Lecture 4

Question 1

List the 3 characteristics of an action potential

Answer 1

• All or none process
• Self-propagating: each region of depolarization serves to generate action potentials on either side
• It is non-decremental: doesn’t decrease in strength

Question 2

List the types of ion channels with regard to what they respond to

Answer 2

• Slow leak channels: always open

- Gated: ligand/gated, requires a chemical messenger or change in membrane potential

Question 3

Describe and compare sodium ion channels with potassium ion channels

Answer 3

Voltage gated sodium channels: Has 4 domains in cylindrical configuration and 2 gates (Activation/Inactivation gates)
- Opening of voltage gated Na+ channels responsible for rapid depolarization phase

Potassium channels: Has carbonyl oxygens on pore loops which form selectivity filter only allowing “naked” K+ ions to pass

• Closed at resting membrane potential, opens upon depolarization
• Kinetics are much slower than fast Na+ channels
• Primary mechanism for repolarization

Question 4

Know voltages associated with opening and closing of channel gates during propagation of action potential

Answer 4

Voltage gated Na+ channels: 2 gates

• Both gates are open between -90mv to +35mv
• Activation gates open as voltage reaches -70mv to -50mv
  - Activation gate is opened and inactivation gate is closed at +35mv to -90mv

K+ channels: has a single gate
- Gate is closed at -90mv, slow activation opens the gate from +35 to -90mv.

Question 5

Distinguish between absolute and relative refractory periods

Answer 5

Absolute refractory period

- Period during which second action potential cannot be elicited no matter how strong the stimulus 
- During this time, most fast Na+ channels are either open or in the inactivated state
- Length of this period determines the maximum frequency of action potentials 

Relative refractory period

 - Stronger than normal stimulus can cause action potential 
 - Mechanism for this is the elevated K+ conductance

Question 6

List 2 ways axons might increase the speed of conduction, explain how each method works

Answer 6

Increase diameter of axon
- Large diameter allows larger area for flow of current, less resistance
Myelination
- Nodes of ranvier, saltatory conduction
- Allows 100x less loss of ions and requires little energy for repolarization