Synapses and Neurotransmitters

This lesson covers the structure and function of synapses as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how electrical impulses are transmitted across a synapse using chemicals called neurotransmitters.

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What Is a Synapse?

A synapse is the junction (gap) between two neurones, or between a neurone and an effector (such as a muscle). Neurones do not physically touch each other — there is a tiny gap between them called the synaptic cleft.

Because electrical impulses cannot cross a gap, the signal must be transmitted across the synapse by a different method — using chemicals called neurotransmitters.

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Structure of a Synapse

The synapse has several key components:

Component	Description
Pre-synaptic neurone	The neurone sending the impulse (before the synapse)
Synaptic cleft	The tiny gap between the two neurones (approximately 20 nm wide)
Post-synaptic neurone	The neurone receiving the impulse (after the synapse)
Synaptic vesicles	Small membrane-bound sacs in the pre-synaptic neurone that contain neurotransmitter molecules
Receptor proteins	Proteins on the membrane of the post-synaptic neurone that are complementary in shape to the neurotransmitter
Mitochondria	Abundant in the pre-synaptic neurone to provide energy (via aerobic respiration) for the release of neurotransmitter

graph LR
    A["Pre-synaptic<br/>neurone"] -->|Electrical<br/>impulse arrives| B["Synaptic vesicles<br/>release<br/>neurotransmitter"]
    B -->|Neurotransmitter<br/>diffuses across<br/>synaptic cleft| C["Receptor proteins<br/>on post-synaptic<br/>membrane"]
    C -->|New electrical<br/>impulse generated| D["Post-synaptic<br/>neurone"]

Exam Tip: Notice that transmission across a synapse converts an electrical signal into a chemical signal, then back into an electrical signal. This is called electrochemical transmission.

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How Does Transmission Across a Synapse Work?

The process of synaptic transmission follows a specific sequence of events:

Step-by-Step Process

1. An electrical impulse arrives at the end of the pre-synaptic neurone.
2. This triggers the synaptic vesicles to move towards and fuse with the pre-synaptic membrane.
3. The vesicles release neurotransmitter molecules into the synaptic cleft by exocytosis.
4. The neurotransmitter molecules diffuse across the synaptic cleft (this takes a tiny fraction of a second).
5. The neurotransmitter molecules bind to receptor proteins on the post-synaptic membrane. The receptors are complementary in shape to the neurotransmitter (like a lock and key).
6. This binding triggers a new electrical impulse in the post-synaptic neurone.
7. The neurotransmitter is then removed from the synaptic cleft — it is either broken down by enzymes, reabsorbed by the pre-synaptic neurone, or diffuses away.

Step	Event	Type of Signal
1	Impulse arrives at pre-synaptic neurone	Electrical
2–3	Vesicles release neurotransmitter	Chemical release
4	Neurotransmitter diffuses across cleft	Chemical (diffusion)
5	Neurotransmitter binds to receptors	Chemical (binding)
6	New impulse generated in post-synaptic neurone	Electrical
7	Neurotransmitter removed	Reset

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Types of Neurotransmitter

There are many different neurotransmitters in the human body. The ones you may encounter include:

Neurotransmitter	Function
Acetylcholine	Found at neuromuscular junctions; stimulates muscle contraction
Serotonin	Involved in mood regulation; low levels linked to depression
Dopamine	Involved in reward, pleasure and motor control
Noradrenaline	Involved in the "fight or flight" response; increases heart rate and alertness

Exam Tip: For the Combined Science specification, you mainly need to understand the general mechanism of synaptic transmission. You do not need to memorise specific neurotransmitter names, but knowing one or two examples (like acetylcholine) can help you write better answers.

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Why Are Synapses Important?

Synapses serve several critical functions in the nervous system:

1. Unidirectional transmission — impulses can only travel in one direction across a synapse (from pre-synaptic to post-synaptic neurone) because neurotransmitter is only released from the pre-synaptic side and receptors are only on the post-synaptic side. This ensures signals travel along the correct pathway.

2. Integration of signals — a single post-synaptic neurone may receive signals from many pre-synaptic neurones. The signals can be added together (summation) to determine whether a new impulse is generated.

3. Amplification — a small signal can trigger the release of a large amount of neurotransmitter, amplifying the response.

4. Protection — synapses can become fatigued (run out of neurotransmitter) if overstimulated, which can protect the nervous system from excessive stimulation.

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Effect of Drugs on Synapses

Some drugs affect how synapses function:

Drug Effect	How It Works	Example
Stimulant	Increases the amount of neurotransmitter in the synapse (e.g. blocks reabsorption or breakdown)	Caffeine, amphetamines
Inhibitor	Decreases neurotransmitter activity (e.g. blocks receptor proteins or reduces release)	Alcohol, benzodiazepines
Mimics	The drug molecule has a similar shape to the neurotransmitter and binds to the receptor	Nicotine (mimics acetylcholine)

Exam Tip: Questions about drugs and synapses often ask you to explain how a drug affects the transmission of impulses. Always refer to the specific step in synaptic transmission that is affected (e.g. "the drug blocks the receptor proteins, preventing the neurotransmitter from binding").

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Speed of Synaptic Transmission

• Synaptic transmission is slower than impulse transmission along a neurone because the neurotransmitter must diffuse across the cleft.
• This introduces a small delay (about 0.5–1 millisecond per synapse).
• The more synapses in a pathway, the slower the overall response time.
• This is one reason why reflex arcs have very few synapses — to ensure the fastest possible response.

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Summary

• A synapse is a junction between two neurones with a tiny gap called the synaptic cleft.
• Impulses cross the synapse using neurotransmitter chemicals (electrochemical transmission).
• The sequence is: impulse arrives → vesicles release neurotransmitter → diffusion across cleft → binding to receptors → new impulse generated.
• Synapses ensure one-way transmission, allow signal integration and can be affected by drugs.
• Synaptic transmission introduces a small delay because neurotransmitter must diffuse across the cleft.
• Neurotransmitter is removed after transmission by enzymes, reabsorption or diffusion.

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Why a Synapse Exists — Adaptation to Function