Action Potential Flashcards
What are the phases of the action potential?
1- Rising (depolarizing) phase
2- Overshoot phase
3- Falling (repolarizing) phase
4- Undershoot phase
What phase incorporates the refractory period?
undershoot (hyperpolarization) phase
Which ion is essential for the generation of an action potential?
Sodium (Na+ channels contribute to the depolarizing phase)
What results if external sodium levels are decreased?
Smaller and slower action potentials
What are the three different voltage sensitive mechanisms that generate action potentials?
1- activation of Na+ conductance
2- delayed activation of K+ conductance
3- inactivation of Na+ conductance
What does the release of K+ from the cell cause?
repolarization and hyperpolarization
Can a cell “fire” action potentials in response to stimulation during the refractory period?
When in the refractory period the cell does not”fire” action potentials in response to stimulation.
Why does hyper polarization occur?
The K+ channels stay open longer than the Na+ channels.
Explain how Hodgkin and Huxley demonstrated how volt gated channels work?
They varied the membrane potential in squid giant axon to measure the changes in membrane conductance through volt gated Na+ and K+ channels.
What two types of voltage-dependent ionic currents were revealed in Volt-Clamp technique?
Early inward current Na+ and Late outward K+
Which drug blocks sodium channels and not potassium channels?
Tetrodotoxin (TTX)
Early current is blocked (because no Na+)
Which drug blocks potassium channels and not sodium channels?
Tetraethylammonium (TEA)
Late current is blocked (because no K+)
What is active current flow?
the gating of the voltage-gated channels and associated influx of Na+ current
what is passive current flow?
The depolarization wave that precedes the action potential.
Is there passive current flow though the membrane?
No, only through channels
What is the purpose of passive current flow?
To discharge the membrane capacitance and lead to sodium channel activation
What occurs during the absolute refractory period?
Na+ channel inactivation (which limits the frequency of firing and allows the signal to be propagated in a single direction)
What is the relative refractory period?
Brief hyperpolarized membrane potential due to open resting and voltage gated K+ channels that ends when the voltage gated K+ channels close
What are the four different colt-sensitive mechanisms of action potentials?
1- activation of Na+ conductance
2- activation of K+ conductance
3- inactivation of Na+ conductance
4- closing of voltage gated K+ channels
Which type of cells form myeline sheaths?
Glial cells (oligodendrocytes in the CNS and Schwann cells in the PNS)
What is the purpose of myelination?
1- increases insulation which leads to reduced leak of passive flow
2- decreased capacitance (which is irreversible to the thickness of the membrane)
What are nodes of ranvier?
gaps in myelin sheath that contain full compliment of Na+ and K+ channels, where action potentials can be generated
What is the conduction velocity of unmylinated nerve as compared to the myelinated nerve?
unmyelinated= 0.5 to 10 m/s
myelinated= about 150 m/s
What is saltatory conduction?
Action potential that jumps from node to node
