Lecture 6 – THE ACTION POTENTIAL

Question 1

Excitable Cells:

Answer 1

• Communication in the nervous system includes: neurones, pain sensors, photoreceptors, cochlear hair cells for hearing, mechanoreceptors, muscles and pancreatic beta cells
• Excitable cells allow sensing of the environment and response to it (movement)
• Most (but not all) excitable cells use ACTION POTENTIALS

Question 2

The Action Potential:

Answer 2

1. Transient reversal of the membrane potential (from the inside-negative resting potential to inside-positive)
2. Duration can vary from a few ms (nerve, skeletal muscle) to a few hundred ms (heart)
3. All or nothing
   - Small (sub threshold) stimulus – no action potential
   - Larger stimulus – fixed size of action potential
   - Body codes stimulus intensity by changes of frequency not size of action potentials, more stimulus the greater the no. of action potentials

Question 3

The Anatomy of an Action Potential:

Answer 3

• blue line indicates a small stimulus which shows no action potential being made
• comes back to membrane potential
• the red box shows a large stimulus which results in an action potential being made as it’s above the threshold level
• the all or nothing event has occurred for the red line
• sodium channels are usually closed at rest and sodium entering at the threshold makes it more positive
• sodium entering is known as positive feedback which encourages more sodium channels to open

Question 4

Action potentials look different in different cells:

Answer 4

• cardiac muscle is 200 times longer than the others

- has a different shape but covered in the later lectures

Question 5

Structure of Na+ Channel:

Answer 5

• Alpha (channel forming) + Beta (accessory) subunit structure (1 unit is shown)
• Alpha subunit is very large (>2000 aa)
• Beta subunits are smaller
• Evolutionary related to K+ and Ca2+ channels

Question 6

Structure of K+ Channel:

Answer 6

• Four subunits are separate proteins (1 unit is shown)
• Overall structure quite similar to Na+ channel
• Green bit is the voltage centre
  Change in membrane potential and the charges will move

Question 7

channels:

Answer 7

Sodium channels open rapidly but inactivate after about 1ms

• Potassium channels open more slowly and can also inactivate
• The ones involved in neuronal action potential don’t really do so are called non-activating
• Positively charged ions can interact with electronegative oxygen atoms – found in pores but water causes it to be unstable

Question 8

Time course of permeability changes during an action potential:

Answer 8

• Blue line represents sodium which shows the channels opening and then closing as they become inactivated
• Green line represents potassium which doesn’t go up that fast but reaches a smaller peak, closes and goes lower than the resting membrane potential as it becomes more negative

Question 9

Feedback loops in the action potential:

Answer 9

1. Positive feedback loop

2. Never ending so sodium channel inactivation stops it

Question 10

The Refractory Period:

Answer 10

• Membrane cannot fire another action potential
• Two types of refractory period:
  1. Absolute – cannot produce another AP
  2. Relative – cell is less excitable so a larger stimulus is needed
• Cannot re-fire due to inactivation of K+
• Important for shutting off positive feedback and preventing capacitance

Question 11

before, peak and refractory period of an action potential

Answer 11

• Just like when we were setting up the membrane potential, the concentrations of sodium and potassium hardly change during an AP.
• This is once again due to capacitance: the change in membrane potential requires the movement of only a few ions
• Under rare circumstances e.g. we’ve poisoned the sodium pump, or rapidly repeating stimulation, you might see some decreases in Ki and increases in Nai.