Questions: Neural Integration and Synaptic Plasticity

Question 1

A postsynaptic neuron receives simultaneous weak stimulation from 200 excitatory synapses, each producing a sub-threshold EPSP. The combined depolarization reaches threshold and the neuron fires. This is an example of:

• A. Temporal summation — multiple EPSPs from rapid successive firing of a single synapse
• B. Spatial summation — simultaneous EPSPs from multiple synapses add together at the axon hillock
• C. LTP — co-activation of many synapses permanently strengthens their connections
• D. Lateral inhibition — inactive neighboring synapses suppress the effects of active ones

Answer: (B)

Spatial summation occurs when EPSPs from different anatomical locations (different synapses) arrive simultaneously and their depolarizations add together at the axon hillock. Each individual contribution is sub-threshold, but the combined effect crosses threshold. Temporal summation is distinct: a single synapse fires rapidly enough that EPSPs arrive before the previous one decays — summation in time rather than space. LTP (option C) is a persistent change in synaptic strength resulting from sustained co-activation; the question describes a single firing event, not a long-term structural change.

Question 2

Why does LTP specifically require coincident presynaptic and postsynaptic activity, rather than being triggered by either one alone?

• A. Because AMPA receptors only open when both glutamate binds and the membrane is simultaneously depolarized
• B. Because the NMDA receptor requires both glutamate binding AND relief of Mg²⁺ block by postsynaptic depolarization before calcium can enter and trigger LTP
• C. Because presynaptic activity releases BDNF that must combine with postsynaptic calcium to initiate LTP
• D. Because temporal summation is required to accumulate enough calcium for LTP induction

Answer: (B)

The NMDA receptor functions as a coincidence detector — a molecular AND gate. It requires TWO simultaneous conditions: (1) glutamate binding from the presynaptic terminal, AND (2) sufficient postsynaptic depolarization to displace the Mg²⁺ ion blocking the channel. Only when both conditions are met does calcium enter and trigger the CaMKII-mediated cascade that inserts more AMPA receptors and strengthens the synapse. Either condition alone is insufficient: glutamate without depolarization leaves the Mg²⁺ block intact; depolarization without glutamate leaves the channel closed. This coincidence requirement implements Hebb's rule ('neurons that fire together wire together') at the molecular level.

Question 3

Long-term potentiation (LTP) is a purely postsynaptic phenomenon — changes in synaptic strength during LTP involve mainly the postsynaptic cell's receptor number and conductance.

• T. True
• F. False

Answer: False

LTP involves both postsynaptic and presynaptic changes. Postsynaptically, CaMKII activation inserts additional AMPA receptors into the synapse and enhances their conductance. But presynaptic changes also occur: retrograde signaling molecules (including endocannabinoids and nitric oxide) travel from the postsynaptic cell back to the presynaptic terminal, increasing neurotransmitter release probability. Changes in vesicle pool size and release mechanisms have been documented. Both sides of the synapse contribute to enhanced transmission in LTP, which is why studies manipulating only postsynaptic factors underestimate the full magnitude of potentiation.

Question 4

According to Hebb's rule, a synapse is strengthened whenever the presynaptic neuron fires, regardless of what the postsynaptic neuron is doing at that moment.

• T. True
• F. False

Answer: False

Hebb's rule specifically requires coincident activity: 'neurons that fire together wire together.' A synapse is strengthened only when the presynaptic neuron fires AND the postsynaptic neuron is simultaneously sufficiently depolarized. If the presynaptic neuron fires when the postsynaptic cell is inactive, the NMDA receptor remains Mg²⁺-blocked and no calcium influx occurs — LTP does not result. In fact, weak presynaptic stimulation without sufficient postsynaptic depolarization can produce long-term depression (LTD) instead. The coincidence requirement ensures that only correlated activity produces synaptic strengthening, allowing neural circuits to encode associations rather than indiscriminately potentiating all active synapses.

Question 5

Explain why the NMDA receptor has been called a 'coincidence detector' and why this property is essential for associative learning.

Model answer: The NMDA receptor acts as a coincidence detector because it requires two simultaneous conditions to open and pass calcium: (1) glutamate binding from the presynaptic neuron, indicating presynaptic activity, and (2) sufficient postsynaptic depolarization to displace the Mg²⁺ blocking the channel, indicating the postsynaptic cell is also active. Only when both neurons are simultaneously active does calcium enter and trigger LTP. This is essential for associative learning because it allows synapses to detect and record co-occurrence: if two inputs reliably fire together, both conditions are met repeatedly, the connection strengthens, and the brain encodes the association. Random, uncorrelated activity leaves the Mg²⁺ block in place and produces no lasting change.

The coincidence detection property implements Hebb's rule at the molecular level and is thought to underlie how the brain forms associations between stimuli, learns cause-and-effect relationships, and stores memories. Without it, all activity would strengthen all synapses indiscriminately — there would be no mechanism to selectively encode which inputs are meaningfully correlated versus which co-activated by chance.