The two key cations involved in the action potential of nervous transmissions are?

Analyze the question: It asks which two positively charged ions (cations) are mainly responsible for the action potential, the electrical signal that travels along a neuron.

Concept: A neuron at rest has a resting membrane potential of about -70 mV. The inside is negative relative to the outside. This is maintained by the sodium-potassium pump, which moves 3 Na⁺ out and 2 K⁺ in using ATP.

Step-by-step solution:

1. Stimulus arrives. The membrane reaches threshold.
2. Depolarisation. Voltage-gated Na⁺ channels open and Na⁺ rushes into the cell. The inside becomes positive (about +30 mV).
3. Repolarisation. Na⁺ channels close and voltage-gated K⁺ channels open. K⁺ rushes out, so the inside becomes negative again.
4. Hyperpolarisation and recovery. The membrane dips slightly below resting level. The Na⁺/K⁺ pump then restores the original ion distribution.

Checking the options:

• A. Fe²⁺ is involved in haemoglobin and enzymes, not nerve impulses. ❌
• B. Mg²⁺ is an enzyme cofactor and part of chlorophyll. It is not a key ion in action potentials. ❌
• C. Na⁺ and K⁺ are the ions that move across the neuron membrane to produce the action potential. ✅
• D. Neither Fe²⁺ nor Mg²⁺ is involved. ❌

Exam tip: Remember "Na in, K out". Sodium goes in to depolarise, and potassium goes out to repolarise. Whenever you see nerve impulse, think Na⁺/K⁺.