Lecture 27: Action potentials: Generation and transmission

Question 1

What is hyperpolarisation?

Answer 1

When the potential inside the cell increases (i.e. membrane potential becomes more negative)

Question 2

What is depolarisation?

Answer 2

When the potential inside the cell decreases (i.e. membrane potential becomes more positive)

Question 3

What causes change in membrane potential?

Answer 3

Change in ion concentrations and membrane permeability (due to gated and non-gated ion channels)

Question 4

What is an action potential?

Answer 4

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

Question 5

What is the function of action potentials?

Answer 5

The frequency of action potentials encodes information (how neurons communicate)

Action potentials are a key element of signal transmission along axons

Question 6

What are the 3 stages of action potentials?

Answer 6

1) Depolarisation - membrane potential becomes positive
2) Repolarisation - membrane potential returns to resting potential
3) After-hyperpolarisation - membrane potential temporarily falls below (more negative) resting potential

Question 7

When do action potentials occur?

Answer 7

In response to a stimuli causing depolarisation above a threshold

Question 8

How does the initial depolarisation occur?

Answer 8

When membrane potential reaches threshold voltage-gated Na+ ion channels are activated causing membrane permeability of Na+ to increase (20x more permeable than K+). This shifts membrane potential towards the equilibrium potential of Na+ which is +60mV causing overshoot

Question 9

What is overshoot?

Answer 9

The difference between maximum membrane potential reached and 0V

Question 10

How does repolarisation occur?

Answer 10

The opening of the Na+ ion channels is short-lasting and soon deactivate -> this causes opening of voltage-gated K+ channels leading to repolarisation as membrane potential shifts towards equilibrium potential of K+ (-80mV)

Question 11

How does after-hyperpolarisation occur?

Answer 11

Membrane potential shifts towards -80mV (Ek+) due to increased permeability to potassium ions

Question 12

Why do sodium ions move into the cell when voltage-gated channels are opened by threshold voltage?

Answer 12

They are in higher concentration outside of the cell than inside (concentration gradient)

The charge inside the cell is more negative than the outside attracting positively charged ions (electrical gradient)

Question 13

When does the influx of Na+ stop following the opening of the voltage-gated ion channels?

Answer 13

1) Inside potential becomes positive (no longer attracts postively charged sodium ions)
2) When Na+ channels close xd

Question 14

What is meant by action potentials being an all-or-none event?

Answer 14

The occurrence of an action potential does not depend on the stimuli intensity rather if the stimulus is strong enough to create a voltage above the threshold this will cause an action potential

Question 15

How are action potentials generated physiologically in CNS neurons?

Answer 15

1. Action potentials are first generated in the axon initial segment (part of axon branching of cell body) which ahs the lowest threshold - acts as the trigger zone
2. Depolarisation to threshold is evoked by excitatory postsynaptic potentials (EPSPs) which spread mainly passively from dendrites
3. Once threshold is met APs are transmitted along the axon away from cell body

Question 16

What are the two pathways for generation of action potentials by electrodes?

Answer 16

1) Outside the axon along the surface from + to -

2) Across the membrane and inside the axon (only path 2 will affect resting membrane potential)

Question 17

What happens due to a-toxin from a scorpion sting?

Answer 17

Molecules bind to voltage-gated Na+ channels making the inactivation gate unable to close

This will increase intracellular concentration of Na+

Action potentials are therefore prolonged as repolarisation cannot occur

Contraction paralysis occurs where muscles are unable to move from their contracted state