Lectures 6, 7, & 8 Flashcards
Define membrane potential (Vm) and potassium equilibrium potential (EK). Which of these, if either, changes significantly during the course of an action potential?
MP - The difference in electric potential between the interior and the exterior of a biological cell.
EP - the voltage at which the chemical force of diffusion and the electrical force balance out, resulting in no net movement of an ion across a membrane.
Membrane potential significantly changes during action potential.
Write Ohm’s law and define conductance (g). Write Ohm’s law for current (I) containing conductance (g) and driving force (Vm-Eion).
Ohm’s Law: I = gV
amount of current (I) = conductance (g) times potential (V)
Conductance is a measure of how easily an electric current can pass through a material. It’s the opposite of resistance, which is how much a material impedes the flow of current.
I = g*(Vm - Eion)
I: Represents the ionic current flowing through the membrane.
g: Represents the conductance of the membrane for a specific ion, essentially how easily that ion can move across the membrane.
Vm: Represents the membrane potential, the voltage across the cell membrane.
Eion: Represents the equilibrium potential for a specific ion, the membrane potential at which there is no net movement of that ion.
Is there a large driving force for sodium ions at resting membrane potential? What ion is membrane most permeable to at rest?
Yes &no, there is a large driving force for sodium ions at resting membrane potential, but due to the low permeability of the membrane to sodium at rest, this force is not typically significant in influencing the overall membrane potential; essentially, there is a strong “desire” for sodium to move into the cell, but the cell membrane largely prevents it from doing so at rest.
BUT - the ion that is most permeable to the membrane at rest is POTASSIUM
How does permeability for potassium (K+) and sodium (Na+) change at resting Vm versus during the rising phase of an action potential?
At resting membrane potential, the permeability for potassium (K+) is significantly higher than sodium (Na+), meaning potassium ions can readily move across the membrane, while sodium movement is restricted; however, during the rising phase of an action potential, the permeability to sodium rapidly increases, allowing a large influx of sodium ions into the cell, while potassium permeability remains relatively low at this stage.
Name two key features for inward and outward currents during the action potential? What is inactivation?
During an action potential, the key features of inward currents are that they are primarily carried by sodium ions (Na+) flowing into the cell, causing depolarization, while outward currents are mainly carried by potassium ions (K+) flowing out of the cell, causing repolarization; the inward current is rapid and occurs early in the action potential, while the outward current is slower and occurs later in the action potential.
Inactivation is the process of a channel closing during an action potential
What is a voltage-clamp and why is it important for studying ionic conductances?
A voltage-clamp is an experimental technique used in electrophysiology to measure the ionic currents flowing across a cell membrane by holding the membrane potential at a set, constant voltage, allowing researchers to study how different ion channels behave under controlled voltage conditions, essentially isolating and analyzing the conductance of specific ions across the membrane.
Why it’s important for studying ionic conductances:
1. Precise control of membrane potential: Unlike in a normal cell, a voltage clamp actively adjusts the current flowing into the cell to maintain a predetermined membrane voltage, enabling the study of how ion channels respond to specific voltage changes without the confounding effects of natural membrane potential fluctuations.
2. Isolation of specific ion currents: By manipulating the extracellular solution and using selective ion channel blockers, researchers can isolate the currents carried by specific ions, like sodium, potassium, or calcium, providing detailed information about their individual conductances.
3. Understanding voltage-gated channels: Voltage-clamp is crucial for studying voltage-gated ion channels, which are critical for neuronal signaling and muscle contraction, as it allows scientists to directly observe how these channels open and close in response to changes in membrane potential.
4. Kinetic analysis: By rapidly changing the membrane voltage, researchers can analyze the kinetics of ion channel opening and closing, including activation and inactivation time constants, which are important for understanding the physiological function of the channels.
What is TTX? Where is it found? How does it prevent action potentials?
TTX stands for Tetrodotoxin, a highly potent neurotoxin primarily found in the organs (like liver and gonads) of pufferfish, but also in some other marine animals like certain types of octopus and shellfish; it prevents action potentials by selectively blocking voltage-gated sodium channels on nerve cell membranes, thus inhibiting the influx of sodium ions necessary for an action potential to occur, leading to paralysis.
How does Batrachotoxin affect voltage-gated sodium channel opening?
- Holding channels open: BTX binds to voltage-gated sodium channels and prevents them from closing, acting like a stent to keep them open
- Shifting activation curve: BTX shifts the activation curve to more hyperpolarized potentials, which means the sodium channel is already open at resting potential
- Impairing inactivation: BTX impairs the inactivation process of voltage-gated sodium channels
- Increasing resting sodium permeability: BTX increases the resting sodium permeability of voltage-gated sodium channels
Recall Na v structure. Which segment is important for voltage-sensing?
The S4 segment is the key part of a Nav channel responsible for voltage-sensing
Where are voltage-gated sodium and potassium channels located in neurons?
Voltage-gated sodium and potassium channels are primarily located along the axon of a neuron, with the highest concentration found at the axon hillock (the initial segment of the axon), where they play a crucial role in initiating the action potential.
Is lidocaine a neurotoxin? How is it similar to TTX?
What is the difference in location and function of low versus high-threshold Nav?
Yes.
Lidocaine and tetrodotoxin (TTX) are both drugs that block voltage-dependent sodium channels, which can have similar effects on action potentials
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What is saltatory conduction of action potentials?
Saltatory conduction is the process by which action potentials propagate along myelinated axons by jumping from node of Ranvier to node of Ranvier. This process is faster than action potential propagation in unmyelinated neurons and is made possible by the myelin sheath that insulates the axon.
What is the difference between inside-out and outside-out electrophysiological recordings?
Inside-out and outside-out electrophysiological recordings differ in how the membrane is prepared and what the recordings can show:
Inside-out : The membrane folds onto itself to form a vesicle, exposing the intracellular surfaces of ion channels. This configuration is useful for studying channels activated by intracellular ligands.
Outside-out : The membrane detaches from the cell and reforms into a vesicle, exposing the outside of the membrane to the bath solution. This configuration is useful for studying the properties of ion channels isolated from the cell
What is optogenetics? How can light depolarize and hyperpolarize neurons?
Optogenetics is a technique that uses light to control neurons by expressing light-sensitive proteins in nerve cells. These proteins, called opsins, can be activated by specific wavelengths of light, which can then depolarize or hyperpolarize neurons.
Here’s how light can depolarize and hyperpolarize neurons:
Depolarization - Blue light activates ChR2, a light-sensitive channel that causes an influx of Na+ ions, which depolarizes the neuron.
Hyperpolarization - Yellow light activates Halorhodopsin, a light-gated chloride pump that hyperpolarizes the neuron, inhibiting its function.
