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34 Cards in this Set

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  • Back
Define Action Potential:
A rapid, transient, reversible depolarization response of an electrically excitable cell membrane in response to a threshold depolarization.
4 Fundamental properties of an Action Potential:
1. It maintains a constant shape and size as it propagates
2. It is caused by a transient increase in electrical conductivity of the membrane
3. Cooling causes the AP velocity to slow down
4. Reduced Na+ externally causes the AP to slow and decrease in magnitude.
What 3 things cause the change in Em during an AP?
-Increased ionic conductance
-Ppl to increased Permeability
-Decreased resistance
What is an AP's amplitude and magnitude of overshoot ppl to?
Extracellular Na concentration
What can action potential overshoot be observed in? What tissue type is it NOT seen in?
Neurons, skeletal muscle, and cardiac muscle.
Not seen in Smooth muscle.
Why don't action potentials ever completely reach calculated Ena levels?
Because inactivation gates close and stop Na influx + K gates open and hyperpolarize the cell.
What is 'AMPLITUDE'?
The distance between resting potential to the peak of AP.
What is 'MAGNITUDE'?
Measured from zero.
What re the 3 major implications of the Sodium Hypothesis?
1. Calculated Na permeability or conductance is higher than for K at the AP peak
2. No detectable change occurs in cytoplasmic or extracellular bulk concentration of EITHER ion.
3. A SMALL amt of Na enters and a SMALL amt of K exits the cell during each AP.
How much does sodium permeability increase at the peak of an AP?
500 fold - from 0.05 to 20.0
What is the purpose of the voltage clamp technique?
To identify and measure the ionic currents that flow during an AP.
2 Things noted about AP by early Hodgkin and Huxley Patchclamp experiments:
1. Neuron currents have an early and late phase
2. When depolarized, ions first flow into cell (Na) and then out (K+).
What is Reversal Potential?
The Em for a specific ion at which there is no inward or outward ionic current.
What causes Reversal Potential?
Equilibrium between the chemical gradient and electrical gradient - so it is the same as the ion's Eequilibrium.
Why is it called "reversal" potential?
Because for Na, as the Em becomes more positive, Na reverses and flows out of the cell instead of down its chemical gradient (into the cell).
How were the individual ions responsible for an AP determined?
By measuring the total current and then individual ion currents and calculating the conductance from Em - Eion
2 ways to measure Ik separately from Ina:
1. Block Sodium current via TTX
2. Substitute positive cation with Choline
How to measure Ina separately from Ik:
Block potassium conductance with TEA
What is TTX?
Tetrodotoxin - a highly toxic poison from puffer fish that blocks fast Sodium channels in nerve and skeletal muscle
What is STX?
Saxitoxin - functions similar to TTX
What is TEA?
Tetraethylammonium - blocks K+ channels
What does cocaine block?
Nerve Na channels - but with lower affinity and specificity than for TTX.
Mechanism of action of TTX, STX, TEA, and cocaine:
Block the Na/K channels like a fat man stuck in a pothole.
Threshold potential:
The critical level of depolarization that when reached leads to a self-generated, rapid, further depolarization.
How is sodium conductance increased during an AP?
By positive feedback - until spontaneous closure of inactivation channels.
the ease of generating an AP
Equation for excitability
Excitability is ppl to 1/Eth-Em
Positive after potential
The undershoot of Em to a more negative value than resting Em
What causes the positive after potential?
Longer transient increase in potassium conductance after sodium has already returned to its normal level.
What is Icap?
Capacitative current
What happens to Icap when the permeability/conductance of Na is increased?
It flows transiently and removes the positive charge from the membrane.
When does Icap flow?
BEFORE sequential ionic currents of Sodium and/or Potassium!
What does the flow of capacitative current DO?
It is what opens voltage dependent ion channels!!
How does extracellular Ca have a stabilizing effect?
Increasing it moves Ethreshold higher and makes it harder to excite a cell