential can occur.
What is another important part in keeping the concentration gradients of Na+ and K+
The Na+-K+-ATPase pump
It removes Na+ from inside and brings in K+ to help restore their concentration gradients
