What event immediately precedes the nerve firing an action potential?

The threshold potential is reached.

Explanation:

The threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neuroscience, threshold potentials are necessary to regulate and propagate signaling in both the central nervous system and the peripheral nervous system.

Most often, the threshold potential is a membrane potential value between - 50 and - 55 mV but this value can be different as it is based upon several factors.

When the Nerve fires an action potential, the threshold potential is reached.