L2: Action Potentials

Question 1

What primarily determines membrane potential?

Answer 1

• The ionic concentration gradients and permeability to those ions, primarily determined by potassium ion leak channels
• If the membrane is not permeable to a particular ion, that ion does not affect the membrane potential

Question 2

Draw a graph for the membrane potential of K+, Na+ and RMP, and a change is Na+ across the membrane?

Answer 2

Question 3

What happens to the membrane potential when sodium channels open?

Answer 3

It depolarises, or becomes more positive, due to sodium influx

Question 4

What is the level of depolarisation proportional to?

Answer 4

The strength of the stimulus applied

Question 5

What is the threshold potential for triggering an action potential?

Answer 5

Approximately -55 millivolts, where sodium influx surpasses potassium outflow

Question 6

What is depolarisation?

Answer 6

The process by which the membrane potential becomes less negative, moving toward the equilibrium potential of sodium due to sodium influx

Question 7

Describe the “all-or-nothing” principle in action potentials

Answer 7

Once the threshold is reached, an action potential occurs. If it isn’t reached, no action potential will form

Question 8

What occurs when the membrane reaches threshold potential?

Answer 8

Voltage-gated sodium channels open rapidly, allowing Na+ ions to enter and further depolarise the membrane, triggering an action potential

Question 9

What is the threshold?

Answer 9

The point at which Na+ entry is greater than K+ efflux

Question 10

What happens when a stimulus opens chloride channels?

Answer 10

Chloride ions enter the cell, causing hyperpolarisation, which moves the membrane potential further from the threshold, potentially inhibiting an action potential

Question 11

Describe the rising phase of an action potential by numbers

Answer 11

1) RMP
2) Stimulus, voltage-gated sodium channels open and the membrane potential starts to become more positive due to sodium influx into the cell
3) Membrane depolarises more, reaching the threshold - VGNa and VGK channels activated to open (but VGK channels are slower to open)
4) VGNa channels open and there is Na+ influx causing depolarisation (approach the equilibrium potential of Na at +60mV) - the peak of the action potential is +30mV - does not reach equilibrium potential of Na as VGK channels are open

Question 12

Describe the repolarisation phase of the action potential by numbers

Answer 12

5) Peak of action potential - VGNa channels inactivate - the VGK channels open stopping Na influx and more potassium efflux happens
6) VGK channels fully open, K+ efflux and repolarisation of the membrane begins
7) VGK channels slow to close and stay open for longer and membrane potential becomes more negative that original RMP - hyperpolarisation occurs
8) Return to resting potential

Question 13

What are te two types of VGNa channels involved in Na+ movement?

Answer 13

Activation and inactivates gates

Question 14

What are the roles of activation and inactivation gates in sodium channels during an action potential?

Answer 14

• The activation gate opens to allow Na+ influx during depolarisation
• The inactivation gate closes to stop Na+ influx and depolarisation, ensuring controlled action potential propagation and allows for repolarisation, the membrane potential becomes more negative

Question 15

What are the VGNa channel gates like at RMP?

Answer 15

• Activation gate is closed
• Inactivation gate on cytoplasmic side is open
• Membrane potential -70mV

Question 16

What are the VGNa channel gates like at depolarisation?

Answer 16

• Activation gate open
• Both gates are now open
• Membrane potential more positive

Question 17

What is the delay on the inactivation gate closing?

Answer 17

Approximately 0.5ms

Question 18

How do voltage-gated sodium channels reset?

Answer 18

They reset when the membrane potential returns to a negative level, making them ready to open for another action potential

Question 19

Why can’t action potentials overlap or occur back-to-back without a reset?

Answer 19

Due to the refractory periods, where channels need to reset to ensure directional flow and prevent overlapping signals

Question 20

What is conductance?

Answer 20

Refers to the ease of ion flow through the channel in the plasma membrane; varies with the state of the channel provided the channels are present and open

Question 21

What is the purpose of the refractory periods in neurons?

Answer 21

They ensure that each action potential is a discrete event and that signals move in one direction along the axon

Question 22

What are the two types of refractory periods?

Answer 22

• Absolute
• Relative

Question 23

What is the absolute refractory period?

Answer 23

• The period during which no new action potential can be generated because VGNa channels are inactivated
• During repolarisation phase

Question 24

What is the relative refractory period?

Answer 24

• The period following the absolute refractory phase where a stronger-than-normal stimulus can generate another action potential
• VGNa channels are starting to reset and membrane potential more negative
• VGK channels open and K+ efflux
• During hyperpolarisation phase

Question 25

Why are refractory periods important?

Answer 25

• Sets frequency action potentials are generated
• Limits rate signals can be transmitted
• Ensures one way travel of action potential

Question 26

Describe how the action potential moves along the axon

Answer 26

It initiates at the axon hillock and propagates as each segment of membrane depolarizes, followed by inactivation of sodium channels, ensuring one-way flow

Question 27

How does an action potential propagate along an axon?

Answer 27

The depolarisation spreads along the axon, opening sodium channels ahead, while inactivated channels behind prevent backward propagation

Question 28

What is the role of the axon hillock in action potential initiation?

Answer 28

It integrates incoming signals; if the combined stimulus is strong enough to reach the threshold, an action potential is generated

Question 29

How does myelination affect the conduction of an action potential?

Answer 29

It speeds up conduction by insulating axons and allowing the action potential to “jump” between nodes of Ranvier

Question 30

What is saltatory conduction?

Answer 30

A form of rapid action potential transmission in myelinated neurons, where the impulse “jumps” from one node of Ranvier to the next

Question 31

What is the significance of nodes of Ranvier in myelinated axons?

Answer 31

They contain concentrated sodium channels, allowing rapid action potential propagation between nodes (saltatory conduction)

Question 32

How does the nervous system increase action potential conduction speed?

Answer 32

By using myelinated axons and concentrating sodium channels at nodes of Ranvier for saltatory conduction

Question 33

How does a stronger stimulus affect action potential frequency?

Answer 33

A stronger stimulus can increase the frequency of action potentials, though each individual action potential remains the same size

Question 34

What role do glial cells play in action potential conduction?

Answer 34

They form the myelin sheath, insulating axons and facilitating faster action potential transmission

Question 35

What happens to action potential conduction in the absence of myelin?

Answer 35

Conduction speed is significantly reduced, as continuous sodium channel activation along the axon is required, slowing the signal

Question 36

What can affect the action potential?

Answer 36

Affected by drugs/toxins that target ion channels