Lecture 22: The Action Potential

Question 1

(1) How does the action potential start?

Answer 1

• With a stimulus causing a local depolarisation
• Small number of Na+ channels open
• Membrane potential depolarises toward the threshold potential (-59mV)
• This triggers another event (opening of all Na+ channels)

Question 2

(2) What happens when all Na+ channels open?

Answer 2

• Further depolarisation occurs
• There’s so much sodium in the cell, that now there is a positive value of +30mV
• After this all sodium channels close and another event occurs (K+ channels open)

Question 3

(3) What happens when the K+ channels open?

Answer 3

• The membrane repolarises as it becomes more negative inside (hyperpolarisation)
• At high negativity the K+ channels close and potassium pumps restore the RMP (recharge the battery)

Question 4

To summarise action potential generation:

Answer 4

• Timing is everything
• Order of channel events (opening/closing)
• There are different types of ion channels (stimulus-gated and voltage-gated)

Question 5

Go over different types of potentials and polarisation

Answer 5

• table ^O^

Question 6

Conduction of the action potential down axons relies on…..this generates……

Answer 6

• The speed of the depolarising electrical signal

- An electric field

Question 7

When the action potential moves along the axon it instantly activates….

Answer 7

• The next set of voltage gated Na+ channels

Question 8

What two parts make up the refractory period?

Answer 8

• Absolute –> no response

- Relative –> large stimulus required

Question 9

What does the refractory period limit?

Answer 9

• The number of action potentials at a given time

Question 10

Sodium channels can be in three different states…this prevents….

Answer 10

• Closed
• Open (at most 0.5ms)
• Inactive Open (open but there is a ‘plug’ which prevents sodium from further entering the cell)
• Backward movement

Question 11

If you are a small invertebrate what is the difference in conduction speed?

Answer 11

• Slower due to unmyelination

Question 12

What is something that humans have to speed up conduction of action potentials?

Answer 12

• Myelinated axons!!!!

- This improves velocity

Question 13

How do myelinated axons improve conduction speed?

Answer 13

• Electrical field around node of ranvier is able to extend to next node of ranvier, travels instantly
• This means that the action potential is able to leap between the nodes of ranvier

Question 14

How long can action potentials travel in 3ms?

Answer 14

• 150mm

- Dominoes vs Jenga