Duration required for multiple synapses in rapid succession to result in a stronger depolarization to trigger an action potential

Duration required for multiple synapses in rapid succession to result in a stronger depolarization to trigger an action potential

• A. The duration between inputs across multiple synapses necessary for summation
• B. The total duration of the stimulus
• C. The latency to trigger an action potential
• D. The recovery time of the photoreceptor

Answer: (A)

The main idea here is temporal summation—the ability of a neuron to accumulate multiple excitatory inputs that arrive in close succession so the combined depolarization reaches the threshold for firing an action potential. Each excitatory postsynaptic potential (EPSP) is brief and decays with the neuron's membrane time constant. If several inputs arrive within that short time window, their depolarizations add up, producing a larger overall depolarization. If the interval between inputs is too long, the first EPSP may decay before the next one arrives, so they don’t summate effectively and an action potential may not be triggered. The option describing the duration between inputs across multiple synapses needed for summation captures this time window for summation. The other choices refer to unrelated aspects—the overall stimulus duration, the lag before an action potential after a stimulus, or the photoreceptor’s recovery time—not the critical timing window for summation.

The main idea here is temporal summation—the ability of a neuron to accumulate multiple excitatory inputs that arrive in close succession so the combined depolarization reaches the threshold for firing an action potential. Each excitatory postsynaptic potential (EPSP) is brief and decays with the neuron's membrane time constant. If several inputs arrive within that short time window, their depolarizations add up, producing a larger overall depolarization. If the interval between inputs is too long, the first EPSP may decay before the next one arrives, so they don’t summate effectively and an action potential may not be triggered. The option describing the duration between inputs across multiple synapses needed for summation captures this time window for summation. The other choices refer to unrelated aspects—the overall stimulus duration, the lag before an action potential after a stimulus, or the photoreceptor’s recovery time—not the critical timing window for summation.