4 - Electrical Signaling by Neurons Flashcards
Why is it difficult for a neuron to move an electrical signal? What type of signal do neurons send?
There is a lot of resistance.
I=V/R ; current = voltage/resistance
Signals are ions.
What are some of the properties of the membrane lipid-bilayer in neurons?
Impermeable to ions, functions as a capacitor that separates positive and negative charges, ions move through channels that act as conductors.
What is the structure of channels in neuron lipid bilayers? What states do they have?
Have several subunits and allow ions to flow through by size and charge.
Can be in an open or closed state.
What is an example of a highly selective channel in a lipid bilayer? What about one with minimal selectivity?
Highly selective: Na+ allowed and K+ not.
Minimally selective: anions vs cations
What does it mean that ion channels are variable resistors?
This means that at a given time there can be more or less channels open, thus varying the level of conductance of the lipid bilayer.
What are the different types of gating for lipid bilayer channels?
Voltage gated
Ligand gated
Thermally gated
Mechanically gated
What does it mean if a plasma membrane is impermeable? What does this say about distribution of ions?
It will not allow ions to flow down their concentration gradients.
Negative and positive charges would be identical in each side.
What is an equilibrium potential and how is it calculated?
The potential at which an ion is in electrochemical equilibrium.
Measured in milivolts and can be calculated using the Nernst Equation.
What values are needed to calculate the equilibirum potential of an ion with the nernst equation?
The concentration of the ion inside and outside the cell.
Is the resting membrane potential solely determined by the K+ gradient? How do you know?
It’s heavily influenced by it, but not completely determined by it.
If it were solely determined by it the resting potential would be -94 mV; but its around -65 or -70 because a small amt of Na+ goes inside and makes it less negative.
What two things influence the passive movement of ions across the membrane and establish the resting membrane potential?
- Electrochemical gradient: the balance between the chemical and electrical gradients.
- Conductance of the channel
What maintains concentration gradients? What is an example of this?
Ion pumps that pump ions against their concentration gradient. Requires energy.
Na+/K+ ATPase.
What does it mean (in terms of the membrane potential) if the membrane is depolarized? What about hyperpolarized?
Depolarized: less negative
Hyperpolarized: more negative
What are the two types of electrical properties of neurons?
Passive: cable properties
Active
What are the cable properties of neurons?
The amplitude of the potential change decays exponentially as it moves away from the source - this is because the capacitor is always charged first.
The change in Vm (velocity) passively spreads in both directs along the axon/dendrite.
What is the length constant?
The distance over which the potential (current) falls by 1-(1/e) or 63% from its original value.
Describe the movement of an action potential (AP)? How is this different from cable properties?
Unlike cable properties where the signal fades, action potentials actively propagate over long distances with the same intensity until they reach the axon terminal where neurotransmitters are released.
Action potentials travel in one direction, unlike cable properties.
What is the function of action potentials?
Release of neurotransmitters for communication.
What are the stages/requirements of an action potential?
Require reaching a threshold, all-or-none response, latency(time from stimulus to AP), refractory period, propagation.
What is the refractory period of an action potential? What are the two parts?
Period at which it’s hard for the neuron to generate another AP.
Absolute refractory period: no way to generate an AP
Relative refractory period: can make AP, but you need a higher current than normal AND the AP is not at the same height.
What limits the repetition rate of action potentials? What is the benefit of this?
The refractory period; this makes sense because it allows neurons to rest and limit their speed.
When is there no AP? What is the absolute upper limit of AP frequency for all types of neurons?
The rising phase and most of the falling phase.
1kHz
Properties of _______ underlie the action potential.
Voltage-gated Na and K channels.
What types of K+ channels are responsible for the resting membrane potential?
Leaky K+ channels; these are different than voltage-gated K+ channels.
When do voltage-gated Na channels close? When do they open back up?
After Na+ comes into the cell. they close and do not open (inactive) back up until the membrane goes back down to resting potential and depolarization occurs.
Do voltage-gated K channels have an inactive state? When do they close?
No; as long as depolarization occurs K+ can go out.
They close at the resting membrane potential.
How does the conductance of Na+ change during an AP?
It quickly increases when the threshold is met and then quickly decreases because of channel inactivation.
How does the conductance of K+ channels change during an action potential? How is this different from the conductance of Na+ channels?
There is a delay in channel opening of K+ channels right after threshold is met, mainly b/c of Na+ going in.
The peak conductance of K+ is when Na+ channels are inactivated.
K+ conductance is slower to rise and decrease compared to Na+ conductance.
Peaks of action potentials will never be beyond ____ _____ of Na+. What is this?
Equilibrium potential - the potential at which there’s no net movement of ions (+40 mV for Na).
The peak voltage reached during AP is dependent on what?
Extracellular Na.
Under normal physiological conditions, refractory periods ensure what?
That action potentials are propagated in only one direction.
Artificially induced action potentials along an axon propagate ______.
Bidirectionally.
How fast an AP is going depends on what?
The length constant.
What is the length constant? What does it depend on?
The distance over which the potential falls by 1-(1/e) or 63% from its original value.
Depends on the Rm (resistance of the membrane) and the RI (longitudinal resistance)
What decreases longitudinal resistance? What effect does this have on the length constant?
Increasing the diameter will decrease the longitudinal resistance, thus increasing the length constant.
What effect does myelin have on membrane resistance? How does it do this?
It increases the resistance by increasing the length constant.
It essentially plugs the “holes” of the ion channels and allows the ions to go further.
What are two things that can increase the length constant?
Increasing the diameter of the neuron .
Increasing the resistance of the axonal membrane: insulating with myelination
Which has a bigger impact on the velocity of an action potential, increasing diameter or myelination?
Myelination.
Other than insulation of axons, what is another important benefit of myelination?
Space-saving.
What produces the rising phase of AP?
Voltage-gated Na+ channels.
What carries current that rapidly repolarizes the membrane to terminate an action potential?
Voltage-gated + channels.
What are channelopathies?
Diseases caused by defective ion channels.
What is hyperkalemic periodic paralysis? What happens during these attacks?
Episodes of weakness and decreased muscle tone (may follow exercise of K-rich filled food)
involved muscle fibers are unable to fire APs (cells depolarized by 30-40 mV)_
What is hyperkalemic periodic paralysis caused by? What does this result in?
Mutation of the muscle voltage-gated Na channels that prevent some % of them from being completely inactive after depolarization.
Results in small but constant inward current which inactivates normal Na channels and renders muscle inexcitable.
What is myotonia and what are the two types?
Having difficulty getting muscle to relax once they have contracted.
Thomsen’s disease and Becker’s disease.
-muscles of affected individuals relax unusually slowly following a sudden contraction
What do myotonic goats faint?
There’s a mutation in the Cl channels that results in decreased conductance that reduces the amt of depolarizing current required to reach threshold.
This means that APs are more easily occuring, which means that they can’t relax when they get excited.
What effect does tetrodotoxin have? Where does this come from?
Puffer fish!
Blocks Na channels of the peripheral nerves and causes weakness and numbness.
Larger doses can result in respiratory paralysis and death.
