Study Guide

Synaptic transmission

CIE A-Level BiologyΒ· 45 min read

1. Structure of a Cholinergic Synapseβ˜…β˜…β˜†β˜†β˜†β± 10 min

πŸ“˜ Definition

Synapse

Preβˆ’synaptic=beforecleft;Postβˆ’synaptic=aftercleftPre-synaptic = before cleft; Post-synaptic = after cleft

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

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.

πŸ“ Worked Example

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

  1. 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. 2

    B is the synaptic cleft, the narrow fluid-filled gap between pre and post-synaptic membranes that neurotransmitter diffuses across.

  3. 3

    C is the post-synaptic membrane. This membrane contains neurotransmitter receptors that initiate the new signal in the receiving cell.

2. Mechanism of Synaptic Transmissionβ˜…β˜…β˜…β˜†β˜†β± 15 min

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.

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

3. Role of Acetylcholinesteraseβ˜…β˜…β˜†β˜†β˜†β± 8 min

πŸ“˜ Definition

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

πŸ“ Worked Example

Explain why nerve gases that inhibit acetylcholinesterase cause muscle paralysis

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

4. Key Functional Properties of Synapsesβ˜…β˜…β˜…β˜†β˜†β± 12 min

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.

πŸ“ Worked Example

Explain why synaptic transmission is always unidirectional

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

5. Common Pitfalls

Wrong move:

Claiming sodium ions enter the pre-synaptic terminal to trigger vesicle fusion

Why:

Students confuse the sodium influx that generates the action potential along the axon with pre-synaptic signalling events

Correct move:

Calcium ions enter the pre-synaptic terminal to trigger fusion of synaptic vesicles with the membrane

Wrong move:

Stating acetylcholine crosses the post-synaptic membrane to cause depolarisation

Why:

Students mix up the role of acetylcholine as a membrane-bound signalling molecule

Correct move:

Acetylcholine only binds to receptors on the post-synaptic membrane surface; it does not enter the cell

Wrong move:

Claiming acetylcholinesterase is located on the pre-synaptic membrane

Why:

Students often misremember the location of the enzyme

Correct move:

Acetylcholinesterase is embedded in the post-synaptic membrane to break down acetylcholine immediately after signalling

Wrong move:

Saying all synapses are excitatory

Why:

Students often forget that inhibitory synapses regulate signal transmission

Correct move:

Synapses can be excitatory (trigger depolarisation) or inhibitory (trigger hyperpolarisation to prevent action potentials)

Wrong move:

Claiming neurotransmitter release does not require ATP

Why:

Students overlook the active process of exocytosis

Correct move:

Release of neurotransmitter via exocytosis is an active process that requires ATP from respiration

6. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 1

    MCQ on synapse structure

  • 2023 Β· 2

    Describe transmission across synapse

  • 2024 Β· 4

    Explain effect of AChE inhibitors

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.