4 - Electrical Signaling by Neurons

Question 1

Why is it difficult for a neuron to move an electrical signal? What type of signal do neurons send?

Answer 1

There is a lot of resistance.

I=V/R ; current = voltage/resistance

Signals are ions.

Question 2

What are some of the properties of the membrane lipid-bilayer in neurons?

Answer 2

Impermeable to ions, functions as a capacitor that separates positive and negative charges, ions move through channels that act as conductors.

Question 3

What is the structure of channels in neuron lipid bilayers? What states do they have?

Answer 3

Have several subunits and allow ions to flow through by size and charge.

Can be in an open or closed state.

Question 4

What is an example of a highly selective channel in a lipid bilayer? What about one with minimal selectivity?

Answer 4

Highly selective: Na+ allowed and K+ not.

Minimally selective: anions vs cations

Question 5

What does it mean that ion channels are variable resistors?

Answer 5

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.

Question 6

What are the different types of gating for lipid bilayer channels?

Answer 6

Voltage gated
Ligand gated
Thermally gated
Mechanically gated

Question 7

What does it mean if a plasma membrane is impermeable? What does this say about distribution of ions?

Answer 7

It will not allow ions to flow down their concentration gradients.

Negative and positive charges would be identical in each side.

Question 8

What is an equilibrium potential and how is it calculated?

Answer 8

The potential at which an ion is in electrochemical equilibrium.

Measured in milivolts and can be calculated using the Nernst Equation.

Question 9

What values are needed to calculate the equilibirum potential of an ion with the nernst equation?

Answer 9

The concentration of the ion inside and outside the cell.

Question 10

Is the resting membrane potential solely determined by the K+ gradient? How do you know?

Answer 10

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.

Question 11

What two things influence the passive movement of ions across the membrane and establish the resting membrane potential?

Answer 11

1. Electrochemical gradient: the balance between the chemical and electrical gradients.
2. Conductance of the channel

Question 12

What maintains concentration gradients? What is an example of this?

Answer 12

Ion pumps that pump ions against their concentration gradient. Requires energy.

Na+/K+ ATPase.

Question 13

What does it mean (in terms of the membrane potential) if the membrane is depolarized? What about hyperpolarized?

Answer 13

Depolarized: less negative

Hyperpolarized: more negative

Question 14

What are the two types of electrical properties of neurons?

Answer 14

Passive: cable properties

Active

Question 15

What are the cable properties of neurons?

Answer 15

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.

Question 16

What is the length constant?

Answer 16

The distance over which the potential (current) falls by 1-(1/e) or 63% from its original value.

Question 17

Describe the movement of an action potential (AP)? How is this different from cable properties?

Answer 17

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.

Question 18

What is the function of action potentials?

Answer 18

Release of neurotransmitters for communication.

Question 19

What are the stages/requirements of an action potential?

Answer 19

Require reaching a threshold, all-or-none response, latency(time from stimulus to AP), refractory period, propagation.

Question 20

What is the refractory period of an action potential? What are the two parts?

Answer 20

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.

Question 21

What limits the repetition rate of action potentials? What is the benefit of this?

Answer 21

The refractory period; this makes sense because it allows neurons to rest and limit their speed.

Question 22

When is there no AP? What is the absolute upper limit of AP frequency for all types of neurons?

Answer 22

The rising phase and most of the falling phase.

1kHz

Question 23

Properties of _______ underlie the action potential.

Answer 23

Voltage-gated Na and K channels.

Question 24

What types of K+ channels are responsible for the resting membrane potential?

Answer 24

Leaky K+ channels; these are different than voltage-gated K+ channels.

Question 25

When do voltage-gated Na channels close? When do they open back up?

Answer 25

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.

Question 26

Do voltage-gated K channels have an inactive state? When do they close?

Answer 26

No; as long as depolarization occurs K+ can go out. They close at the resting membrane potential.

Question 27

How does the conductance of Na+ change during an AP?

Answer 27

It quickly increases when the threshold is met and then quickly decreases because of channel inactivation.

Question 28

How does the conductance of K+ channels change during an action potential? How is this different from the conductance of Na+ channels?

Answer 28

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.

Question 29

Peaks of action potentials will never be beyond ____ _____ of Na+. What is this?

Answer 29

Equilibrium potential - the potential at which there's no net movement of ions (+40 mV for Na).

Question 30

The peak voltage reached during AP is dependent on what?

Answer 30

Extracellular Na.

Question 31

Under normal physiological conditions, refractory periods ensure what?

Answer 31

That action potentials are propagated in only one direction.

Question 32

Artificially induced action potentials along an axon propagate ______.

Answer 32

Bidirectionally.

Question 33

How fast an AP is going depends on what?

Answer 33

The length constant.

Question 34

What is the length constant? What does it depend on?

Answer 34

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)

Question 35

What decreases longitudinal resistance? What effect does this have on the length constant?

Answer 35

Increasing the diameter will decrease the longitudinal resistance, thus increasing the length constant.

Question 36

What effect does myelin have on membrane resistance? How does it do this?

Answer 36

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.

Question 37

What are two things that can increase the length constant?

Answer 37

Increasing the diameter of the neuron . Increasing the resistance of the axonal membrane: insulating with myelination

Question 38

Which has a bigger impact on the velocity of an action potential, increasing diameter or myelination?

Answer 38

Myelination.

Question 39

Other than insulation of axons, what is another important benefit of myelination?

Answer 39

Space-saving.

Question 40

What produces the rising phase of AP?

Answer 40

Voltage-gated Na+ channels.

Question 41

What carries current that rapidly repolarizes the membrane to terminate an action potential?

Answer 41

Voltage-gated + channels.

Question 42

What are channelopathies?

Answer 42

Diseases caused by defective ion channels.

Question 43

What is hyperkalemic periodic paralysis? What happens during these attacks?

Answer 43

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)_

Question 44

What is hyperkalemic periodic paralysis caused by? What does this result in?

Answer 44

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.

Question 45

What is myotonia and what are the two types?

Answer 45

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

Question 46

What do myotonic goats faint?

Answer 46

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.

Question 47

What effect does tetrodotoxin have? Where does this come from?

Answer 47

Puffer fish! Blocks Na channels of the peripheral nerves and causes weakness and numbness. Larger doses can result in respiratory paralysis and death.