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Az elő videóból megtudtuk,

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hogy magas Ca-ion koncentráció esetén
az izomsejtben

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a Ca-ionok hozzákötődnek a troponin fehérjéhez,

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amely úgy változtatja meg az alakját,

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hogy a tropomiozin elmozdul az útból,

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így a miozinfejek végig tudnak haladni az aktinon,

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és összehúzódik az izom.

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Tehát a magas Ca-koncentráció

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összehúzódást okoz.

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Alacsony Ca-koncentrációtól

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a troponinmolekulák visszanyerik eredeti konformációjukat,

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és visszajuttatják (húzzák) a tropomiozint

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a miozin fejek útjába,

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és az összehúzódás megszűnik.

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Ez okozza tehát az izomösszehúzódást,

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az alacsony Ca-koncentráció pedig az elernyedést.

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Ebből ered a következő kérdés:

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miként váltakozik az izomban

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a magas Ca-koncentráció és összehúzódás

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illetve az alacsony Ca-koncentráció és elernyedés.

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Vagy még pontosabban:

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Hogyan idéz elő az idegrendszer az izomban

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magas Ca-koncentrációt és összehúzódást

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illetve alacsony Ca-koncentrációt és elernyedést.

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Ennek megértéséhez

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ismételjük át azt, amit korábban a neuronokról tanultunk!

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Ide rajzolok egy axonvégződést,

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amely nem egy másik neuron dendritjével alkot szinapszist

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hanem egy izomsejttel.

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Így ni, mindjárt kész.

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Ez egy szinapszis egy izomsejttel.

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Feliratozom, hogy érthető legyen.

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Ez itt az axon,

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vagyis az axonvégződés.

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Ez a szinapszis (kimentem az ábrából)

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Rémlik a neve az idegsejtes videókból

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ez a hézag a szinaptikus rés,

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ez a szinapszis előtti idegsejt (preszinaptikus neuron),

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ez pedig a szinapszis utáni (posztszinaptikus) sejt,

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ami ebben az esetben nem idegsejt,

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ez pedig az izomsejt membránja.

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Készítek majd egy videót

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az izomsejt szerkezetéről,

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most csak vázlatosan mutatom,

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hogy megértsük a Ca-ion koncentráció szabályozását.

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Ez tehát az izomsejt membránja,

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más néven szarkolemma.

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Ez tehát az izomsejt membránja,

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ez pedig egy betűrődés az izomsejt membránjában.

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Az izomsejt felszínéről nézve

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olyan lenne, mint valami lyuk vagy horpadás,

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de ezen a keresztmetszeti rajzon

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olyan mintha benyomtuk volna

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mondjuk egy tűvel

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akkor ilyesmit kapnánk

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benyomódik a membrán,

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ez az úgynevezett T-tubulus.

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A T betű jelentése transzverz (átlós)

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mivel merőleges a membránra.

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És itt van a legfontosabb rész az egész videóban

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a legfontosabb sejtalkotó,

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egy sejtalkotó az izomsejtben,

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a szarkoplazmatikus retikulum.

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Ez tulajdonképpen olyan, mint az endoplazmatikus retikulum,

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de ennek a feladata a tárolás.

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Mig az endoplazmatikus retikulum a fehérjeszintézis helye,

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és rioszómák kapcsolódnak hozzá,

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ez a sejtalkotó viszont kimondottan csak tárolást végez.

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A szarkoplazmatikus retikulumnak

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Ca-ion pumpák találhatók a membránjában.

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Ezek ATP-ázok,

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vagyis ATP felhasználásával működtetik a pumpákat.

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ATP kapcsolódik hozzájuk,

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esetleg Ca-ion is (ezt rózsaszínnel jelölöm).

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Az ATP hidrolizál ADP-re és foszfátcsoporta,

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ettől megváltozik a fehérje konformációja,

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és Ca-ionokat pumpál a sejtbe.

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