What mechanism enables triggering an action potential when a single synapse is not enough?

• A. Spatial summation
• B. Temporal summation
• C. Inhibitory summation
• D. Neurotransmitter depletion

Answer: (A)

When one excitatory input isn’t enough to reach the threshold, multiple inputs can converge on the same postsynaptic neuron and their depolarizations add up. This is spatial summation: several excitatory inputs from different synapses arrive at roughly the same time, and their excitatory postsynaptic potentials combine to reach the threshold to trigger an action potential. If their combined depolarization crosses threshold, voltage-gated sodium channels open and the neuron fires.

Temporal summation is a related idea but involves rapid successive firing at the same synapse, causing potentials to accumulate over time. Inhibitory summation would make firing less likely by hyperpolarizing, and neurotransmitter depletion would reduce the signal, also making an action potential less likely.