Action Potentials

Question 1

what is the resting potential?

Answer 1

-65 mV

Question 2

what is depolarization?

Answer 2

rising phase due to Na+ influx (membrane potential gets less negative –> more positive from -65 mV to +40 mV)

Question 3

what is hyperpolarization

Answer 3

falling phase due to K+ efflux (membrane potential becomes more negative from +40 mV to below -65 mV)

Question 4

overshoot phase

Answer 4

depolarization above the threshold value

-greater depolarization produces more spikes at higher frequency

Question 5

undershoot phase

Answer 5

afterhyperpolarization phase with refractory period (cannot be stimulated) and relative refractory period (needs greater stimulation)
-due to open voltage-gated K+ channels that gradually close and return to resting membrane potential

Question 6

relationship between sodium and depolarization

Answer 6

lowered external Na+ results in smaller and slower APs
-important control is to return the external solution to normal (so if they are in very hypotonic solution without Na+ influx, return it to a higher Na+ solution and APs will return)

Question 7

4 voltage-sensitive mechanisms during action potentials

Answer 7

1. activation of Na+ conductance (Na+ influx down concentration gradient) –> depolarization
2. delayed activation of K+ conductance (K+ efflux down concentration gradient) –> hyperpolarization
3. inactivation of Na+ conductance (later merely close)
4. closing of voltage-gated K+ channels

Question 8

how were changes in Na+ and K+ conductances discovered?

Answer 8

voltage-clamp recordings via voltage and ligand-gated channels to measure change with time and membrane potential

• injects current into the cell that is equal and opposite to the current flowing through the voltage-gated channels
• negative feedback loop prevents voltage across the membrane from changing

Question 9

how to use voltage-clamp recordings

Answer 9

the amount of current injected by clamp to keep voltage constant is a measure of the current flowing across the membrane
-routinely used during development of new drugs

Question 10

what 2 currents the voltage-clamp technique reveals

Answer 10

1. early inward current (Na+)
2. late outward current (K+)
   both change with time

Question 11

tetrodotoxin (TTX)

Answer 11

blocks early Na+ channels without affecting K+ channels

-from puffer fish

Question 12

tetraethylammonium bromide (TEA)

Answer 12

blocks late K+ channels without affecting Na+ channels

-also an ACh receptor blocker

Question 13

AP propagation

Answer 13

requires both active and passive current flow

• active: gating of voltage-gated channels and associated Na+ influx
• passive: depolarization wave that precedes AP (Na influx travels further to depolarize the other areas)
• -discharging membrane capacitance leads to Na+ channel activation

Question 14

myelination

Answer 14

wrapping of glial cells in cell membranes around axon (equivalent to increasing membrane thickness 100x)
-increases insulation to reduce leak of passive flow and decrease capacitance

Question 15

capacitance equation

Answer 15

C = area/distance (distance = total thickness)

decreases with myelination

Question 16

conduction in myelinated fibers

Answer 16

1. fast, passive potentials between nodes of Ranvier
2. generation of AP in nodes (boosting stations)
3. saltatory conduction (APs jump from node to node)
   unmyelinated is 0.5 to 1.0 m/s, but myelinated is 150 m/s

Question 17

nodes of Ranvier

Answer 17

gap in myelin sheath separated by 1 or 2 mm

• contain full complement of Na+ and K+ channels
• generate APs