Excitability of muscles
Excitability of muscles is the ability of any living cell or tissue to exhibit an electrochemical change (action potential) to a stimulus. However, nerve and muscle cells are highly excitable cells than other cells.
Events of action potential of muscle fiber
The action potential is caused by a sequence of changes or events occurring in the membrane permeability to Na+ and K+ ions. When an excitable cell is stimulated, action potential aids the transmission of impulses through nerve and muscle fibres. Action potential is the rapid changes in the membrane potential from its normal negativity to positive potential inside the cell membrane which last for few milliseconds, and then returns back to its original resting negative potential level.
Polarised membrane is the resting cell membrane with a normal negative resting membrane potential.
Depolarization stage is the first event of action potential is characterised by rapid increase in the permeability to Na+ ions (5000 folds) to interior of the cell generating more positive electrical potential inside of the cell.
This is followed by a gradual inactivation of Na+ channels (closure) that occur within another few milliseconds and the membrane becomes impermeable to Na+ ions. It is associated with gradual opening of voltage gated K+ channels to allow K+ ions outflow to the exterior of the cell membrane. The potential inside cell is re-established to its normal resting level (- 75mV). This stage is called as the repolarisationstage.
Higher concentration of K+ ions in the exterior of the cell towards the end of the action potential continues for a short period creates more negativity inside referred to as hyper-polarised state. At this state, re-excitation of the cell will not occur.
The final event is characterised by electrogenic pump mechanism, which aids in the transport of three Na+ ions to the exterior for every two K+ ions to interior of the cell and create the normal resting potential (- 75 mV) on the inside of the cell membrane.
The whole action potential lasts about 1 – 2 milliseconds in most nerves, but longer in many muscle cells.
Channel systems of cell membrane
Voltage – gated channels of the cell membrane
The voltage-gated Na+ channel causes both depolarisation and repolarisation of the nerve/muscle membrane during the action potential. The voltage-gated K+ channel also plays an important role in establishing the repolarisation 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 (- 75 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 –75 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- 80 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.
Electro-chemical changes during action potential Spike potential (over shoot)
It is the steep change in the negativity of the cell from – 75 mV to more positivity of + 40 mV in the membrane potential due to rapid increase in the permeability of the Na+ ions to the interior of the cell.
In large fibres, the electrical potential during depolarisation stage overshoots slightly positive beyond the zero level whereas in the small fibres and many CNS neurons the potential does not over shoot beyond zero.