Chapter 2: ACTION POTENTIALS

Question 1

How are voltage gated Na+ channels structured?

Answer 1

With transmembrane domains and ion-selective pores

Question 2

What happens to the membrane potential during hyperpolarization or depolarization?

Answer 2

• hyperpolarization: increase in membrane potential (becomes more negative)
• depolarization: decrease in membrane potential (becomes more positive)

Question 3

What condition must be reached to trigger an action potential ?

Answer 3

-the threshold of excitation must be reached (-60mV)

Question 4

What happens during an action potential ?

Answer 4

• threshold of excitation is reached
• depolarization:
  1. sodium channels open, Na+ rushed in, interior of the cell becomes more positive
  2. potassium channels begin to open (later because less sensitive, need more depolarization) but K+ pushed our because interior positively charged
• refractory state (sodium channels close)
• hyperpolarization: K+ continues to leave so membrane inside continues to become more negative.
• K+channels close, membrane comes back to resting potential

Question 5

What is action potential caused by?

Answer 5

-brief increase in permeability of membrane to Na+, then brief increase of permeability of membrane to K+

Question 6

Can an action potential change of size and duration?

Answer 6

No, it can spread across different branches of the axon but it always has the same size and duration (all or none law)

Question 7

What does the rate law state?

Answer 7

The principle that variations on the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires.

Question 8

What is used to study action potentials and what aspects are studied?

Answer 8

• oscilloscope

- rising phase, overshoot, falling phase, undershoot

Question 9

What are optogenics?

Answer 9

• biological technique to control the activity of neurons or other cell types with light.
• Example: ion channel channelrhodopsin opens in response to blue light. The neuron firing of the rat is controlled by blue light delivered by the optic fiber.

Question 10

What happens to concentration of Na+ during depolarization and concentration of K+ in repolarization?

Answer 10

• depolarization: influx of Na+

- repolarization: efflux of K+

Question 11

How does Na+ get into the cell?

Answer 11

• With voltage gated Na+ channels

- Pore loop gated channels highly selective for Na+

Question 12

What are the main properties of Na+ channels ?

Answer 12

• They open quickly
• Then they inactivate (for about 1 ms)
• Don’t open again until repolarization
• They must be de-inactivated

Question 13

What does potassium conductance serve as ?

Answer 13

-delayed rectifier: serves to rectify or reset membrane potential

Question 14

How are potassium and sodium gates structured?

Answer 14

-four separate polypeptide subunits join to form a pore

Question 15

How does initiation to an action potential start? (Hodgkin cycle)

Answer 15

• Membrane potential increases
• Increase Na+ influx
• depolarization
• More Na+ influx
• This is called the explosive or Hodgkin cycle

Question 16

What kind of signals can cause depolarization? (reach of treshold)

Answer 16

-generator potentials can be chemical signals or electric signals

Question 17

Why does Na+ never reach equilibrium?

Answer 17

• ENa+= 62 mV

- Heads toward it but never happens because Na+ channels close before it reaches equilibrium

Question 18

How do Na+ channels close and open?

Answer 18

• Two gates: M and H
• M is closed at rest and opened by depolarization
• H closes once peak Vm is reached

Question 19

What does the refractory period refer to ?

Answer 19

Refers to the time after AP when no other AP can occur

Question 20

What is absolute RP ?

Answer 20

-Na+ channels are inactivated, no AP is possible

Question 21

What is relative AP?

Answer 21

• Some but not all Na+ channels are activated (can open)
• K+ channels are open
• AP can occur but requires a strong depolarization

Question 22

How does the AP travel down the axon?

Answer 22

• travels down in segments

- positive charges spread across the membrane to depolarize the next segment

Question 23

What are the two directions of conduction of an action potential?

Answer 23

• orthodromic: normal conduction, in one direction, from soma to axon terminal
• antidromic: experimental conduction, from axon terminal to soma.

Question 24

What is the typical length and conduction velocity of an action potential?

Answer 24

• length: 2 msec

- velocity: 10m/sec

Question 25

What factors influence the conduction velocity?

Answer 25

• spread of AP along membrane
• path of the positive charge
• axonal excitability (diameter the bigger the faster, number of voltage-gate channels
• myelin

Question 26

What are the two paths that the AP can take?

Answer 26

• inside axon: faster

- across axonal membrane: slower

Question 27

What does the cable theory state?

Answer 27

-AP conduction velocity increases with increased axonal diameter

Question 28

How does saltatory conduction work?

Answer 28

-AP skip from node to node, travel faster in myelinated parts and regenerate in nodes of Ranvier

Question 29

What are the advantages of saltatory conduction?

Answer 29

• speed: passive conduction is faster
• saves energy because current occur only at nodes
• Na+ and K+ pumps have less to compensate for

Question 30

Where can the spike-initiation zone of an AP be located?

Answer 30

• differs depending on neuron
• can be at sensory nerve endings
• or at axon hillock