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Coordination and Irritability

(b) Rapid repolarisation All the three phases of the action potential
This is the process that changes the occur within a few milliseconds (ms) for
membrane potential back to negative the action potential to begin in a giant axon,
inside and positive outside. A fraction the membrane potential must shift from
of a second after the sodium gates open, its resting potential of -70 mV to about
depolarisation of the axon membrane +40 mV. If the membrane depolarises
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causes the potassium gates to open too, less than that, an action potential does not
and potassium diffuses out of the cell. occur. However, if this threshold potential
Since potassium is positively charged, the is reached, sodium ion channels in the
inside of the cell becomes less positive and axon membrane open. Ions, thus rush into
starts the process of repolarisation, then, it the axon following their electrochemical
returns to its original resting potential.
gradients. The inside of the membrane
becomes less negative and then positive
(c) Hyperpolarisation with respect to the outside. When the
This occurs after the impulse has been membrane potential reaches about +40 mV,
transmitted; the action potential falls down a rapid change occurs and the repolarisation
and the sodium gates close immediately. phase begins (Figure 4.8). The change
But potassium gates delay to close, causing
more K ions to exit the axon and this makes is caused by the closure of sodium ion
+
the membrane slightly more negative than channels and the opening of potassium ion
the resting potential. Hyperpolarisation of channels.
the axon overshoots into more negative
potential than the original potential.
Action Refractory
50 Depolarisation potential period

Membrane Potential (mV) -50 0 Threshold potential Resting potential
Repolarisation

-100 Resting potential Hyperpolarisation
0 1 2 3 4 5 6 7
Time (milliseconds)
Action potential in a neurone

Figure 4.8: Graph showing the propagation of nerve impulse across a membrane

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