Action potentials

Question 1

All cells have a resting membrane potential, but only two cell types alter their membrane potential in response to stimuli. What cells are they and what is this process called?

Answer 1

Muscle cells and Neurons, such ability is excitability or irritability.

Question 2

What are ion currents?

What limits where they can flow?

Answer 2

Ions will diffuse down their electrochemical gradient once the membrane is permeable, this flow of ions is called an ion current.
The limit to where these currents can flow is whether there are the correct ion channels to allow ion flow in a given area. (in other words ion channels do not appear everywhere in the cell, where they do not appear there can be no action potentials.

Question 3

When voltometers are measuring the potential, and it becomes less negative, is the inside or outside becoming less negative?

Answer 3

The inside of the cell (influx of Na+)

Question 4

Define:
Hyperpolarization:
Depolarization:
Repolarization:

Answer 4

Hyperpolarization: inside the cell becomes more negative, inhibitory signal.
Depolarization: inside the cell becomes more positive, excitatory signal.
Repolarization: A return to the resting membrane potential

Question 5

What is an oscilloscope:

Answer 5

A device which allows measure of varying voltage.

Question 6

Is the magnitude of all cells resting potentials the same? If not give counter examples.

Answer 6

No.
Neuron on average -70 mV
Cardiac cell on average -85 mV

Question 7

Explain the structure of the voltage gated ion channel (example being a Na+ channel)

Answer 7

The channel itself spans the membrane, and can be in an open and closed formation (independent of the ball and chain mechanism, the part which opens and closes are called the gates)
A ball and chain also exists which is capable of blocking the bottom of the channel.

Question 8

What are two causes of hyperpolarization:

Answer 8

Negative charges entering the cell.

Positive charges leaving the cell.

Question 9

Voltage gated channels exist in the axons, true or false?

Answer 9

True, the conduct action potentials. The gate is the part of the channel, internal, which can open and close.

Question 10

What is a non-gated ion channel called?

Answer 10

A leakage channel

Question 11

Are channels for K predominately gated or ungated?

Are channels for Na predominately gated or ungated?

Answer 11

This is a bad question. The majority of potassium channels are still probably gated, but there are some leakage channels. This is why the membrane is more permeable to K+ then Na+.
Na channels are all gated, and the gates are closed at membrane potential.

Question 12

Na channels are all gated and these gates are closed at resting membrane potential, but Na still gets into the cell. How?

Answer 12

Sodium gated channels occasionally flicker open, allowing some entry of sodium.

Question 13

What does the slight inward movement of Na result in?

Answer 13

A resting membrane potential which is slightly more positive then the equilibrium potential for Potassium.

Question 14

Why are cable properties not a reliable way of relaying charge down an axon?

Answer 14

Internal resistance (there are things in the way of the charges, an analogous situation would be pouring water through a tube filled with sand vs one filled with gravel, there will be a greater equivalent radius of the tube with gravel better enabling flow) and charges leak out of the membrane (this is affected by channel proteins and is mitigated by myelin sheaths, which decrease flow out of the cell), capacitance is also a factor but we will not focus much on it (myelin also decreases capacitance).

Question 15

What example threshold does he use to trigger an action potential? What cell type is this most indicative of (from those you’ve learned about)

Answer 15

-55 mV is threshold. Resting potential is -70 mV, which is indicative of neurons (though not all neurons)

Question 16

What happens when a neuron reaches threshold in the axon initial segment?

Answer 16

Na+ voltage gated channels open (the gate mechanism within the channel opens)

Question 17

How is depolarization achieved in an action potential?

How is repolarization achieved in an action potential?

Answer 17

Open Na+ channels

Open K+ channels (letting K+ out)

Question 18

What does the term electrochemical gradient refer to?

Answer 18

The combined electrical (voltage) gradient and the chemical (concentration) gradient acting on an ion.

Question 19

Why can the neuron be quickly refired if it has just allowed Na+ in and K+ out?

Answer 19

Because it is operating in a relatively small area at the surface of the membrane (where the capacitance is occurring), the majority of the cell is not undergoing any real change in charge and enough Na and K are allowed to flow to just change this small region.

Question 20

What are the steps/configurations for a Na channel depolarization?

Answer 20

Channel closed:
(resting membrane potential)
Channel Open:
depolarization (action potential)
Channel inactivated:
Possible ball and chain mechanism blocks channel, lasts ~ 1 millisecond (refractory period, channel is technically open but nothing can pass because of the ball/chain)

Question 21

Describe an action potential:

Answer 21

Depolarization: Na channels open, cell temporally gains + charge.
Depolarization: Na channels are inactivated by the ball and chain mechanism, K+ channels open (voltage gated, activated by depolarization of the cell), the cell quickly loses its positive charge.

Question 22

Is any ATP necessary to fire an action potential?

Answer 22

No ATP is directly used within an action potential, but ATP was used to set up the conditions (Na+ and K+ conc.) and it will be used to return the cell to its normal conditions.

Question 23

What is termed the overshoot?

What is termed the undershoot?

Answer 23

The membrane potential continues to drive itself towards an increasingly positive number past a voltage of zero, termed the overshoot.
The undershoot refers to a hyperpolarization of the membrane on the downstroke of an action potential.

Question 24

Why does the cell not reach Na+ equilibrium potential during an action potential?
Why does a cell not stay at K+ equilibrium potential as its resting potential?

Answer 24

Na+ inactivation (ball and chain, this is positive feedback and they would stay open without this mechanism) occurs prior to reaching +66 volts.
Some leakage of Na+ occurs even at resting potential, resulting in a potential slightly more positive then the resting potential for Na.

Question 25

After-hyperpolarization (undershoot) doesn’t reach K+ equilibrium potential. Why?

Answer 25

The K+ channels close before they can reach their potassium potential (they do this naturally, this is negative feedback).

Question 26

What codes for the strength of an action potential to a cell?

Answer 26

Action potentials are frequency modulated. The frequency of action (number per a second)
Also the number of cells firing. This is taken into account by recruitment.
The duration and amplitude of action potentials are constant and do not code any information.

Question 27

What is another name for action potentials?

Answer 27

Spike potentials (because they happen so fast)

Question 28

How fast does an action potential occur?

Answer 28

about 3 msec

Question 29

Define recruitment:

Answer 29

Recruitment is a measure of strength of a stimuli. Different axons will require different thresholds to fire (smaller cells fire sooner, it has to do with cable properties). As the strength of the stimuli increases cells with higher action potentials will start to fire, gaining more and more axons firing, this recruitment codes for strength of an action potential as well as the frequency of firing.

Question 30

Define the word refractory:

Answer 30

Resistant to process or stimuli

Question 31

Refractory period (two types):

Answer 31

Absolute refractory period: occurs during the upswing and downswing, lasts about a millisecond, no activation can happen because of inactivation of Na+ channels by ball and chain mechanism or a change in the channel.
Relative refractory period: occurs during the undershoot/after-hyperpolarization. This is the result of K+ diffusing out of the cell, only a very strong stimuli can fire an action potential during the relative refractory period (this will result in no closed voltage ion gate gate configuration, aka open -> inactivated -> open).

Question 32

Explain different inactivation mechanisms to me. Is only the ball and chain method utilized?

Answer 32

No. Inactivation can also be a result of a change in the configuration of the channel closing it. Regardless of the change it will last for a fixed period of time. Different channel types use different mechanisms.

Question 33

How far can cable properties send a signal?

Answer 33

About 1 mm, this is why they are only used for the build up of EPSPs and IPSPs.

Question 34

In an unmyelinated neuron what is conduction of an action potential called?

Answer 34

Saltatory conduction.

Question 35

What does conduction without decrement mean/refer to?

Answer 35

It means that the action potential will not loose any strength over distance.

Question 36

Conduction in an unmyelinated neuron.

Answer 36

Stimulation will cause a depolarization of the soma/dendrites. This will be a graded potential trasnmitted by cable properties. If these properties build up in sufficient levels in the axon’s initial segment (where enough voltage gated Na+ channels exist for a action potential to begin), then an action potential will start. The action potential will have an influx of Na from the opening of voltage gated sodium channels. This influx will move by cable properties to (which are limited to 1-2 millimeters) to the next region, and cause depolarization there. Na channels will be inactivated around 30 mV, and vg K channels will open at about the same time. These channels will repolarize the membrane, and will eventually undershoot (get too low), during this time they will eventually shut off due to a lack of activation (the voltage has dropped. This time will a relative refractory period, an actual refractory period is caused by the inactivation of Na channels. This is relatively slow as an action potential must occur in every fraction of the axon. These action potentials are potentially slower then cable properties (if cable properties are optimized by increased distance in a capacitor aka myelin sheaths.)

Question 37

Myelin has three affects on the nature of action potentials (as defined by John) what are they?

Answer 37

Decreased capacitance (faster cable properties)
Decreased permeability of membrane to ions, leading decreased external resistance (faster cable properties)
Stops action potentials from occurring along it (which is why the nodes of ranvier exist)

Question 38

What must be the maximal spacing of the nodes of ranvier?

Answer 38

1-2 mm. This is as far as cable properties can effectively transmit potentials.

Question 39

Nodes of ranvier what are they like?

Answer 39

1-2 um segments. High concentration of Na+ channels (which are almost absent elsewhere on myelinated axons).

Question 40

Is cable-like spread of charge faster or are action potential faster?

Answer 40

The answer is cable properties, because they will have to wait for influx of Na+ from the action potential as a limiting step in the speed of their spread.

Question 41

Why is conduction faster in larger neurons?

Answer 41

They are normally myelinated.

They have less internal resistance.

Question 42

Speed of thin unmyelinated axon (human):

Speed of thick myelinated axon (human):

Answer 42

1 m/s

100 m/s

Question 43

Are there large unmyelinated axons or small myelinated axons?

Answer 43

Generally no. Methods for increasing speed are often used together (size and myelination)

Question 44

What is the result of hypocalcemia in the interstitial fluid around nerve cells?

Answer 44

Increased Na channel permeability and causes repetitive potentials to be fired from a single event.
This will make the extracellular fluid more negative, and depolarize the membrane. This can lead to spontaneous action potentials which can result in muscle contractions (hypocalcemic tetany)

Question 45

In addition to hypocalcemic tetany what other affects can hypocalcemia have on the nervous system?

Answer 45

Certain psycological disorders result from low Ca2+ levels, a restoration of Ca levels will result in loss of the mental disorder.