---
title: "The Reflex Arc and Synapses"
description: "A withdrawal reflex runs receptor to sensory neurone to relay neurone to motor neurone to effector, with no decision from the conscious brain in this arc. Covers the chemical synapse gap and why that "
canonical: https://lightmysky.com/learn/science/the-reflex-arc-and-synapses-mt_evlgCVxTnK
source: https://lightmysky.com/learn/science/the-reflex-arc-and-synapses-mt_evlgCVxTnK.md
retrieved: 2026-11-12
---

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# The Reflex Arc and Synapses

A withdrawal reflex runs receptor to sensory neurone to relay neurone to motor neurone to effector, with no decision from the conscious brain in this arc. Covers the chemical synapse gap and why that gap is the slow step.

Subject: Science · Area: The Human Body · Ages 14 to 15
Page: https://lightmysky.com/learn/science/the-reflex-arc-and-synapses-mt_evlgCVxTnK

## Ready when they can

- Puts the parts of a reflex arc in order for a given stimulus and response.
- Explains what crosses a synapse and why that step takes time.
- Says why a reflex protects the body better than a decided response would.

## Lesson: Pull away before you think

Touch something hot and your hand jerks back before you feel the pain. That is a withdrawal reflex: a fast, automatic response that skips your conscious brain in this arc. Skipping thought trades thinking for speed, which saves you from burns.

**Example.** The signal runs a fixed track called the reflex arc. The receptor senses heat, the sensory neurone carries the message in, the relay neurone passes it across the spinal cord, the motor neurone carries the order out, and the muscle effector pulls your hand away.

Between neurones in this withdrawal arc sit tiny gaps called chemical synapses. A chemical messenger crosses each gap, and that crossing adds a small delay. Inside one neurone the signal is electrical, and its travel time depends on distance while its speed depends on axon diameter and myelination, but every chemical gap adds a chemical delay.

**Tip.** Your brain learns about the burn only after your hand is moving. If a question asks why this withdrawal reflex beats a decided response, say it is faster because the loop runs through the spinal cord alone.

**Recap.** Receptor to effector through the cord, with slow chemical synapses, pulls you clear before thought.

## Practice

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

## Needs first

- [Neurons & Brain Structure](https://lightmysky.com/learn/science/neurons-and-brain-structure-mt_okUMpHsV-P)

## Opens up

- [The Resting Potential and the Action Potential](https://lightmysky.com/learn/science/the-resting-potential-and-the-action-potential-mt_0sh4w30ocz)
- [The Brain: Regions and the Evidence for What They Do](https://lightmysky.com/learn/science/the-brain-regions-and-the-evidence-for-what-they-do-mt_BA_DGCDic8)
- [Hormones and Negative Feedback](https://lightmysky.com/learn/science/hormones-and-negative-feedback-mt_OBa6BHmW29)
- [Regional Anatomy of the Central Nervous System](https://lightmysky.com/learn/science/regional-anatomy-of-the-central-nervous-system-mt_S9f5cUS8NZ)