'A' Level Medical Option Questions - The Heart - ECG

Q3. The graph below shows the variation of membrane potential difference with time of a nerve fibre, known as an action potential.

(a) Complete the graph by adding suitable axes, units and scales.

  • axes: time/ms, action potential/mV
  • time scale can range about 5 ms - usually about 2-3 ms for central peak
  • action potential scale +30 mV to –70 mV

3 marks

(b) Describe the processes involved in the production of such an action potential when a nerve is stimulated.

  • Na+ ions move into the cell .
  • The pd rises (from –70 to 0) , this is called depolarisation
  • K+ ions move out of the nerve . The pd returns/falls to –70/resting potential, this is called repolarisation
  • Na+ moving from 0 to +30 called reverse polarisation
  • To restore starting equilibrium of ions, the Na/K pump operates

3 marks

Total 6 marks