L16 Action Potentials Initiation and Conduction Flashcards
what kind of AP conduction is seen in unmyelinated axons?
continuous conduction
what kind of AP conduction is seen in myelinated axons?
saltatory conduction
what drives the membrane potential towards Ena?
an increased influx of Na+ ions into the cell down its electrochemical gradient
describe the influence of the influx of Na+ ions on the rising phase of the AP?
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
describe the influence of the efflux of K+ ions on the repolarizing phase of the AP
depolarization causes the slow opening of K+ channels = delayed efflux of K+ down their electrochemical gradient = repolarization
recognize the different time courses of change in sodium and potassium conductance
fast time course for Na+ channels opening
slower time course of the opening of K+ channels
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
prolonged depolarization inactivated voltage gated Na+ channels
what are the characteristics of the Na+, K+ and Ca2+ voltage gated channels?
- 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
what is refractory period?
when the sodium channel reverts from inactivated state to the normal closed state
what is the absolute refractory period?
when all Na+ channels are inactivated
what is the relative refractory period?
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
where are voltage gated Na+ and K+ channels in unmyelinated axons?
distributed uniformly
where are voltage gated Na+ and K+ channels in myelinated axons?
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
what does conduction velocity depend on?
fiber diameter
why are unmyelinated axons slower than myelinated axons?
- small diameter
- continuous conduction require that all of the axonal membrane must be driven to theshhold sequentially
conduction velocity is _____x the fiber diameter in unmyelinated axons
2
conduction velocity is ____x the fiber diameter in myelinated axons?
6
both unmyelinated and myelinated axons propagate AP without decrement =?
all of none!
what are the advantages of myelin?
- 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!
what can impair impulse conduction?
- TTX and saxitoxin
- local anesthetics
- demyelination
what does TTX do?
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
what does saxitoxin do?
blocks Na+ channels at extracellular site to!
you get similar symptoms of TTX but it results in flaccid and respiratory paralysis
what do local anesthetics do
- block Na+ channel at intracellular site
- they block small myelinated axons first, then unmyelinated axons then large myelinated axons
pain perception is blocked!
what does demyelination do?
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
what is frequency related block?
failure of some impulses in a train to propagate through the demyelinated region
what is total conduction block?
failure to excite the node
what is “crosstalk” between axons?
during impulse conduction some current circulating may deplarize an adjacent demyelinated hyperexcitable axons
