Channels system of cell membrane of heart
Na – K leakage channels and Voltage gated Na channels and K channels are the Channels system of cell membrane of heart.
Na – K leakage channels
These channels are 100 times more permeable to K+ ions than Na+ ions and are exceedingly important in determining the level of normal resting membrane potential.
Voltage – gated channels of the cell membrane
The voltage-gated Na+ channel causes both depolarization and repolarization of the nerve/muscle membrane during the action potential. The voltage-gated K+ channel also plays an important role in establishing the repolarization of the membrane. These two voltage-gated channels are present in the cell membrane-
Voltage – gated Na+ channel
The voltage gated Na+ channels have two gates, the external gate or activation gate which opens to outside of the cell. The other gate is at the interior end and opens to inside of the cell referred as internal gate or inactivation gate.
The activation gate is closed during resting stage (- 90 mV), while the inactivation gate is opened. This prevents free passage of Na+ ions from outside to interior of the cell.
During depolarisation stage, the resting membrane potential drops from –90 mV or to – 70 to – 50 mV causes opening of the activation gate due to conformational changes results in increased Na+ ion permeability as much as 500 to 5000 folds into the cell through the channel system.
A gradual increase to positivity following the opening of the activation gate also closes the inactivation gate comparatively at a slow speed.
The time lapse between the activation and inactivation of the channels causes the passage of Na+ ions to the interior of the cell for few milliseconds.
During repolarisation stage, the inactivation gate is completely closed and prevents Na+ ion entry from outside to the interior of the cell membrane. The inactivation gate will not reopen until the disturbed membrane potential returns nearly to the original resting membrane potential level of -70 to- 90 mV.
Voltage – gated potassium channel
This channel has only one gate at the interior of the membrane. It may either close or open to the interior of the cell.
During resting stage this gate is inactivated and prevents the passage of K+ ion to the exterior of the cell through this channel system. When the electrical potential drops towards zero, Na+ channels get inactivated causes the activation of K+ channels to allow increased K+ ions diffusion to outside of the cells.