Lecture 19: Action Potentials

Question 1

what is the role of an axon?

Answer 1

to send output signals to the next neuron and generate action potentials

Question 2

define depolarisation of membrane potential.

Answer 2

inside of the membrane becomes more positive due to Na+ coming into the neuron

Question 3

define repolarisation of membrane potential.

Answer 3

return to the resting membrane potential from being depolarised

Question 4

define hyperpolarisation of membrane potential.

Answer 4

inside becomes more negative due to K+ leaving the neuron.

Question 5

what is graded potential?

Answer 5

this is the initial trigger that changes the membrane potential in a neuron.

Question 6

what is an action potential?

Answer 6

if a graded potential causes a depolarisation that is big enough, an action potential can be triggered.

Question 7

what is the threshold for an action potential?

Answer 7

-50 mV.

Question 8

explain the role of voltage gated sodium and potassium channels in action potentials.

Answer 8

they mediate the ion flow during an action potential. the voltage-gated Na+ channels have two gates and alternate between three different states.

Question 9

what are the activation states of voltage-gated Na+ and K+ channels?

Answer 9

Na+ channels:
- closed at the resting state
- open at depolarisation
- inactivated, channels blocked by gates soon after they open
- open and close quickly

K+ channels:
voltage-gated K+ channels have one gate and two states.
- closed when triggered by repolarisation - resting state - no K+ exits the cell
- open by depolarisation - allowing K+ to exit the cell.
- open and close slower

Question 10

what is the process of ion flow in action potential generation?

Question 11

how does local anaesthetics work?

Answer 11

local anaesthetics block Na+ ion flow through voltage gated Na+ channels by keeping the Na+ channels in the inactivated state. therefore, Na+ channels are not open preventing depolarisation and the triggering of an action potential.

Question 12

compare absolute and relative refractory period?

Answer 12

absolute refractory period is when no further action potentials can be generated, Na+ channels are open and then inactivated.
relative refractory period is when an action potential can be generated if a large stimulus is applied.

Question 13

what are the factors that influence the speed of an action potential propagation?

Answer 13

1. axon diameter - thicker axons propagate axons faster due to less resistance to current flow
2. temperature - reactions occur faster at higher temperatures
3. degree of myelination - insulates the membrane and speed up propagation, allowing for conduction.

Question 14

process of action potential in myelinated fibres?

Answer 14

voltage gated ion channels are localised at gaps in the myelin. action potential is faster than non-myelinated fibres.

Question 15

process of action potential in unmyelinated fibres?

Answer 15

action potentials are depolarised in membrane as sodium flows in and electrical current flows to the adjacent part of the axon. the adjacent part of the axon is depolarised and Na+ channels are opened. another AP is triggered in this adjacent region and is continued down the axon.

Question 16

what’s an example of what happens when myelin breaks down?

Answer 16

multiple sclerosis. immune system starts to attack their own myelin and impulse conduction slows and eventually ceases. symptoms include pain, numbness, tingling, difficulty walking, fatigue, muscle spasms.