Download Electrochemical Impulses

Electrochemical Impulses
Comparing Impulses to Electricity
One of the greatest differences between nerve impulses and electricity is that nerves use
cellular energy to generate current where electricity requires an outside source. The
cytoplasm of the cell is very resistant to impulses, therefore nerve impulses travel at a
much slower rate than electricity. Finally, nerve impulses remain strong from start to
finish, where as electricity loses strength as it moves through the circuit.
Nerve Impulses – are electrochemical messages that are created by the movement of
ions through the cell membrane. When the cell becomes excited there is a rapid change
in the movement of charges across the membrane (-70 mV to 40mV).
Resting Membrane – is when there is a steady charge difference across the cell
membrane. The cell membrane is not being stimulated.
Action Potential – are nerve impulses. The reversal of charge across a nerve cell
membrane starts an action potential.
Nerve cell are charged where no other cell in the body is. The reason why nerve cell are
different is because they have a rich supply of ions (+/-) both inside and outside the cell’s
membrane. The electrochemical impulse is caused by an unequal concentration of
positive ions across the membrane.
A potassium pump is found in the membrane and it pulls K+ ions into the nerve cell,
while a sodium pump pushes Na+ out of the cell.
A high concentration of K+ or Na+ ions on either side of the membrane cause the
diffusion of ion from high concentration to low concentration.
Therefore, K+ diffuses outside the nerve cell and Na+ diffuses into the nerve cell. As you
can see positive ions are moving in and out of the cell but the diffusion of these two ions
are not equal. This unequal diffusion causes the nerve impulse (or action potential).
In a resting membrane, more channels are open to K+ than Na+. Therefore, more K+
diffuses out of the cell than the amount of Na+ that is being diffused into the cell. The
more rapid the diffusion of K+ outside makes the outside more positive than the inside of
the cell. Therefore, the cell loses a greater number of ions than it gains. Scientist believe
that is happens because there are more potassium gates in the cell membrane than sodium
gates.
An excess of positive ions accumulate along the outside of the membrane, while an
excess of negative ions accumulate along the inside of the membrane. The resting
membrane is therefore referred to as charged or polarized. This separation of charges in
the membrane means that there is the potential for work to be done.
Upon excitation, nerve cells become more permeable to Na+ than K+. The Na+ ions then
rush into the cell by diffusion and cause a charge reversal or depolarization. Once the
voltage inside the cell becomes positively charged the Na+ gates close and Na+ no longer
is able to enter the cell.
The K+ gates now open and K+ ions begin to diffuse out of the cell. Eventually, the
diffusion out of the cell causes the cell to return to normal. However, the Na+ and K+
ion are now on the side of the resting phase opposite to their position before
depolarization occurred. Once again, the sodium-potassium pumps will resort the
condition of the resting membrane by moving Na+ out and K+ in the neuron. The
process of restoring the original polarity is called repolarization.
Nerve conducting impulses cannot be activated until the repolarization has occurred. The
period of time required for the nerve cell to become repolarized is called the refractory
period.