Which of the following is the correct sequence of events that follows a threshold potential (stimulus)?

A) The membrane becomes depolarised.

B) Sodium channels open and Na⁺ ions diffuse inward.

C) The membrane become repolarised.

D) Potassium channels open and K⁺ ions diffuse out.

The correct sequence is B - A - D - C.

(B) Sodium channels open and Na⁺ ions diffuse inward: When the membrane potential reaches the threshold level (stimulus), voltage-gated sodium channels in the neuron's cell membrane open. This allows sodium ions (Na⁺) to rush into the neuron, causing a rapid depolarisation of the membrane.

(A) The membrane becomes depolarised: As sodium ions enter the neuron, the membrane potential becomes more positive, moving away from the negative resting potential. This rapid change in potential is the depolarisation phase of the action potential.

(D) Potassium channels open and K⁺ ions diffuse out: After a short delay, voltage-gated potassium channels open. This allows potassium ions (K⁺) to move out of the neuron, reversing the depolarisation and contributing to the repolarisation of the membrane.

(C) The membrane becomes repolarised: As potassium ions leave the neuron, the membrane potential returns to its negative resting value. This repolarisation phase is crucial for the neuron to reset and be ready for another action potential.