---
title: "Synaptic Plasticity and the Molecular Rules of Change"
description: "A synapse strengthens when its input arrives just before the target cell fires, and weakens when the timing is reversed. Calcium entry through receptors that need both transmitter and depolarisation i"
canonical: https://lightmysky.com/learn/science/synaptic-plasticity-and-the-molecular-rules-of-change-mt_HldevO4Lzw
source: https://lightmysky.com/learn/science/synaptic-plasticity-and-the-molecular-rules-of-change-mt_HldevO4Lzw.md
retrieved: 2026-09-12
---

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# Synaptic Plasticity and the Molecular Rules of Change

A synapse strengthens when its input arrives just before the target cell fires, and weakens when the timing is reversed. Calcium entry through receptors that need both transmitter and depolarisation is what detects the coincidence.

Subject: Science · Area: The Human Body · Ages 19 to 21
Page: https://lightmysky.com/learn/science/synaptic-plasticity-and-the-molecular-rules-of-change-mt_HldevO4Lzw

## Ready when they can

- Explains how one receptor type acts as a detector of two conditions at once.
- Predicts strengthening or weakening from the order of firing.
- Links a lasting change in strength to a change in receptor number or transmitter release.

## Lesson: Inputs that fire together wire together

Imagine you are watching one link between two cells. When the input fires just before the target, the link strengthens and holds that gain. When the target fires first and the input lands after, the link weakens instead. Order is the record of cause and effect.

Now meet the detector that reads that order. One receptor type demands two things at once: chemical bound from the sender plus a fired up membrane in the receiver. Only when both hold does its block lift and calcium flood in. That calcium pulse says this input helped cause the firing.

**Example.** Suppose the same puff of chemical must hit harder tomorrow. The link inserts extra receiving spots into the membrane, so the current grows, and it can also release more chemical per spike. Weakened links do the reverse and remove spots or cut release. These edits persist long after the calcium fades.

**Tip.** Here is how you use all this. Only a real all or nothing spike opens the detection window, so quiet inputs that miss it get weakened. Long trains of matched firing drive lasting strengthening, while reversed or lonely firing drives weakening. Causes get strengthened, the rest get trimmed.

**Recap.** You check the order of firing to predict whether a link grows or shrinks.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Transmitter Systems and Two Kinds of Receptor](https://lightmysky.com/learn/science/transmitter-systems-and-two-kinds-of-receptor-mt_yXaSbrBxua)

## Opens up

- [Memory Systems: What Amnesia Separates](https://lightmysky.com/learn/science/memory-systems-what-amnesia-separates-mt_oNFXpEHl8b)