In a motor neuron receiving thousands of synaptic inputs, temporal summation is occurring. A single presynaptic neuron fires 3 action potentials in rapid succession (50 ms apart). Why does this cause greater depolarization than a single impulse?
- A Rapid firing causes presynaptic facilitation via Ca²⁺ depletion at the terminal
- B Each EPSP decays slowly; successive EPSPs summate before the previous one fully dissipates, raising membrane potential progressively ✓
- C The postsynaptic membrane becomes hypersensitive through upregulation of NMDA receptors
- D Temporal summation requires at least two different presynaptic neurons firing simultaneously
Correct answer: B. Each EPSP decays slowly; successive EPSPs summate before the previous one fully dissipates, raising membrane potential progressively
Explanation
Each EPSP has a time constant of several milliseconds; it does not instantly decay. When a second EPSP arrives before the first has fully dissipated, the depolarizations algebraically summate. Successive impulses at short intervals (50 ms) produce cumulative depolarization that may reach threshold. By definition, temporal summation involves the same presynaptic neuron firing repeatedly, while spatial summation involves multiple neurons firing simultaneously.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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