P. 87. Ex. IV

1. resting potential; 2. escape movements; 3. external environment;

4. sodium ions; 5. potential difference; 6. electrical charge;

7. light intensity; 8. to respond quickly; 9. nerve fibres; 10. to reach a peak.

p. 88. Ex. VII. The inside / interior (of the neurone); the size of the potential; to respond quickly; resting condition; potential difference; to reach the threshold level; to obey the law; unequal distribution; to measure; complete reversal of charge.

p. 88. Ex. IX. During the resting potential, the voltage-gated sodium and potassium ion channels are closed. When a stimulus is applied, sodium ion channels open rapidly, sodium ions move in, and the inside becomes more positive. If the stimulus reaches the threshold level, an action potential occurs. When the action potential reaches its peak, the sodium ion channels close slowly and potassium ion channels open slowly. Sodium ions stop moving into the cell but potassium ions diffuse more rapidly out. These changes cause the potential difference to drop. When the membrane returns to its resting potential, potassium ion channels close, but because they do this slowly, the potential dips below the resting level. Finally, when the potassium ion channels are closed, the membrane returns to its resting condition.