Lecture 5: Action Potential

Question 1

Can action potentials happen during the refractory and relative refractory phases?

Answer 1

refractory - no

relative refractory - requires greater stimulation

Question 2

What happens to action potentials with lowered external Na+?

Answer 2

smaller, slower action potentials

Question 3

What three voltage-sensitive mechanisms generate action potentials?

Answer 3

1. activation of Na+ conductance
2. delayed activation of K+ conductance
3. inactivation of Na+ conductance

Question 4

The undershoot of the membrane potential is due to open _____, which gradually close, and the _____ is restored

Answer 4

• voltage gated K+ channels

- resting membrane potential

Question 5

What does activation of Na+ conductance lead to? How does this happen?

Answer 5

• depolarization of membrane potential
• voltage-dependent Na+ channels open; ions rush down conc gradient INTO the cell; + charge accumulates inside cell and across membrane

Question 6

What does delayed activation of K+ conductance lead to? How does this happen?

Answer 6

• hyperpolarization of membrane potential

- K+ ions rush OUT of cell down conc gradient; reduction of + charge inside cell membrane

Question 7

What does inactivation of Na+ conductance lead to? How does this happen?

Answer 7

• refractory period
• voltage-dependent Na+ channels transition into non-conductive state as voltage-gated K+ channels begin to close; cell is refractory = doesn’t fire action potentials in response to stimulation; inactivation gates open, Na+ gates close

Question 8

What does a stimulus do?

Answer 8

depolarizes membrane potential above threshold

Question 9

Getting an action potential during relative refractory period depends on _____

Answer 9

K+ channel (K+ conductance)

Question 10

To measure changes in Na+ and K+ conductances, the ______ approach was used.

Answer 10

voltage-clamp

Question 11

The amount of current which must be injected by the voltage clamp is a measure of the ______

Answer 11

current flowing across the membrane

Question 12

What two types of voltage-dependent ionic currents did the voltage clamp technique reveal?

Answer 12

1. early inward Na+

2. late outward K+

Question 13

Which ion channel is blocked by tetrodotoxin (TTX)?

Answer 13

Na+

Question 14

Which ion channel is blocked by tetraethylammonium bromide (TEA)?

Answer 14

K+

-also blocks Ach receptor

Question 15

What phase occurs from Na+ channel inactivation?

Answer 15

absolute refractory period

Question 16

Which part of action potential propagation involves active flow?

Answer 16

gating of voltage-gated channels and associated Na+ influx

Question 17

Which part of action potential propagation involves passive flow?

Answer 17

depolarization

Question 18

APs first reverse the sign of the mem pot at their peak (1_____; gNa increases) and then 2______ the mem pot (undershoot; gK increases), yielding a 3____ period during which it is impossible, then hard (4____) for the axon to produce another AP. Undershoot then 5____ (gK decreases) and mem pot returns to normal

Answer 18

1. overshoot
2. hyperpolarizes
3. refractory
4. relative refractory
5. dissipates

Question 19

The length constant is a measure of ______. It is related to the resistance of the _______ and resistance to _______.

Answer 19

• efficiency of electrical conduction
• membrane
• axial conduction

Question 20

The time it takes for depolarization to spread is determined by both the ____ and the _____ per unit length of axon

Answer 20

• axial resistance

- membrane capacitance

Question 21

How is conduction velocity increased?

Answer 21

• increase axon diameter

- decrease capacitance

Question 22

How does myelination affect capacitance?

Answer 22

decreases capacitance

Question 23

what is myelination?

Answer 23

wrapping on glial cells around axon

Question 24

What cells form myelin in the CNS/PNS?

Answer 24

CNS - oligodendrites

PNS - Schwann cells

Question 25

Where are action potentials generated in myelinated axons?

Answer 25

nodes of ranvier

Question 26

What does saltatory conduction mean?

Answer 26

APs jump between nodes of Ranvier