Synaptic Plasticity and the Molecular Rules of Change
Synaptic strength can change with activity. In a common timing rule, repeated input-before-target firing favours strengthening and reversed timing favours weakening. At many glutamate synapses, NMDA receptors detect overlapping transmitter binding and depolarisation, allowing calcium entry that can initiate change.
What a learner can do afterwards
- Explains how NMDA receptors detect overlapping glutamate binding and local depolarisation, with a suitable co-agonist bound.
- Uses a stated spike-timing rule to predict strengthening or weakening without treating that rule as universal.
- Links a lasting change in strength to a change in receptor number or transmitter release.
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At a synapse, a sending cell signals to a receiving cell. A spike is a brief electrical signal. Synaptic plasticity changes how strongly the connection transmits a signal. Long-term potentiation (LTP) is a lasting increase in strength; long-term depression (LTD) is a lasting decrease. In a common spike-timing rule, repeated close input-before-target spike pairs favour LTP. Reversing their order favours LTD. Other synapses can follow different rules.
You have met receptors that open ion channels. At many glutamate synapses, NMDA receptor channels respond to overlapping binding and voltage conditions. Glutamate and a co-agonist, such as glycine, must bind. The receiving membrane must also depolarise enough to relieve a magnesium block. Depolarisation means becoming less negative. Calcium can then enter. Glutamate binding alone at resting voltage does not remove the block.
Calcium entry through NMDA receptors can start signalling that changes synaptic strength. It does not guarantee LTP. The calcium signal's size and time course, and the synapse's state, affect which pathways respond. Some pathways add AMPA receptors during LTP; others remove them during LTD. AMPA receptors are glutamate-gated channels that carry much of the fast excitatory current at these synapses. Calcium entry may also produce no lasting change.
Strength changes can involve either side of a synapse. Adding working AMPA receptors can let the same glutamate release open more channels and produce a larger current under matched conditions. Removing receptors can reduce the response. Moving receptors into or out of the membrane is receptor trafficking. The sending terminal may instead change transmitter release per spike. A larger response alone does not identify which mechanism changed.
NMDA channels can pass calcium when glutamate and a co-agonist are bound and local depolarisation relieves the magnesium block. Summed inputs can provide this depolarisation without a target spike. Input-before-target timing does not prove that one input caused the spike alone: other inputs may contribute. An active input need not strengthen, and an input outside a timing window need not weaken.
Apply a timing rule only where it fits. NMDA receptors detect overlapping conditions; calcium signalling can lead to changes in receptor response or transmitter release.
Lesson, questions and flashcards written for this dataset. OpenStax Biology and Anatomy and Physiology were consulted for scientific background.
Readings and flashcards written for this dataset, with OpenStax consulted for scientific background. Linked videos are from Khan Academy.
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What a timing experiment can show
An experiment can compare a synapse's response before and after a repeated firing pattern. A lasting increase in strength is LTP. A lasting decrease is LTD. These names describe the measured outcome.
In a common timing protocol, the input spikes shortly before the target on each repetition. Synapses that follow this rule tend to strengthen. Repeating the reversed order tends to weaken them. The rule has a scope: different synapses and experimental conditions can give different results.
Firing first does not prove that the input was the only cause of the target spike. Other inputs may have helped. A timing record and a strength measurement answer different questions, and neither alone establishes sole causation.
Checking the NMDA receptor conditions
At many glutamate synapses, NMDA receptors connect chemical input with the receiving membrane's voltage. Glutamate and a co-agonist, such as glycine, must bind to the receptor.
At resting voltage, magnesium blocks much of the channel's current. Glutamate binding alone does not remove that block. Sufficient local depolarisation relieves it. When the binding conditions and depolarisation overlap, calcium can enter through the channel.
The calcium signal can activate pathways inside the receiving cell. Its size and time course help determine the result, along with the synapse's state. Detecting calcium entry therefore does not establish that strengthening occurred. Later measurements are needed to distinguish a lasting increase, a lasting decrease and no lasting change.
Local voltage without a target spike
A spike is a brief electrical signal. Inputs can also produce smaller changes in the receiving membrane's voltage without making the cell spike. Several input responses can add together.
NMDA receptor block depends on voltage near the receptor. A target spike can contribute depolarisation, but summed input responses can also provide enough local depolarisation without that spike.
Suppose glutamate and a co-agonist are bound and the local membrane is sufficiently depolarised. NMDA-mediated calcium entry is possible even if no target spike occurs. This statement concerns channel conditions. It does not predict a particular lasting strength change without more information about the activity, calcium signalling and synapse.
Distinguishing release from response
A strengthened synapse may have changed on the sending side, the receiving side, or both. A larger response alone does not identify the mechanism.
The sending terminal can change transmitter release per spike. The receiving membrane can change its response to the same release. At many glutamate synapses, adding working AMPA receptors can allow more channels to open and carry a larger fast excitatory current. Removing receptors can reduce that response. Receptor trafficking means moving receptors into or out of the membrane.
To investigate the mechanism, compare release and receiving-side responses under matched conditions. The calcium signal that initiates plasticity and the change that expresses the later response are separate parts of the process.
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