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action potential generation in skeletal muscle cells
the sarcolemma
is the plasma membrane of the skeletal muscle. It separates
the extracellular space from the intracellular space
Sodium ions are highly concentrated
in the extracellular space outside of the muscle cell
whereas potassium ions
are highly concentrated inside the cell in the intracellular space
this concentration of ions
is what is seen at rest. Embedded in the sarcolemma
are voltage-gated sodium channels, potassium, sodium,
ATP-ase pumps and voltage-gated potassium channels
in order to initiate an action potential
along the sarcolemma a stimulus from a neuron at the neuromuscular
junction
causes a wave of positive charge to reach voltage-gated sodium channels on the
sarcolemma
the increase in positive charge causes the voltage-gated sodium channels to
open
positively-charged sodium ion flood into the cell following their concentration
gradient
This causes the inside of the membrane near to the open
sodium channel to become more positively charged. This process
is the depolarization of the sarcolemma. The build-up a positive charge
on the inside of the membrane eventually causes the voltage-gated sodium
channels to close
at the same time voltage-gated potassium channels open
since they open only when the inside of the membrane is at its maximum positive
charge
Positively charged potassium ions flow
rapidly out of the muscle cell following their concentration gradient
this decreases the level of positive charge
inside the sarcolemma. This return of the membrane to resting levels of charge
is called repolarization. Once the sarcolemma has been fully repolarized
the potassium channels close. A sodium potassium pump
uses ATP energy to restore the concentration gradient
of sodium and potassium to their normal levels