Neurons - Generation and Transmission of Action Potentials

Question 1

Hyperpolarization

Answer 1

When the resting membrane potential becomes more negative (moves closer to Ek than ENa)

Question 2

Depolarisation

Answer 2

When the resting membrane potential becomes more positive (moves closer to ENa than Ek)

Question 3

What is an action potential?

Answer 3

A brief fluctuation in membrane potential caused by a transit opening of voltage-gated ion channels, which spreads, like a wave, along axon.

Question 4

What are the interchangeable terms for action potentials?

Answer 4

Spike, nerve impulse, discharge

Stick to action potential

Question 5

When do action potentials occur?

Answer 5

After the membrane potential reaches a certain voltage called the ‘threshold’

Question 6

What is the significance of the action potential?

Answer 6

Information is coded in the frequency of action potentials

A.P’s can be regarded as a form of language by which neurons communicate

Question 7

What other key element are action potentials involved in?

Answer 7

Signal transmission along (often very long) axons that can be up to 0.5/1m long

Question 8

Does the membrane depolarise or hyperpolarise to reach the threshold for a membrane potential?

Answer 8

It depolarises

Question 9

The membrane potential voltage threshold to begin an action potential

Answer 9

~-55mV

Question 10

What is the slow depolarisation evoked by?

Answer 10

A stimulus

Question 11

What happens in the first stage of the action potential?

Answer 11

After the membrane potential reaches threshold it undergoes a fast (ie in milliseconds) depolarisation to ~ +30mV (overshoot)

Question 12

Voltage of the “overshoot” in an action potential

Answer 12

+30mV

Question 13

Stage 2 of an action potential

Answer 13

Repolarisation where the membrane potential regains its original polarity

Is coming back to a negative charge and hence becoming less positive

Question 14

Stage 3 of an action potential

Answer 14

After-hyperpolarization

Question 15

What stages of an action potential are the ‘Absolute Refractory Period’?

Answer 15

stages 1 and 2

Question 16

What occurs in the ‘Absolute Refractory Period’?

Answer 16

The action potential cannot respond to another stimulus at this point

This means that the nerve cell is when a nerve cell is NOT excitable

Question 17

What stages of an action potential are the ‘Relative Refractory period’?

Answer 17

stage 3

Question 18

What is the Relative refractory period?

Answer 18

the part of an action potential where a strong stimulus could invoke an action potential but may not reach as high a point as it has to depolarise a lot more

Question 19

What happens to the channels in the cell membrane once the threshold has been reached (stage 1 of an action potential)

Answer 19

• There is a sudden activation(opening) of the voltage gated Na+ channels causing a fast depolarization as the cell becomes more permeable to Na+
• At this point the ratio of K+:Na+ goes from 40:1 to 1:20 as the membrane potential shifts towards that of Ena (+60mV) but this is SHORT lasting

Question 20

What can the stimulus be?

Answer 20

Physical = electrical current, mechanical stretch 
Chemical = drug or synaptic excitation

Question 21

What happens to the channels in the cell membrane in stage 2 and 3 of an action potential?

Answer 21

The voltage gated sodium channel quickly inactivates

This is followed by the opening of the voltage gated K+ channel, leading to re-polarisation and after-hyperpolarization.

The membrane potential shifts towards EK+ and the ration becomes 100:1 (K+ and Na+ respectively) before returning to the original 40:1

Question 22

Two mechanisms for stopping the excessive influx of Na

Answer 22

Voltage gated Na+ channels:

Activation and Inactivation gate

Question 23

What is the key role of the voltage gated Na+ channel?

Answer 23

When the voltage threshold is reached, sodium channels open and Na+ ions move into the cell along BOTH the concentration gradient and electrical gradient

The influx of Na+ slows down and then stops when:

1. The inside potential becomes positive and moves towards ENa, and thus attracts less Na+ ions
2. Na+ channels inactivate

Question 24

What are the key components of the voltage sensor?

Answer 24

Has polar charges
Is extremely sensitive
Undergoes a conformational change
Is closed so the solute cannot pass through

Question 25

Why does Na+ go inside the cell?

Answer 25

1. Due to the presence of the electrical gradient

2. Due to the concentration gradient, ie from an area of high to low concentration

Question 26

What is Voltage gated Na+ channels activiation gate

Answer 26

Gate is normally closed.

When the threshold is met, the voltage sensor detects a change in membrane potential causing the gate to open, thus allowing Na+ ions to flow into the cell and causing rapid depolarisation.

It inactivates once the membrane potential within the cell becomes positive and hence no Na+ ions are no longer travelling down the conc gradient

Question 27

What is Voltage gated Na+ channels inactiviation gate

Answer 27

Closed when the inner membrane potential is negative.

Once a depolarisation of the membrane potential to the threshold is detected, the gate is retracted.

When the membrane potential becomes positive, the gate closes the channel so the membrane potential can re-polarise

Question 28

The voltage of resting membrane potential

Answer 28

-70mV

Question 29

Voltage of action potential “peak”

Answer 29

+30mV

Question 30

Total amplitude of action potential

Answer 30

100 mV

Question 31

What does “graded” mean

Answer 31

The small sub-thresholds of depolarisation/hyperpolarisations that by themselves do not cause an action potential to occur

Question 32

What event is each action potential?

Answer 32

An all or none event