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11 Cards in this Set
- Front
- Back
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What does hyperpolarizing and depolarizing mean?
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Hyperpolarizing: making the cell more negative
Depolarizing: making the cell less negative or more positive |
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What are the features or characteristics of a graded potential?
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1. They propagate decrementally after stimulation
2. They can be hyperpolarized or depolarized 3. They can be summed |
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What events are needed to initiate an action potential?
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1. Threshold stimulus moves the potential difference to a threshold potential.
2. This opens a popn of voltage-gated Na channels which increases the conductance/permeability of Na (gNa) and depolarizes the membrane 3. This results in an explosive positive feedback loop which recruits more Na channels = Hodgkin cycle. |
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What is the Hodgkin cycle?
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The fact that an initial threshold stimulus leads to the depolarization of a membrane which leads to an explosive positive feedback of Na recruitment. This leads to a massive increase in pNa.
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What are the events of the action potential?
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1. Rapid Depolarization: Threshold potential reached which allows for inc and massive pNa in cell.
Na channels are inactivated right after they open to allow ions in irrespective of what happens to the original stimulus. 2. Rapid repolarization/hyperpolarization: a. K channels are triggered by depolarization too but there is a delay in their opening as they need more stimulus to get depolarized. K channels close after repolarization 3. After hyperpolarization: Due to the newly opened voltage-gated K channels plus the leak K channels, there is a summed activity of repolarization which leads to a longer deflection below the threshold potential. |
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What are the differences between an action potential and a graded potential?
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Action potential: all-or-nothing response, not decremental, are always depolarizing, and cannot be summed
Graded Potential: Can be propagated decrementally, stimuli can be summed, and they can be depolarized AND hyperpolarized. |
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What are two ways used to prolong the action potential (this is a bad thing)?
Give examples of how nature uses these |
1. Retard inactivation of Na channels
2. Block opening of voltage-gated K channels Some toxins produce these effects. Ex. Scorpion toxin acts on Na channels, while sea anemone toxins act on K channels |
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What is the difference between the absolute refractory period and the relative refractory period of an excitable cell?
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Absolute refractory time: This occurs from the initiation of hyperpolarization to after-hyperpolarization where the Na channels are completely inactive and there is no chance for them to open no matter the stimulus.
Relative refractory period: this is the second inactive state of the Na channels where they can be stimulated by subsequent depolarization. |
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What is referred to as the inherent stochastic variability in the activity of indv. Na channels?
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This describes the fact that during the second inactive state of Na channels, not all the Na channels open at the same time; some take longer than others. So a larger than normal stimulus to open enough channels to fire an action potential from the reduced pool of channels
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How does the RATE of depolarization affect Na channels?
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They are highly sensitive to RATE of depolarization.
If the membrane depolarizes slowly, Na channels begin to shift back to the inactive and unavailable state which reduces the number of channels needed to fire a new action potential. |
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What does it mean that a difference between action potential and graded potential is that action potential only depolarizes and graded potential depolarizes or hyperpolarizes?
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the ‘rising phase of the AP, which reflects opening of the voltage-gated Na channels, can ONLY result in depolarization of the membrane (i.e., Na will enter, not exit, the cell). Hence, APs are ‘depolarizing’ events, whereas graded potentials can be either depolarizing or hyperpolarizing (e.g., open a K+ channel, somehow, and the cell will hyperpolarize by some amount).
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