NDA (II) 2022 General Ability Test · Question 73 of 150

Correct — D, sodium and potassium. A nerve impulse is not electrons running along the fibre; it is a travelling change in the voltage across the neuron's membrane, and that voltage is created by sodium and potassium ions moving through it. At rest the cell is kept sodium-poor and potassium-rich inside by the sodium–potassium pump. When the neuron fires, sodium ions rush inward and reverse the membrane voltage, and potassium ions then flow outward to restore it. Both ions are indispensable, so only the option naming both is complete.

• (a)sodium — Sodium influx does start the impulse, but on its own it explains only half the cycle. Without potassium moving the other way the membrane could neither reset after firing nor hold its resting voltage in the first place.
• (b)potassium — Potassium efflux is what repolarises the neuron and sets the resting potential, but the impulse itself is triggered by sodium rushing in. Naming potassium alone leaves out the event that starts the signal.
• (c)iron — Iron is essential in haemoglobin and in respiratory enzymes, and its shortage causes anaemia, but it plays no part in carrying the nerve's electrical signal.

Neurons keep an electrical charge difference across their membrane, the resting potential, maintained by the sodium–potassium pump, which drives three sodium ions out for every two potassium ions it brings in, using ATP. A stimulus opens sodium channels; sodium floods in and the membrane briefly reverses polarity, producing an action potential. Potassium channels then open and potassium leaves, restoring the resting state. The disturbance travels along the axon as each patch of membrane sets off the next.

The examiner is separating two ideas students often merge — the metals that carry oxygen and the ions that carry signals. Iron belongs to the first group, sodium and potassium to the second. Notice also that the correct option is the inclusive one, which is unusual — here it is right because the two ions do genuinely different jobs in the same cycle, depolarisation and repolarisation, rather than because 'both' is a safe hedge. Calcium is the next ion to learn in this chain, because it triggers neurotransmitter release at the synapse and starts contraction in muscle.

• Nerve impulses travel as action potentials — brief reversals of membrane voltage caused by ion movement, not by electrons flowing along the nerve.
• The sodium–potassium pump moves 3 Na⁺ out for every 2 K⁺ in, using ATP, and maintains the resting potential.
• Depolarisation is a rapid inflow of sodium ions; repolarisation is an outflow of potassium ions.
• Calcium ions take over at the synapse, triggering release of neurotransmitter into the gap between neurons.
• Iron's role in the body is oxygen transport in haemoglobin and electron transfer in respiratory enzymes, not nerve conduction.

• Picking a single ion when the mechanism needs both.
• Imagining nerve signals as electrons travelling down the fibre like current in a wire.
• Confusing iron's oxygen-carrying role with the ion movements behind nerve conduction.

As a direct 'which ion' item like this, as a statement set on the sodium–potassium pump, or as part of a broader question on which minerals the body needs for which function.

• practice — not a real PYQ

  The resting potential of a neuron is maintained mainly by

  • (a)the sodium–potassium pump
  • (b)haemoglobin in the blood
  • (c)calcium binding to troponin
  • (d)the myelin sheath

  Answer(a) the sodium–potassium pump — it uses ATP to keep sodium out and potassium in, holding the membrane voltage.

• practice — not a real PYQ

  Which ion rushing into a neuron begins the action potential?

  • (a)Potassium
  • (b)Sodium
  • (c)Chloride
  • (d)Iron

  Answer(b) Sodium — its rapid inflow depolarises the membrane and starts the impulse.