M&R Session 4 - Electrical Excitability

Question 1

Roughly how long is the duration of an action potential in an axon or skeletal myocyte?

Answer 1

0.5ms

Question 2

Roughly how long is the duration of an action potential in a cell within the sino-atrial node?

Answer 2

100ms

Question 3

What is the sequence of events in an axon action potential?

Answer 3

1. Local changes in membrane potential cause some voltage-gated Na channels to open. Small Na influx, results in depolarisation of membrane potential (moves towards E(Na) of +61mV)
2. This depolarisation continues until the membrane reaches its threshold potential, at which point an AP is fired.
3. Voltage-gated Na channels open, large Na influx.
4. Rapid depolarisation up to +30mV, where Na channel inactivate and voltage-gated K channels open. K efflux.
5. There is rapid repolarisation past resting membrane potential (hyperpolarisation).
6. Na/K/ATPase reverses ion concentrations, returning them to normal and restoring resting membrane potential.

Question 4

What is “Absolute Refractory Period” and how long does it last?

Answer 4

The period during which the Na channels are inactivated and so are unable to be excited. Lasts about 1ms (wheyyyyy)

Question 5

What is “Relative Refractory Period?”

Answer 5

Immediately following ARP, there is a period where the Na channels are recovering from inactivation and are able to be excited in strong stimulus. Excitability increases as the number of Na channels recovered from inactivation increases.

Question 6

What is the advantage of having an Absolute Refractory Period?

Answer 6

It allows for unidirectional propagation of the action potential

Question 7

What is accommodation?

Answer 7

In long-lasting stimuli, the threshold over which membrane potential must be in order to fire an AP becomes more positive.

As Na channels open in insufficient numbers to reach threshold, then inactivate, the membrane potential must reach a more positive potential to initiate an AP.

Question 8

What is the basic structure of voltage-gated ion channels?

Answer 8

Na+ -> 4x repeating sequences of 6 TMDs = 1 alpha subunit

K+ -> similar, but 4x separate subunits of 6 TMDs.

Question 9

How do local anaesthetics work and give an example of one.

Answer 9

Bind to and block Na+ channels

eg. Procaine

Question 10

What are the five key points about Action Potentials?

Answer 10

1. Change in voltage across a membrane
2. Depends on ionic gradients (pre-AP) and membrane permeability
3. Only occurs ince threshold potential is reached (this is not a static value)
4. All or nothing. You cannot have half an AP
5. Propagated without loss of amplitude.