# Synaptic transmission

> CIE A-Level Biology · Unit 16: Control and Coordination
> Source: https://www.owlsprep.com/study/cie-9700-u16-synaptic-transmission/

This sub-topic explains how electrical nerve impulses are transmitted across synapses between neurones. It covers cholinergic synapse structure, the mechanism of signal transmission, and key roles of neurotransmitters and enzymes.

**Prerequisites:** [Structure of mammalian neurones](https://www.owlsprep.com/study/cie-9700-u16-neurone-structure/); [Generation of action potentials](https://www.owlsprep.com/study/cie-9700-u16-action-potentials/)

## Learning objectives

- Describe the structure of a cholinergic synapse
- Outline the step-by-step process of synaptic transmission
- Explain the importance of acetylcholinesterase in synaptic function
- Discuss key functional properties of synapses

## Structure of a Cholinergic Synapse

**Synapse** — A specialised junction that allows transmission of a nerve impulse from one cell to another

*Notation:* Pre-synaptic = before cleft; Post-synaptic = after cleft

*Example:* Neuromuscular junctions between neurones and muscle cells are cholinergic synapses

All synapses share a common core structure. The narrow fluid-filled gap between the pre- and post-synaptic membranes is called the synaptic cleft (~20-30 nm wide). The pre-synaptic axon terminal has voltage-gated calcium channels and many synaptic vesicles filled with neurotransmitter. The post-synaptic membrane has specific receptor proteins for the neurotransmitter, and is often folded to increase surface area.

**Worked example:** Identify the three key labelled structures: A (pre-cleft gap region), B (gap between membranes), C (post-synaptic membrane region)

1. A is the axon terminal of the pre-synaptic neurone. This region stores neurotransmitter in synaptic vesicles and contains voltage-gated calcium channels in its membrane.
2. B is the synaptic cleft, the narrow fluid-filled gap between pre and post-synaptic membranes that neurotransmitter diffuses across.
3. C is the post-synaptic membrane. This membrane contains neurotransmitter receptors that initiate the new signal in the receiving cell.

## Mechanism of Synaptic Transmission

Synaptic transmission converts an electrical signal (action potential) into a chemical signal (neurotransmitter) and back to an electrical signal in the post-synaptic cell. This process follows a consistent sequence of events.

**Worked example:** Outline the full sequence of events from arrival of an action potential at the pre-synaptic membrane to generation of a new action potential in the post-synaptic neurone

1. 1. The action potential depolarises the pre-synaptic membrane, causing voltage-gated calcium ion channels to open.
2. 2. Calcium ions diffuse down their concentration gradient into the pre-synaptic axon terminal.
3. 3. Calcium ions trigger synaptic vesicles filled with acetylcholine to fuse with the pre-synaptic membrane.
4. 4. Acetylcholine is released into the synaptic cleft via exocytosis, an active process requiring ATP.
5. 5. Acetylcholine diffuses across the cleft and binds to specific receptors on the post-synaptic membrane.
6. 6. Binding opens sodium ion channels, allowing sodium influx that depolarises the post-synaptic membrane. If threshold is reached, a new action potential is generated.

> **tip**
>
> Always remember calcium (not sodium) enters the pre-synaptic terminal first — this is one of the most common exam mistakes.

## Role of Acetylcholinesterase

**Acetylcholinesterase** — A membrane-bound enzyme that hydrolyses acetylcholine into choline and ethanoic acid (acetate)

*Example:* This reaction stops signal transmission to allow the synapse to reset

**Worked example:** Explain why nerve gases that inhibit acetylcholinesterase cause muscle paralysis

1. 1. Normally, acetylcholinesterase breaks down acetylcholine immediately after it binds to post-synaptic receptors.
2. 2. If acetylcholinesterase is inhibited, acetylcholine remains bound to receptors permanently.
3. 3. This causes continuous opening of sodium channels and sustained depolarisation of the post-synaptic membrane.
4. 4. Muscle cells remain contracted, leading to fatigue and eventual paralysis as no new signals can be transmitted.

> **info**
>
> CIE often uses toxins or pesticides that inhibit acetylcholinesterase as a context for application questions.

## Key Functional Properties of Synapses

Synapses have important properties that allow them to regulate nerve transmission. The most fundamental of these is unidirectional transmission: signals can only travel from pre-synaptic to post-synaptic, never the reverse. Synapses also allow summation (combining multiple small signals to reach threshold) and integration of information from multiple inputs.

**Worked example:** Explain why synaptic transmission is always unidirectional

1. 1. Unidirectional transmission means the impulse can only travel one way across the synapse.
2. 2. Neurotransmitter is only stored in synaptic vesicles in the pre-synaptic neurone, so can only be released from the pre-synaptic side.
3. 3. Receptor proteins that bind neurotransmitter are only located on the post-synaptic membrane.
4. 4. This means a new electrical signal can only be generated in the post-synaptic cell, so transmission can never be reversed.

## Common pitfalls

- **Wrong:** Claiming sodium ions enter the pre-synaptic terminal to trigger vesicle fusion
  - Why it fails: Students confuse the sodium influx that generates the action potential along the axon with pre-synaptic signalling events
  - Correct: Calcium ions enter the pre-synaptic terminal to trigger fusion of synaptic vesicles with the membrane
- **Wrong:** Stating acetylcholine crosses the post-synaptic membrane to cause depolarisation
  - Why it fails: Students mix up the role of acetylcholine as a membrane-bound signalling molecule
  - Correct: Acetylcholine only binds to receptors on the post-synaptic membrane surface; it does not enter the cell
- **Wrong:** Claiming acetylcholinesterase is located on the pre-synaptic membrane
  - Why it fails: Students often misremember the location of the enzyme
  - Correct: Acetylcholinesterase is embedded in the post-synaptic membrane to break down acetylcholine immediately after signalling
- **Wrong:** Saying all synapses are excitatory
  - Why it fails: Students often forget that inhibitory synapses regulate signal transmission
  - Correct: Synapses can be excitatory (trigger depolarisation) or inhibitory (trigger hyperpolarisation to prevent action potentials)
- **Wrong:** Claiming neurotransmitter release does not require ATP
  - Why it fails: Students overlook the active process of exocytosis
  - Correct: Release of neurotransmitter via exocytosis is an active process that requires ATP from respiration

## Cheatsheet

| Structure | Location | Core Function |
| --- | --- | --- |
| Voltage-gated Ca²⁺ channels | Pre-synaptic membrane | Open on depolarisation, allow Ca²⁺ entry |
| Synaptic vesicles | Pre-synaptic axon terminal | Store acetylcholine neurotransmitter |
| Synaptic cleft | Between membranes | Gap for neurotransmitter diffusion |
| Acetylcholine receptors | Post-synaptic membrane | Bind ACh, open Na⁺ channels |
| Acetylcholinesterase | Post-synaptic membrane | Hydrolyse ACh to reset synapse |

## What's next

Understanding synaptic transmission is the foundation for studying all nervous system function in CIE A-Level Biology. It connects directly to muscle contraction, reflex actions, and brain function, and is a common topic for extended response questions in Paper 4. Synaptic function is also often the context for questions on drugs, toxins, and cell signalling, which test your ability to apply knowledge to new situations. Mastering this topic will prepare you for more complex topics in control and coordination and help you score highly on synapse-related exam questions.

- [Hormonal communication](https://www.owlsprep.com/study/cie-9700-u16-hormonal-communication/)

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