Nervous System: Action Potential Generation and Transmission part 1

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

Action Potentials

Answer 3

A brief fluctuation in membrane potential caused by the transient opening of voltage-gated ion channels which spread along an axon

Question 4

The membrane potential voltage threshold to begin an action potential

Answer 4

~-55mV

Question 5

What are action potentials the key element of in neurons?

Answer 5

The process of signal transmission along axons

Question 6

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

Answer 6

It depolarises

Question 7

Stage 1 of an action potential

Answer 7

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

Question 8

Voltage of the “overshoot” in an action potential

Answer 8

+30mV

Question 9

Stage 2 of an action potential

Answer 9

Repolarisation where the membrane potential regains its original polarity

Question 10

Stage 3 of an action potential

Answer 10

After-hyperpolarization

Question 11

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

Answer 11

stages 1 and 2

Question 12

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

Answer 12

stage 3

Question 13

What is the Absolute Refractory Period?

Answer 13

when a nerve cell is NOT excitable i.e. a stimulus would not cause another action potential to occur

Question 14

What is the Relative refractory period?

Answer 14

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 15

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

Answer 15

• There is a sudden activation 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 16

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

Answer 16

the voltage gated sodium channels inactivate which is followed by the opening of voltage gated K+ channels leading to a repolarisation and after-hyperpolarization and the membrane potential shifts towards EK+ and ratio becomes 100:1 before returning to original 40:1

Question 17

Two mechanisms for stopping the excessive influx of Na+

Answer 17

Voltage gated Na+ channels have an activiation and inactivation gate

Question 18

What is Voltage gated Na+ channels activiation gate

Answer 18

This is a gate that is normally closed. When threshold is met, the voltage sensor detects the change in membrane potential causing the gate to open, allowing Na+ ions to flow into the cell causing a rapid depolarisation. It inactivates once membrane potential within the cell becomes positive so Na+ ions are no longer travelling down conc gradient

Question 19

What is Voltage gated Na+ channels inactiviation gate

Answer 19

Is closed when the inner membrane poential is negative . Once a depolarization of the membrane potential to threshold is detected, the gate is retracted. When the membrane potential becomes positive the gate closes the channel so the membrane potential can repolarise

Question 20

Voltage of resting membrane potential

Answer 20

-70mV

Question 21

Voltage of action potential “peak”

Answer 21

+30mV

Question 22

Total amplitude of action potential

Answer 22

100mV

Question 23

What does “graded” mean

Answer 23

These are small subthreshold depolariseation/hyperpolaeisaitons that by themselves do not cause an action potential to occur

Question 24

When is an action potential “automatic”

Answer 24

Once the membrane potential has reached the threshold of -55mV

Question 25

What path does current flow through

Answer 25

the once of LEAST resistance

Question 26

Direction of current with charge outside membrane

Answer 26

Current flows from positive to negative

Question 27

Rule #2

Answer 27

Wehn the current generated from an outside source flows through the cell membrane from
outside > inside = hyperpolarisation (MP is more -ve)
inside > outside = depolarisaiton (MP is more +ve)

Question 28

Where in the neuron are action potentials first generated

Answer 28

The action initial segment (axon hillock)

Question 29

Why are action pottentials generated in the action initial segment (axon hillock) first?

Answer 29

It has the lowest threshold (is a trigger zone) because of the high density of voltage gated Na channels

Question 30

What are excitatory postsynaptic potentials (EPSPs)?

Answer 30

is the change in membrane potential of the postsynaptic neuron cell that causes the depolarisation of the axon initial segment to threshold. They are caused by synaptic transmission of stimuli from prestynaptic axons to dendrites and cell bodies

Question 31

When do action potentials go once generated in the axon initial segment?

Answer 31

Down to cell body/along axon