L4: action potentials - Levy

Question 1

the farther above threshold the stimulus the greater the frequency of action potential firing. This relationship holds until…

Answer 1

until the fiber reaches its maximum frequency of firing

Question 2

what limits the frequency of action potential firing?

Answer 2

refractory period

Question 3

what approximately is resting potential in vertebrates?

Answer 3

-70 mV

Question 4

describe the “all-or-none” property of an action potential

Answer 4

once threshold is reached, an amplitude will always attain a fixed amplitude that is independent of stimulus strength

Question 5

describe the “time-limited” property of an action potential

Answer 5

action potential duration does not vary with duration of stimulus – action potential repetitions will change according to duration of stimulus and frequency will change according to strength of stimulus until maximum frequency is reached (due to refractory period)

Question 6

describe the “non-decremental” property of an action potential

Answer 6

unlike local potentials, an action potential propagates along the cell membrane without decrement, which is valuable for conveying info over long distances

Question 7

describe the “voltage dependence” of an action potential

Answer 7

when a critical voltage is reached across the membrane, voltage-dependent cahnnels open and allow a series of passive ion movements to produce the action potential

Question 8

when in the course of an action potential is the absolute refractory period

Answer 8

from the beginning of the action potential to the point when it has repolarized to a value near threshold (before undershoot)

Question 9

when in the course of an action potential is the relative refractory period

Answer 9

from repolarization to threshold through undershoot to depolarization back to resting potential

Question 10

what happens to the threshold voltage during the relative refractory period

Answer 10

threshold is raised

Question 11

what is the orthodromic direction

Answer 11

from receptive region to presynaptic region of neuron

Question 12

what is the antidromic direction

Answer 12

from presynaptic region to receptive region of neuron

Question 13

T/F concentration gradients change significantly during the course of an action potential

Answer 13

false - concentration gradients hardly change during the course of an action potential

Question 14

how is conductance related to current and driving force?

Answer 14

g = I / (Vm - E)

Question 15

why is the slope of depolarization steep?

Answer 15

g Na+ is increasing because voltage gated Na+ channels are opening

Question 16

why is the slope of repolarization steep?

Answer 16

g K+ is increasing because voltage gated K+ channels are opening

Question 17

this technique used a constant Vm and allowed the measurement of ion currents and calculation of voltage dependence of ion conductances

Answer 17

voltage clamp method

Question 18

the puffer nerve poison tetrodotoxin (TTX) blocks ___

Answer 18

blocks voltage gated Na+ channels and therefore Na+ current due to with no effect on K+ current

Question 19

tetraethylammonium ions (TEA) block ___

Answer 19

the delayed voltage gated K+ channels and therefore the delayed K+ current but have no effect on Na+ current

Question 20

this substance blocks Na+ current with no effect on K+ current

Answer 20

TTX puffer nerve poison tetrodotoxin

Question 21

this substance blocks the delayed K+ current but have no effect on Na+ current

Answer 21

TEA tetraethylammonium ions

Question 22

describe the entire sequence of events that gives rise to an action potential

Answer 22

• local depolarization activates voltage-gated Na+ channels, allowing influx of Na+ and depolarization
• local depolarization also increases K+ driving force (Vm - EK+), influx K+ and repolarization
• threshold is reached when sufficient voltage Na+ channels are opened and overcome outward K+ current. Na+ influx depolarizes further and opens more Na+ voltage channels in positive feedback
• Na+ channels are inactivated and I Na+ turned off
• (with some overlap to previous step) delayed K+ voltage channels open and efflux, repolarize, hyperpolarize, and then close to depolarize back to normal Vm resting
• absolute refractory is about threshold to threshold when Na+ channels are either open or inactivated
• relative refractory is from re-threshold to Vresting when K+ channels are still open and Na+ are changing from inactivated to closed

Question 23

what are the voltage sensors that open voltage gates?

Answer 23

groups of charged amino acid residues on transmembrane a-helices

Question 24

what confers channel selectivity for either Na+ or K+ ions?

Answer 24

particular geometric arrangement of negative amino acids at narrowest part of pores

Question 25

what are 4 conformations of Na+ voltage gated channel over course of action potential

Answer 25

• rest (closed)
• activated (open)
• inactivated
• inactivated and closed
• back to rest (closed)

Question 26

this period of an action potential is due to inactivation of Na+ voltage gates

Answer 26

absolute refractory

Question 27

explain the absolute refractory period

Answer 27

so few Na+ channels have recovered that even if all of them were opened, inward current would not exceed outward K+ current that keeps cell polarized

Question 28

explain the relative refractory period

Answer 28

fewer than normal Na+ channels are available, so a larger fraction of them must be opened to carry sufficient inward current to offset considerable outward K+ current. hence, a larger depolarizing stimulus (threshold) is necessary

Question 29

T/F like the Na+ channel, the K+ channel has an inactivation gate

Answer 29

false - contrary to the Na+ channel, the K+ channel has no activation gate

Question 30

when do the K+ channels open and close?

Answer 30

• delayed open after action potential depolarization

- begin to close during hyperpolarization and finish closing when back to resting membrane potential

Question 31

when do the Na+ channels open and close?

Answer 31

• open - depolarization
• inactivate - near top of depolarization peak
• back to rest (closed) ~ re-threshold before hyperpolarization

Question 32

why doesn’t an action potential re-invade a region it has immediately passed through?

Answer 32

region becomes refractory and cannot generate an action potential for a certain time

Question 33

myelin consists of

Answer 33

fused membranes of satellite cells wrapped around the axon

Question 34

nodes of Ranvier

Answer 34

regularly spaced gaps in myelin

Question 35

what are 3 functions of myelin?

Answer 35

• decrease leakage current from axoplasm during conduction
• faster conduction through saltatory conduction
• require less metabolic energy to maintain Na+ K+ concentration gradients (less room for leak channels, less need for Na K pump)

Question 36

saltatory conduction is

Answer 36

faster conduction jumping from nodes of Ranvier between myelin