Neurons fire when they reach which threshold?

• A. Resting potential
• B. Synapse
• C. Threshold
• D. Myelin sheath

Answer: (C)

The key idea is that firing depends on reaching a specific membrane-potential level called threshold. At rest, a neuron's membrane is about -70 millivolts. When enough excitatory input depolarizes the membrane to the threshold (roughly -55 mV in many neurons), voltage-gated sodium channels open, triggering a rapid depolarization that becomes an action potential. This all-or-none event means once threshold is reached, the neuron fires; if it isn’t reached, no action potential occurs. The resting potential is just the baseline, the synapse is simply the site where signals are received, and the myelin sheath speeds up signal transmission but does not determine whether the neuron fires.