Week 4: Ion Channels and Action Potential Flashcards
(21 cards)
How does the depolarization of nerve cells occur? What repolarizes the cell?
The opening of voltage-gated sodium channels and the influx of sodium ions into the cell, making the cell less and less negative. After the peak of depolarization (the overshoot), sodium channels close and voltage-gated potassium channels open to allow K+ to flow out and restore the RMP. This is called the undershoot, as the K+ channels take a while to close.
What is the RMP of an axonal cell, and what ion’s resting potential is it closest to?
The RMP of an axon is about -70 mV, which is close to the RMP of K+. This is because the membrane has higher permeability to potassium, and a greater number of sodium leak channels.
What happens to membrane permeability at the voltage threshold?
The permeability of the membrane to Na+ goes up significantly at threshold, and the AP can begin.
How do you measure the ion flux(es) that make up the action potential? What does a negative slope versus positive slope mean on a voltange clamp curve of action potential?
Using a voltage clamp to hold the membrane potential at a fixed value, even when conductance is changing, researchers affixed several wires to an axon and amplifier to keep the voltage balanced across the membrane of a giant squid axon. A negative slope value means a positive ion is moving into the cell, and a positive slope means a positive ion is moving outwards.
What effects do TEA and TTX toxins have on membrane potential, respectively?
TEA blocks K+ channels, preventing repolarization of the membrane, and TTX blocks Na+ channels, preventing depolarization of the membrane at all. Use of these toxins helped scientists see that over time, the Na+ channels open, then shut soon after, and that there is a delay in K+ channel opening, and a longer period of time over which they work to allow for repolarization.
Briefly explain how the voltage-gated K+ channels open and close.
Voltage-gated K+ channels open when voltage is changed, and only close when the RMP is reached once again–their closing is NOT time-dependent, but rather voltage-dependent.
Briefly explain how the voltage-gated Na+ channels open and close.
The Na+ channels open once a certain membrane threshold is reached (-50 mV), but close and inactivate soon after (less than 5 ms total between open and close), and are thus considered both voltage- and time-dependent channels.
What is the relevance of the inactivation period on voltage-gated Na+ channels? The closed period after inactivation?
It occurs very soon after sodium channels open, and ensures that another action potential cannot immediately be sent through the same axon. During the closed period after inactivation of Na+ channels, it is difficult to re-initiate an AP because the K+ channels have re/over-established an RMP (~-75-80mV compared to the normal -70mV)
What are common disorders or diseases that affect nerves?
Hyperkalemic paralysis (muscle weakness) and paramyotonia congenita (bouts of sustained muscle tensing) are diseases that arise from mutations in the Na+ channel. Issues with this channel have driven docs to study the affects of cocaine, lidocaine, procaine, and tetracaine on the channel, which act as local anesthetics.
What are the absolute and relative refractory periods?
The absolute refractory period occurs when Na+ channels are inactivated, and are thus not able to open. NO AP can be generated at all during this time. The relative refractory period occurs after Na+ channels are re-activated (but closed) and K+ leak channels have hyperpolarized the membrane. This makes it difficult, but not impossible, for cells to undergo an AP.
What are the two ways membrane areas nearby an area that is depolarized can carry the AP forward?
Lateral spreading of Na+ ions that have rushed into the membrane is the first way APs can be carried down the line, and saltatory conduction at the Nodes of Ranvier also help speed up this process by opening Na+ channels at points just far away from one another to keep the AP moving.
Why can’t an AP travel backwards?
Inactivation of Na+ channels prevent an AP from traveling backwards.
Based on the flux of Na+/K+, when is an AP actually generated?
An AP is generated when the influx of Na+ is greater than the efflux of K+. This means the amount of K+ leak channels is an important determinant of threshold values.
How does calcium regulate the activation of voltage-gated Na+ channels?
High [Ca2+] confers decreased excitability on membranes by Na+, and low [Ca2+] confers increased excitability. Activation of voltage-gated sodium channels is shifted to a MORE depolarized value (requires more/higher stimulation) due to the shielding effect of extracellular Ca2+
What is the difference between an action potential down a neuron and a localized action potential?
AP down a neuron propogates down the line in an “all or nothing” fashion, with the amplitude of voltage remaining the same as it travels. The AP in a localized stimulation, as in muscle, can still travel, but only a short distance, and the voltage of the AP decreases over that distance.
What can change the conduction velocity of an axon?
The internal diameter and resistance of an axon (resistance usually mediated by myelination), as well as the conductance (/concentration) of Na+ channels in the axonal membrane.
What are the key equations for calculating the space constant of a neuronal cell? What is the space constant?
λ = sqrt(Rmembrane/Rinternal)
where Rinternal = 1/diameter
The space constant is the distance at which the voltage of the propogated AP signal has dropped by 63% of it’s original value (to 37% of OG value).
What can affect the resistance of a membrane?
The presence/absence of phospholipids, the number of channels present, and the amount/degree of myelination. MORE myelination = MORE membrane resistance, but that is ultimately outweighed by the decrease in capcitance that results from myelination.
What cells generate myelin? What are some of their characteristics?
Oligodendrocytes in the CNS and Schwann cells in the PNS. They allow for saltatory conduction of the AP, and lower membrane resistance.
Define capacitance, and discuss how it relates to the time constant of APs.
The time constant is equal to a 63% rise in voltage, and is calculated via:
τ = Rmem x Cmem
Capacitance is stored charge across the membrane. The lower the capacitance of a membrane, the lower the time constant and the faster the AP will propagate.
What is a biological means to decrease capacitance of a membrane?
Myelination by oligodendrocytes or Schwann cells decreases membrane capacitance, and allows for faster conduction of APs.
