17 Flashcards
What is the membrane potential change stimulated by?
By the depolarizing current pulse arrive above a certain threshold (when depolarization becomes independent from the size and duration of the Curren impulses)
What is the principle of The voltage-clamp technique?
the membrane potential of a cell is kept constant regardless of the magnitude and the direction of the ionic currents flowing through the membrane.
What is The simplest way of achieving voltage-clamp ?
The implementation of two microelectrodes into a single cell.
→ One of these electrodes (electrode 1) is used to measure the actual membrane potential (with respect to a ground electrode in the bath solution surrounding the cell)
→ The difference between the command and the actual membrane potentials is calculated
→ A current is injected-into the cell through the second electrode (electrode 2) with appropriate polarity and magnitude to cancel this difference.
Why is voltage-clamp technique is suitable for measuring individual ionic currents?
Activation of voltage-gated ion channels at the new membrane potential and the flow of the appropriate current is expected to change the membrane potential since charges are moving across the cell (in this case positive charges are flowing into the cell).
→ However, the change in the membrane potential is prevented by the injection of a current into the cell through electrode 2
→ the membrane potential remains clamped at the command potential. The current injected into the cell through electrode 2 can be measured and its amplitude is equal to the membrane current
Describe phase 1
Slow depolarization of the membrane to reach the depolarizing threshold
→ Opening of the voltage-gated Na+ channels, Na+ influx overrides background K+ efflux
Describe phase 2
Depolarization phase
→ Rapid opening of voltage gated Na+ channels, na+ influx
What does this graph show?
Typical curves for specific Na+ and K+ conductivity changes during an action potential
Describe phase 3
Peak potential
→ Inactivation of voltage gated Na+ channels and delayed opening of voltage gated K+ channels
→ K+ efflux balances na+ influx
Describe phase 4
Repolarization
→ Voltage-gated Na+ channels are inactivated
→ Voltage gated K+ channels are open
→ Na+ efflux
Describe phase 5
Hyperpolarizing after potential
→ Voltage-gated Na+ channels close with a delay
→ K+ efflux dominates until the membrane potential returns to the resting value
What is absolute refractory period?
The interval of time during which a second action potential cannot be initiated, no matter how large a stimulus is repeatedly applied.
What is refractory period?
the interval of time during which a second action potential can be initiated, but initiation will require a greater stimulus than before.
Describe patch-clamp technique
- Ideal for measuring ionic currents under voltage-clamp condition
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What is Propagation of action potential?
What affects conduction velocity of an action potential?
Axon diameter, internode distance, and myelin sheath thickness all influence the speed of action potential propagation.
How are the time constant and the space constant related to propagation velocity of action potentials
The relationship between conduction velocity and axon diameter
Describe saltatory conduction
Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.
→ The uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin
