Lecture 4 - Action Potentials Flashcards
What are the 3 characteristics of an action potential
- It is all-or-none: it will either occur or not occur.
- It is self-propagating: each region of depolarization serves to generate action potentials on either side.
- It is non-decremental: it does not decrease in strength
Describe the 3 main types of ion channels (pertaining to their openings)
- Always open channels (Slow-Leak Channels - Filter ions through slowly)
- Ligand-Gated Channels - require the attachment of a chemical messenger such as a neurotransmitter or hormone to a receptor.
- Voltage Gated Channels - require a change in the membrane potential.
Describe the molecular structure of voltage-gated Sodium channels
- Thought to have 4 domains in cylindrical formation
- Each domain has 6 hydrophobic transmembrane segments (S1-S6).
- The S4 segment within each domain has a high positive charge. - The inactivation gate is associated with an intracellular hydrophilic linkage between domains three and four.
Voltage gated sodium ion channels have two gates: The activation gate, and the inactivation gate. Describe the membrane voltage potentials at which each gate is opened and closed.
- Activation Gate Closed/Inactivation Gate Opened: -90mV
- Both Gates are Open anywhere between -90mV and +35mV
- Except not quite because the activation gate starts to (fully) open between -70mV and -50mV
- Also apparently as the voltage Drops from +35 to -90, the activation gate is still open, but the inactivation gate is closed.
- Presumably, once we get back to -90, Activation gate Closes and Inactivation gate opens
In voltage potassium gated channels, how does the channel prevent sodium from crossing through the potassium channel?
- The passage for the potassium channel is too small for either hydrated potassium ions, or hydrated sodium ions
- However, in the start of the passage, there are loops in the pore helices with attached carbonyl oxygens. This forms the selectivity filter.
- Hydrated sodium ions are still to small to hit this selectivity filter, but larger hydrated potassium can be dehydrated by this filter, allowing the newly created ion to slide through.
Describe the voltage gate membrane potentials for potassium channels
- Unlike sodium, there is only a single gate in the potassium channel. It’s closed at a resting potential of -90mV.
- The gate opens as voltage travels down from +35mV back down to -90mV. Very. Very. Slowly. (At least compared to sodium gates)
Reminder Slide:
How does sodium Leave the cell? How Does Potassium enter and leave?
- Sodium Leaves the cell via the Sodium-Potassium Pump(not a channel! A Pump), which utilizes ATP to send 3 Na+ ions out of the pump, while at the same time, pulling 2 K+ ions Into the Cell
- Potassium has an easier time leaving the cell, using the voltage gated potassium channel
What are the 4 stages of polarization used to generate an action potential? Describe each stage
- Resting Stage - Cell’s chilling at -90mV membrane potential
- Depolarization Stage - Voltage Jumps, Membrane becomes more permeable to sodium ions, but potential can often overshoot if it’s a large axon.
- Repolarization Stage - Sodium Channels begin to close again within a few/ 10,000ths of a second, but potassium channels are still opening
- Sodium and Potassium Conductance - Yeah…more on this later
In a cell, when the voltage gate jumps from -90 to +35, when does the potassium channel gate open?
Immediately, but it takes a long time for the gate to open, and as the voltage begins to drop back down to -90, the gate slowly begins to close at (roughly) the same speed
Describe Positive Afterpotential
Following Repolarization, the membrane potential enters a phase called positive afterpotential, or Hyperpotential, where the resting potential drops even lower thatn -90mV (usually around -94). This hyperpotential phase restabilizes to -90mV over a the course of a few miliseconds (longer than it sounds).
What causes the sudden voltage changes in membrane potential
The release of sodium from the cell.
There are 3 conformational states of the sodium channel, Closed, when Conductance is low, Open, where conductance is high, and membrane potential begins to increase, (this causes more sodium channels to chain-react open…I think) And when membrane potential is positive enough, the inactivation gate closes and the cell begins to repolarize. (Once opened, the activation gate CANNOT close until after the inactivation gate has closed)
Extra details about the sodium channels:
- What are the Letter Names for the 2 Sodium Gates?
- What is the other name for the depolarization phase
- What is the name of that phenomenon for when More Sodium gates open in response to the potential rising?
- Activation Gate = m-gate, Inactivation gate = h-gate
- The upstroke phase
- Positive Feedback Response - Cells have many channels and each one has a different specific “voltage” for exactly when it’s respective activation gate (m-gate) opens.
Note: Understand the difference between Gate Status: Open, Closed, and Inactivated
- What Is the primary cause for Repolarization?
2. What is the cause of the hyperpolarization phase
- While a high enough membrane potential causes the sodium gate to inactivate, it’s the Potassium gate that Repolarizes the cell and brings the potential back down to -90ish through slow-leak channels and slow opening and closing gate.
- As Potassium channel gates closed, it briefly brings down potential below -90mV in a phase also called the Undershoot phase. (Aka, I’m not totally sure)
List the two factors that can affect propagation speed
- Myelination Level
- Fiber Size
Why is myelination important for action potential propagation?
- Sphingomyelin is the principle lipid found in myelin sheaths.
- Schwann cells are cells that form the myelin sheaths in peripheral axons.
- Nodes of Ranvier are the unmyelinated junctions between Schwann cells.
- Increases velocity of nerve transmission. Why? Induces the “jumping” type of conduction; Jumping’s faster than walking
- Allows 100x less loss of ions and requires little energy for repolarization
