voltage clamp Flashcards
What is the function of a voltage clamp?
Allows the Vm to be changed/controlled while measuring ion currents
A voltage clamp is a technique used in electrophysiology to hold the membrane potential of a cell at a set value while measuring ionic currents that flow across the membrane.
What does the voltage-dependence of ion currents refer to?
Na+ and K+ permeabilities depend on membrane potential (and vice versa)
This relationship indicates that the permeability of these ions changes with the membrane potential, influencing their movement across the membrane.
Define permeability in the context of ion channels.
Ability of ions to cross the membrane; directly proportional to the total number of open channels for a given ion in the membrane
Permeability is a key factor in determining how easily ions can move across the cell membrane.
What types of voltage-dependent ion currents are activated during depolarization?
Early and late
These currents are characterized by their distinct activation and inactivation kinetics during an action potential.
When multiple ions contribute to the membrane potential, what does Vm not equal?
Ex
Ex refers to the equilibrium potential for a specific ion, which may differ when multiple ions affect the membrane potential.
What is the electrochemical driving force (VDF) formula?
VDF = Vm - Ex
This formula helps to determine the net movement of an ion across the membrane based on its current membrane potential and equilibrium potential.
What occurs when a channel is opened regarding ion movement?
Ions will immediately rush in/out
This rapid movement is driven by the electrochemical gradients established for the ions involved.
How does the sign of VDF help predict ion flow direction?
The sign (+/-) of VDF and the valence of the ion can be used to predict the direction of ion flow across the plasma membrane (into or out of the cell)
Understanding the charge of the ion (cation or anion) is crucial in predicting its movement under the influence of the electrochemical driving force.
For cations, what does VDF > 0 indicate?
Efflux
This means that when the electrochemical driving force is positive, cations will move out of the cell.
For cations, what does VDF < 0 indicate?
Influx
When the electrochemical driving force is negative, cations will move into the cell.
What are the characteristics of changes in membrane conductance during an action potential?
Time- and voltage dependent
This means that the conductance of the membrane changes in response to the timing of the action potential and the voltage across the membrane.
What are the characteristics of action potentials (APs)?
- Are all-or-none (feedback cycles)
- Travel down the axon as a wave of depolarization
- Proceed in one direction (due to refractory period)
- Can vary in frequency
None
What explains the self-sustained nature of action potentials once initiated?
The feedback cycles during the action potential
Feedback cycles contribute to the regenerative nature of action potentials.
What is the role of voltage-dependent Na+ and K+ conductances in action potentials?
They explain the properties of action potentials and how they propagate
Active and passive current flow results from these conductances.
What is the refractory period in relation to action potentials?
It is the time interval in which an action potential will not fire in a given region when given a 2nd stimulus
This period includes the undershoot phase.
What are the two types of refractory periods?
- Absolute refractory
- Relative refractory
Absolute refractory means no AP can fire; relative refractory requires a larger stimulus.
What happens during the undershoot phase of the refractory period?
K+ channels are still open
This contributes to the inability to fire another action potential.
Fill in the blank: The action potential shape and all-or-none behavior enable _______.
[long-distance signalling]
This is crucial for effective communication within the nervous system.
True or False: Action potentials can propagate backwards.
False
The refractory period prevents re-excitation of the same membrane segment.
What limits the number of action potentials that a neuron can produce per unit time?
The refractory period
This limitation is referred to as the maximum AP firing rate.
What is the consequence of K+ channels being still open during the refractory period?
It contributes to the undershoot phase, affecting the ability to generate subsequent APs
This phase is critical in understanding the timing of AP generation.
How does the action potential propagate down the axon?
As a wave of depolarization
This propagation is essential for transmitting signals along the nerve.
What is the self-sustained nature of an action potential (AP)?
The AP exhibits a regenerative nature once initiated and the threshold is reached.
This means that once the AP starts, it continues without needing additional stimuli.
What does the shape of the action potential demonstrate?
The all-or-none behavior of the AP.
This indicates that an AP either occurs fully or not at all, with no partial responses.
What type of feedback does Na+ represent in the action potential?
Positive feedback.
This is crucial for the rapid depolarization phase of the AP.
What type of feedback does K+ represent in the action potential?
Negative feedback.
This is important for the repolarization phase of the AP.
What is the consequence of action potential propagation?
Enables long-distance signaling.
This allows neurons to communicate over large distances within the body.
What results from active and passive current flow in an AP?
The propagation of the action potential.
Active flow involves ion channels, while passive flow pertains to the movement of charge along the membrane. active flow” refers to the rapid movement of ions across the neuronal membrane through voltage-gated channels, primarily sodium channels, which actively generate the electrical signal by causing a large depolarization, while “passive flow” describes the gradual spread of electrical charge along the membrane due to local currents, which occurs without the opening of voltage-gated channels and progressively weakens as it travels further from the initial site of depolarization
What are the two types of voltage-dependent conductances involved in an action potential?
- gNa+
- gK+
These conductances refer to the permeability of the membrane to sodium and potassium ions, respectively.
In what direction is the action potential polarized?
One direction only.
This unidirectional flow is essential for proper signal transmission.
What is the refractory period?
The time interval in which an AP will not fire in a given region when given a 2nd stimulus.
This period is crucial for regulating the frequency of action potentials.
What phases are contained within the refractory period?
- Undershoot phase (K+ channels still open)
- K+ channels still open + inactivation of gNa+
These phases contribute to the limitations on firing rates.
What is the consequence of the refractory period on action potentials?
Limits the number of APs that a neuron can produce per unit time.
This is important for preventing excessive firing and maintaining signal integrity.
What does the absolute refractory period refer to?
A time interval in which an AP will not fire in a given region when given a 2nd stimulus.
This ensures that the neuron cannot be re-excited immediately.
What does the relative refractory period require?
A larger stimulus to elicit an AP.
This indicates that while the neuron can fire again, it is less responsive than normal.
