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17. Coordination and Control eLearn.Punjab

Nerve Impulse

Nerve impulse is a wave of electrochemical changes, which travels along the length of the neuron
involving chemical reactions and movement of ions across the cellmembrane. Electrical potential
is a measure of the capacity to do electrical work. It represents a type of stored energy which is-
manifested during separation of charges across a barrier. In the case of neuron, the charges are
positive and negative ions, and the charge separating barrier is the plasma membrane. The electrical
potential that exists across a cell membrane is known as membrane potential. A typical neuron
at rest is more positive electrically outside than inside the cell membrane. This net difference in
charge between the inner and the outer surface of a non-conducting neuron is called the resting
membrane potential. The major factors which are involved in resting membrane potential are:

1. Sodium and potassium ions: Of the many kinds of ions present in the nerve cells and the

surrounding fluid, sodium (Na+) and potassium (K+) ions are the most important. Sodium ions are
tenfold higher in concentration outside than inside the membrane surface, whereas potassium ions
are twenty times more concentrated inside than outside. All the neurons have very active sodium
and potassium pumps located in their cell membranes. Driven by the splitting of ATP, these pumps
transport Na+ out and K+ into the cell, both against their respective concentration gradients. For
every two K+ that are actively transported inward, three Na+ are pumped out. So inside becomes
more negative than the outside ofthe cell membrane o f neurons. (Fig. 17.4)

2. Negative organic ions: The large negative organic ions (such as proteins, organic acids etc)

are much more inside the membrane than outside, where they are only in negligible concentration.
This makes the inside of neuron membrane more negative.

3. Leakage of K+ ions from neurons: The cell membrane is virtually impermeable to all ions

except K+. Since the membrane is slightly permeable to K+, some of it leaks out of the cell. The loss
of this positive ion from the neuron by diffusion accounts for more negative’charges inside than
outside the cell membrane of neuron.

4. No conduction of nerve impulse

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