Chapter 11- Action potentials

Question 1

Are leakage channels permanently open?

Answer 1

Yes

Question 2

How do gated channels open and close?

Answer 2

The protein within the gated channel changes shape.

Question 3

What causes gated ion channels to open and close?

Answer 3

1- In response to an electrical stimulus

2- In response to a specific chemical binding to the gate

3- By mechanical deformation

Question 4

For action potentials, what is the key mechanism?

Answer 4

The voltage gate

Question 5

How are voltage gated ion channels activated?

Answer 5

Activated by changes in electrical membrane potential near the channel.

Question 6

What is the molecular structure of an ion channel?

Answer 6

A collection of proteins that cross the cell membrane and which contain a central pore through which ions can travel.

Question 7

How do you depolarise a cell membrane?

Answer 7

Open the gated sodium channels to let the positively charged sodium ions flow back in.

Question 8

A local change in voltage causes what to happen?

Answer 8

Causes gated channels to open, allowing positive sodium ions to enter.

Question 9

A small, localised change in voltage will result in what?

Answer 9

This will only open a few channels and the incoming sodium will just get pushed back out by the sodium potassium pump.

Question 10

What happens if there is a high change in voltage and enough channels open?

Answer 10

The sodium potassium pump will be overwhelmed and so more sodium ions will be able to enter inside the membrane.

Question 11

The more sodium ions that enter…

Answer 11

The more voltage gated sodium channels open, so even more sodium ions enter.

This creates a positive feedback loop- when the product of a reaction leads to an increase in that reaction.

Question 12

What is the threshold of voltage gated sodium channels?

Answer 12

-55mV

Question 13

What happens if you exceed the voltage gated sodium channel threshold?

Answer 13

The membrane is depolarised.

Question 14

When it comes to sodium channel inactivation, sodium channels contain what type of mechanism?

Answer 14

Ball and chain mechanism- a ball of amino acids connected to the main protein.

Question 15

When it comes to sodium channel inactivation, after a fixed amount of time following the gate opening, what happens?

Answer 15

The ball binds to the inside of the channel, blocking the flow of sodium ions completely.

Question 16

When it comes to sodium channel inactivation, is the process time dependent or voltage dependent?

Answer 16

Time dependent

Question 17

Mutations resulting in sluggish sodium channel inactivation can result in what?

Answer 17

Epilepsy

Question 18

In the depolarised membrane, both electrochemical forces want to do what?

Answer 18

Push potassium out

Question 19

How would you return the neuron back to its resting state?

Answer 19

Open up gated potassium channels, allowing positive ions to leave the cell, restoring the negative membrane potential.

During the repolarisation phase, the sodium channel remains closed.

In the final phase, the potassium channels close, and the sodium channel resets.

Since it takes time to close all the potassium channels, there is a slight overshoot (more negatively charged than normal) in which the membrane becomes hyperpolarised before its true resting potential (-70mV) is restored.

Question 20

What is the absolute refractory period?

Answer 20

The interval of time during which a second action potential cannot be initiated, no matter how large a stimulus is repeatedly applied.

Question 21

What is the relative refractory period?

Answer 21

Where inactive sodium channels become active in order to accept the second signal.

Question 22

In order to serve as a signal, the action potential must travel the length of what?

Answer 22

The length of the axon

Question 23

The axon can be quite long. What are the solutions to transmitting action potentials along an axon?

Answer 23

1- To periodically regenerate the action potential, via more voltage gated ion channels that are spread along the axon.

• In this way, the initial action potential is propagated (regrown) along the length of the axon.
• This is process is known as continuous conduction.
• This works but it is very slow.

2- Myelination- coating the axon in a layer of insulating fat that limits voltage decay.

• Provided by Schwann cells and oligodendrocytes.
• The signal can travel more quickly because it only needs to be regenerated occasionally.
• It is regenerated at the periodic gaps in the myelin, known as nodes of Ranvier.
• This is called saltatory conduction- describes the way an electrical impulse skip from node to node down the full length of the axon.