What is the process by which subthreshold stimuli add together to evoke a response?

• A. Summation
• B. Conduction
• C. Propagation
• D. Activation

Answer: (A)

Summation is the process at work when several subthreshold inputs add up to trigger a response. Each input produces a small depolarization (an EPSP) that by itself isn’t enough to reach the threshold for firing an action potential. But when inputs occur close together in time (temporal summation) or come from multiple nearby synapses at once (spatial summation), their depolarizations accumulate. If the combined effect reaches the threshold at the axon hillock, an action potential is generated. This concept also explains how inhibitory inputs can oppose excitation: if enough IPSPs offset the EPSPs, the net depolarization may not reach threshold.

Conduction refers to the movement of an impulse along the axon after initiation, and propagation is the spreading of that impulse along the membrane—these describe movement, not the additive process. Activation isn’t the term used for this integrative process.