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