Week 2: Action Potential Flashcards
What is extracellular recording of the action potential?
The ionic movement across the membrane (current) can be detected without impaling the neuron using extracellular recording. In extracellular recordings, an electrode (either a thin glass capillary filled with salt solution, or just an insulated metal wire) is placed next to the neuron, while the ground is placed further away. When the action potential occurs, positive charge flows away from the recording electrode so that a brief, alternating volatage between it and ground is shown. They can be observed by connection to an oscilloscope or to a loud speaker (they go “pop”).
How is the firing frequency of action potentials by a neuron is influenced by the injection of charge (current) into the cell?
A larger current will increase the frequency of action potentials. (I.e. increased frequency of APs codes for higher magnitude of stimulus.)
Firing frequency reflects the magnitude of the depolarizing current
Define threshold.
The critical level of depolarization that must be reached in order to trigger an action potential.
Action potentials are caused by depolarization of the membrane beyond threshold.
At this point, enough volatage-gated sodium channels are open so that the relative ionic permeability favours sodium over potassium.
Define absolute refractory period.
During this period, new action potentials cannot be generated. This stops backward propagation, and is accounted for by the inactivation of voltage-gated sodium channels.
Define relative refractory period.
During the relative refractory period, for a new action potential to be generated, more current than normal is required.
The membrane potential is hyperpolarised until the voltage-gated potassium channels close.
How is the patch clamp technique used?
Patch clamp technique (Neher & Sakman) enables
single channel currents to be measured
List the factors that affect conduction velocity in axons.
• Speed of action potential along membrane -Dependent on axon structure • Path of the positive charge -Inside the axon (faster) -Across the axonal membrane (slower) • Axonal excitability -Axonal diameter (bigger = faster) - Number of voltage-gated channels
Describe the changes in the ionic permeability of the membrane during the rising (depolarizing) phase. (In terms of the opening and closing of different membrane ionic channels.)
Na+ channels open.
K+ channels remain open.
Inward Na+ current
Outward K current
Na+ current dominates
Rising phase of action potential is explained
by an inward sodium current.
Describe the changes in the ionic permeability of the membrane during the overshoot phase. (In terms of the opening and closing of different membrane ionic channels.)
Na+ channels open.
K+ channels remain open.
The relative permeability of the membrane greatly favours sodium, so the potential approaches ENa, which is a positve value (greater than 0mV).
Describe the changes in the ionic permeability of the membrane during the falling (repolarizing) phase. (In terms of the opening and closing of different membrane ionic channels.)
Na channels inactive
K+ channels still open.
K efflux occurs (K+ flows outward)
Falling phase of action potential is explained by an outward potassium current
The voltage-gated potassium channels finally open (triggered 1ms earlier by depolarisation). Great driving force on K+ to leave cell.
Describe the changes in the ionic permeability of the membrane during the undershoot (after- hyperpolarization) phase. (In terms of the opening and closing of different membrane ionic channels.)
The voltage-gated potassium channels have increased the membranes K+ permeability, so the potential approaches EK+. The membrane is hyperpolarised relative to the resting potential until the voltage-gated K+ channels close again.
How was the voltage clamp technique used by
Hodgkin & Huxley to identify these ion channels in the squid axon membrane?
The voltage clamp technique allows the membrane potential to be “clamped” at a value.
What effect does membrane potential have on voltage-gated sodium channels?
Na+ channels closed at -80 to -65mV, and open at -40mV.
The molecule twists open (a conformational change) at threshold (-40mV) to allow Na+ through.
How is the patch clamp technique used?
Patch clamp technique (Neher & Sakman) enables
single channel currents to be measured.
What is inactivation of the voltage-gated sodium channel?
The inactivation of the voltage-gated sodium channel occurs about 1ms after they open and causes the absolute refractory period.
The inactivation involves a globular portion of the protein swinging up to occlude the pore.
What is deinactivation of the voltage-gated sodium channel?
The globular portion of the voltage-gated sodium channel swings away, and the pore closes by movement of the transmembrane domains.
List the functional properties of voltage-gated sodium channels.
Found that Na+ channels closed at -80 to -65mV, and open at -40mV. They open without delay and stay open for about 1 millisecond then are deactivated and cannot reopen until the membrane returns to a negative value near the threshold.
They are highly selective for Na+ (12x more permeable to Na+ than K+), with pore loops as selectivity filter. Part of Na+ sphere of hydration may be stripped of and retained H2O may chaperone Na+ through the channel.
How is action potential conducted down the axons?
The influx of postive charge spreads to depolarise adjacent sections of the membrane, which at threshold, causes the voltage-gated Na+ channels to open.
Describe how local anaesthetics blocks action potential conduction.
Blockade of the voltage-gated Na channel
What is intracellular recording of the action potential?
Intracellular recording requires impaling the neuron or axon with a microelectrode. The goal of intracellular recording is to measure the potential difference between the tip of the intracellular electrode and another electrode placed in the solution bathing the neuron (ground). The electrode is connected to an amplifier to compare the difference.
State the difference between intracellular and extracellular recording of the action potential.
Intracellular recording requires impaling the neuron or axon with a microelectrode. The goal of intracellular recording is to measure the potential difference between the tip of the intracellular electrode and another electrode placed in the solution bathing the neuron (ground). The electrode is connected to an amplifier to compare the difference.
The ionic movement across the membrane (current) can also be detected without impaling the neuron using extracellular recording. In extracellular recordings, an electrode (either a thin glass capillary filled with salt solution, or just an insulated metal wire) is placed next to the neuron, while the ground is placed further away. When the action potential occurs, positive charge flows away from the recording electrode so that a brief, alternating volatage between it and ground is shown. They can be observed by connection to an oscilloscope or to a loud speaker (they go “pop”).
State the difference between intracellular and extracellular recording of the action potential.
Intracellular recording requires impaling the neuron or axon with a microelectrode. T
In extracellular recordings, an electrode (either a thin glass capillary filled with salt solution, or just an insulated metal wire) is placed next to the neuron, while the ground is placed further away.
During depolarization, which ion has an increased permeability across the cell membrane?
Chloride
Potassium
Sodium
Sodium
During repolarization, which ion has an increased permeability across the cell membrane?
Calcium
Potassium
Sodium
Potassium
What happens during an action potential?
Cell potassium concentration is changed appreciably.
Cell sodium concentration is changed appreciably.
There are no measurable changes in any cell ion concentrations.
There are no measurable changes in any cell ion concentrations.
Which statement regarding the relative refractory period is true?
a. The relative refractory period is when a stimulus of greater than normal intensity may elicit a new action potential.
b. The relative refractory period is when the voltage-sensitive sodium channels cannot be reactivated under any conditions.
c. The relative refractory period occurs before the absolute refractory period.
a. The relative refractory period is when a stimulus of greater than normal intensity may elicit a new action potential.
