Final answer:
Upon reaching the threshold potential, voltage-gated Na+ channels open first in a giant squid axon, initiating an action potential that eventually leads to depolarization followed by repolarization of the membrane.
Step-by-step explanation:
The threshold potential is a critical level to which the membrane potential must be depolarized to initiate an action potential. When a giant squid axon's membrane potential reaches the threshold potential (-49 mV), voltage-gated Na+ channels open first.
This opening triggers a rapid influx of sodium ions into the cell, causing further depolarization. Once the membrane potential reaches approximately +40 mV, an action potential peaks, and the sodium channels close, preventing further entry of sodium.
Subsequently, voltage-gated K+ channels open, allowing potassium ions to leave the cell, which leads to repolarization. The cell eventually returns to its resting membrane potential via the action of the Na+/K+ pump and potassium leakage channels. The correct option is that the first channels to open are the sodium ion channels.