L16 Action Potentials Initiation and Conduction

Question 1

what kind of AP conduction is seen in unmyelinated axons?

Answer 1

continuous conduction

Question 2

what kind of AP conduction is seen in myelinated axons?

Answer 2

saltatory conduction

Question 3

what drives the membrane potential towards Ena?

Answer 3

an increased influx of Na+ ions into the cell down its electrochemical gradient

Question 4

describe the influence of the influx of Na+ ions on the rising phase of the AP?

Answer 4

at threshold, Na+ causes the rising phase of the impulse

at threshold, the influx of Na+ is greater than the efflux of K+ = more Na+ channels open= more depolarization

Question 5

describe the influence of the efflux of K+ ions on the repolarizing phase of the AP

Answer 5

depolarization causes the slow opening of K+ channels = delayed efflux of K+ down their electrochemical gradient = repolarization

Question 6

recognize the different time courses of change in sodium and potassium conductance

Answer 6

fast time course for Na+ channels opening

slower time course of the opening of K+ channels

Question 7

explain how the structural features of voltage-gated Na+ channels are related to the influence of depolarization on the closed and open states of this channel

Answer 7

prolonged depolarization inactivated voltage gated Na+ channels

Question 8

what are the characteristics of the Na+, K+ and Ca2+ voltage gated channels?

Answer 8

• four identical subunits
• 6 transmembrane spanning segments
• segment 4 = charged components = voltage sensing element for channel protein
• pore loop that endows the channel pore with its particular ion selectivity

Question 9

what is refractory period?

Answer 9

when the sodium channel reverts from inactivated state to the normal closed state

Question 10

what is the absolute refractory period?

Answer 10

when all Na+ channels are inactivated

Question 11

what is the relative refractory period?

Answer 11

following absolute refractory period, when an impulse can be evoked by a LARGE stimulus

*some sodium channels are in the normal closed state and can be opened
the large number of K+ channels remain open and tend to hold the potential close to Ek = opposes excitiation of the axon

Question 12

where are voltage gated Na+ and K+ channels in unmyelinated axons?

Answer 12

distributed uniformly

Question 13

where are voltage gated Na+ and K+ channels in myelinated axons?

Answer 13

Na channels = axon initial segment, presynaptic terminales and node of Ranvier

K channels = paranodal and juxtaparanodal regions

*impulses only occur at nodes of rangier
repolarized at paranodal regions

Question 14

what does conduction velocity depend on?

Answer 14

fiber diameter

Question 15

why are unmyelinated axons slower than myelinated axons?

Answer 15

• small diameter

- continuous conduction require that all of the axonal membrane must be driven to theshhold sequentially

Question 16

conduction velocity is _____x the fiber diameter in unmyelinated axons

Answer 16

2

Question 17

conduction velocity is ____x the fiber diameter in myelinated axons?

Answer 17

6

Question 18

both unmyelinated and myelinated axons propagate AP without decrement =?

Answer 18

all of none!

Question 19

what are the advantages of myelin?

Answer 19

• enhanced velocity of conduction because impulses are generated only at nodes of Ranvier
• large velocity is achieved in fibers with relatively small diameters - space saving without fall in velocity
• confining impulses to nodes means that Na+ loading during intense impulse traffic is minimized in myelinated axons giving a huge energy saving!

Question 20

what can impair impulse conduction?

Answer 20

• TTX and saxitoxin
• local anesthetics
• demyelination

Question 21

what does TTX do?

Answer 21

blocks Na+ channels at extracellular site

pufferfish = paralysis, convulsions, mental impairment, cardiac arrhythmia, numbness of lips and tongue, complete paralysis then death within 4-6 hours

Question 22

what does saxitoxin do?

Answer 22

blocks Na+ channels at extracellular site to!

you get similar symptoms of TTX but it results in flaccid and respiratory paralysis

Question 23

what do local anesthetics do

Answer 23

• block Na+ channel at intracellular site
• they block small myelinated axons first, then unmyelinated axons then large myelinated axons
  pain perception is blocked!

Question 24

what does demyelination do?

Answer 24

agents destroy myelin and leave axons bare
causes redistribution of Na+ and K+ channels along the axons

*frequency related block
total conduction block
“crosstalk” between axons

Question 25

what is frequency related block?

Answer 25

failure of some impulses in a train to propagate through the demyelinated region

Question 26

what is total conduction block?

Answer 26

failure to excite the node

Question 27

what is “crosstalk” between axons?

Answer 27

during impulse conduction some current circulating may deplarize an adjacent demyelinated hyperexcitable axons