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Coordination and Irritability

(c) Action potential
Sodium ions move inside the membrane when a stimulus reaches a resting neurone. The
gated ion channels on the resting neurone’s membrane open suddenly to allow the Na+
that was on the outside of the membrane to rush into the cell. While this happens, the
neurone changes from being polarised to being depolarised. After more positive ions enter

inside the membrane, the inside becomes positive, causing depolarisation (Figure 4.11).
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Na +
Figure 4.11: Propagation of nerve impulse across a membrane (action potential)

(d) Repolarisation
Localised electrical circuits are established, causing further influx of sodium ions and
so progression of the impulse. Behind the impulse, potassium ions begin to leave the
axon along the concentration gradient, hence repolarisation begin to occur due to the
outward flow of K ions. The depolarisation speeds forward, triggering an action potential
+
(Figure 4.12).

Figure 4.12: Propagation of nerve impulse across a membrane (localisation)

During repolarisation, potassium ions move outside, while sodium ions stay inside the
membrane. After repolarisation, the inside of the cell becomes flooded with Na ; the gated
+
ion channels on the inside of the membrane open to allow K to move to the outside of
+
the membrane. With K moving to the outside, the membrane’s repolarisation restores
+
electrical balance, although it is the opposite of the initial polarised membrane that had
Na gates close. Otherwise, the membrane could not repolarise (Figure 4.13). Then Na
+
+
ions are actively forced out of the axoplasm in the process called sodium pump. However,
since K ions are also involved in this process, the process is best called cation pump.
+

Figure 4.13: Propagation of nerve impulse across a membrane (repolarisation)

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