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Posts Tagged ‘sodium channel’

Human body is a collection of trillions of cells.  Life  is nothing but , a bundle of energy flowing across each of these cells  .Every  organ  has a  specailised mode of communication among themselves and others. When a cell is in an excited state , there is a  likelihood of spontaneous electrical activity.This can happen in nerve cells, cardiac cells , GI tract,  or virtually in  any cell  which has a porous cell membrane and ionic fluxes across it .

  • Each cell membrane has a resting membrane potential . It  varies between -60 to – 90mv in most cells. When this potential increases there a propensity for  arrhythmias in heart  and convulsions in the brain , peristalsis in intestines and so on .
  • Drugs  like local anesthetic lignociane acts by blocking the  Na+ channels and there by neural activation .Similarly magesium  acts on these channels to reduce the excitability of these cells.
  • We know,  the sharp ascending stroke of cellular  action potential is mediated by Na + .Blockage of this channel blunts the action potential voltage and thus  the  early and late after depolarisation is prevented
  • Magnesium sulphate’s anticonvulsant action is directly  attributable  to this membrane stabilising action

Thus , membrane stabilising action  can be termed as “membrane sedating”  action

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Here is  the link  to  one of  the best illustration for cardiac action potential  which I  have stumbled upon !

Spend some time on the following illustration  depicting  the  normal  cardiac  action potential that  explain the ionic movements . Understand why a cardiac muscle has two refractory periods , why there is a sustained dome for  myocardial action potential  and this is missing in SA and AV nodal potential ?

Click below to reach the online book

Textbook in  Medical Physiology And Pathophysiology

Essentials and clinical problems Copenhagen Medical Publisher

Note :

Red curve indicates electrical action potential .Blue depicts the mechanical contraction . Both red and blue curves together form the electromechanical systole. Realise  ,   QT interval  represents electro mechanical systole . It  includes both cardiac depolarisation and repolarisation .

There is a inherent tendency for our brains  to equate depolarisation with systole and repolarisation with diastole .It is totally a wrong perception. Please , be aware of this !

Identify the gap between the  red and blue curves that represent 50%  of ARP  .This is the time the myocytes can not be stimulated whatever be the  power of stimuli because the Na  channels are closed .

Understand ,the above action potential  represents only half of the cardiac cycle as diastole is not fully illustrated here .Recognise  the fact ,  diastole begins at the end of phase 3  and  goes into phase 4 as diastolic depolarisation  by a slow Na current.

After learning   the basics of action potential   read about the antiarrhythmic drugs . You will get to understand it better .

Learn  which drug acts on which receptors or channels and what does it do  to the various intervals  .For example ,  any drug that is prolonging an action potential  duration is fraught with risk of  ventricular arrhythmia as it is synonymous with prolonging QT interval (Eg Class 1 A /1B/Class 3) .

Sicilian gambit is the receptor & channel based classification for anti arrhythmic drugs . (Sicilian gambit 2 )

  • Understand the paradox of  QT interval getting shortens with Class 1 B (ligocaine /Mexilitine ) while 1 A  does the opposite !

Class 1   Drugs blocks sodium channels .The blockage  occurs in  a complex rate dependent fashion . It blunts the slope  the phase 0 and hence prolongs the action potential .

Class 2 . Beta blockers

Class 3 . Blocks K + Channels and hence prolongs the AP

Class 4  .Calcium blockers

Finally  don’t forget to say thanks to Copenhagen medical  publishers  for this excellent illustration .

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