Flashcards in 2.2.4. Action Potentials Deck (34)
Cells that can produce Action Potentials
1. Cardiac muscle
2. Skeletal muscle
3. Neurons (most)
4. Smooth muscle (some)
The duration of an action potential is longest in which type of cell?
(1-2 ms compared to 0.5 ms in Neurons and Cardiac muscle)
What is largest peak amplitude of an action potential?
Sodium Conductance Changes during an AP
1000-fold increase and automatically returns to resting levels
Potassium Conductance Changes during an AP
30-fold increase and returns to resting levels (NOT automatically)
What establishes the resting potential?
Non-gated Na and K channels (Na leaks into the cell and K leaks out of the cell)
Do non-gated Na and K channels change during an action potential
How many gates does the voltage-gated Na channel have?
1. M-gate: is closed at the resting potential, but opens RAPIDLY with depolarization
2. H-gate: open at resting potential, but closes SLOWLY with depolarization
How many gates does the voltage-gated K channel have?
Just one and it is closed at resting potential, but open VERY SLOWLY with depolarization
Driving force and Conductance at Resting Potential of Na vs. K
Na: Driving force is large and conductance is small
K: Driving force is small and conductance is medium
Na Channel during depolarization
For a brief period both gates are open and sodium enters the cell
K Channel during depolarization
Begins to open very slowly (very little K leaves the cell at this time)
Na Channel near peak of AP
Some of the slowly closing gates have closed, but most are still open (conductance is still large, but driving force is low AKA inward Na current is decreased but still present)
K Channel near peak of AP
A large fraction of the slowly opening gates are now open (driving force is large, but conductance is small).
Outward K current is large (stops depolarizations and begins to repolarize cell back toward the resting potential of -90 mV)
Na Channel past peak of AP
Rapidly open gate remains open but inward Na current decreases because slowly closing gate is closed
K Channel past peak of AP
All of the slowly opening gates are now open
Na Channel during repolarization
Both gates are closed
K Channel during repolarization
Some of the gates are beginning to close (outward current decreases but still continues)
Na Channel Repolarized
Slowly closing gate reopens. Rapidly opening gate does not reopen b/c membrane potential is below threshold
K Channel Repolarized
Most slowly opening gates are closed
Both Channels at the Resting Potential
Na: gates are unchanged (rapidly opening is closed, and slowly closing is open)
K: all gates closed
Control of Sodium Conductance
Depolarization leads to increased conductance which leads to more sodium entry into the cell which leads to further depolarization which leads to increased conductance....
What stops the Sodium Conductance Cycle?
When the bottom gate closes (slowly closing gate), it cuts off sodium conductance
Control of Potassium Conductance
Depolarization leads to increased conductance of K which leads to K efflux, resulting in hyperpolarization
Hyperpolarization and Depolarization ARE NOT CONNECTED
Sodium Potassium ATPase Pump
During each action potential cells gain a small amount of Na and loose a small amount of K
Na-K ATPase pumps: pump sodium out of the cell while pumping potassium back into the cell (consuming energy in the process)
THIS IS KEY: NA-K PUMP DOES NOT REPOLARIZE THE CELL DURING AN ACTION POTENTIAL (cell is repolarized by K leaving the cell)
Action Potential Propagation
AP propagates to the left and not to the right (in the area ahead of the AP, Na channels are ready to be activated on depolarization)
In the area just to the right of the peak of the AP, Na channels are in various states of inactivation (they cannot be reactivated until being reset by hyperpolarization)
Further to the right, the Na channels are reactivated (prepared to produce a new AP when activated)
What determines how soon the new AP is created?
Primarily a function of axon diameter and myelination
The amount of current leaking out, in part, determines how far along the axon the new AP is generated (leaker = closer the new AP is to the original = slower conduction velocity)
Increases with increasing axon radius (vertebrates generally use myelination)
Myelinated CV is proportional to axon radius
Non-myelinated CV is proportional to the square root of axon radius
Conduction in a Myelinated Axon
Even though there are some voltage-gated Na and K channels under they myelin, neither can pass current since they are blocked
What channels are concentrated at the Nodes of Ranvier?
Voltage-gated Na channels
AP jumps from node to node (typically in groups of 3-5)
Multiple Sclerosis (MS)
Most common demyelinating disease. Current leaks out between nodes AND voltage-gated K channels (normally blocked by myelin) are exposed
At best, AP is slower. At worst, AP is blocked