Synapses are specialised junctions that enable communication between neurones or between a neurone and an effector cell, ensuring rapid, coordinated responses essential for nervous system function.

Structure of a Synapse

A synapse is the point where one neurone communicates with another or with an effector. The small gap between the two cells is called the synaptic cleft, typically around 20–40 nm wide.

Main Components

• Presynaptic neurone – transmits the impulse towards the synapse; its end forms the presynaptic knob, rich in mitochondria and synaptic vesicles containing neurotransmitters.

• Synaptic cleft – the extracellular gap separating the two membranes, across which neurotransmitters diffuse.

• Postsynaptic neurone (or cell) – has a postsynaptic membrane containing specific receptor proteins complementary to the neurotransmitter.

Adaptations of the Presynaptic Knob

• Contains numerous mitochondria to provide ATP for neurotransmitter synthesis and vesicle recycling.

• Has voltage-gated calcium ion channels that open in response to an arriving action potential.

• Possesses synaptic vesicles that store neurotransmitters such as acetylcholine in cholinergic synapses.

• Contains enzymes and transport proteins to synthesise and recycle neurotransmitters efficiently.

Transmission Across a Cholinergic Synapse

A cholinergic synapse uses the neurotransmitter acetylcholine (ACh). It is the most common type in the mammalian nervous system, especially at neuromuscular junctions.

Labeled diagram of a cholinergic synapse showing Ca²⁺-triggered vesicle fusion, ACh release into the synaptic cleft, binding to postsynaptic ligand-gated ion channels, and AChE hydrolysis with choline re-uptake. Demonstrates one-way transmission and neurotransmitter clearance. Extra detail includes presynaptic M₂ autoreceptors beyond OCR’s required depth. Source.

Sequence of Events

1. Arrival of an action potential

   • The impulse reaches the presynaptic knob, causing voltage-gated calcium channels to open.

2. Calcium ion influx

   • Ca²⁺ ions diffuse into the presynaptic knob down their electrochemical gradient.

3. Vesicle fusion and neurotransmitter release

   • Calcium ions stimulate synaptic vesicles to fuse with the presynaptic membrane.

   • Acetylcholine is released into the synaptic cleft by exocytosis.

4. Diffusion and receptor binding

   • ACh diffuses across the cleft and binds to specific receptor sites on the postsynaptic membrane, usually associated with ligand-gated sodium ion channels.

5. Postsynaptic depolarisation

   • Binding causes these sodium channels to open, allowing Na⁺ ions to enter, generating an excitatory postsynaptic potential (EPSP).

   • If the EPSP reaches the threshold potential, a new action potential is generated in the postsynaptic neurone.

6. Neurotransmitter breakdown

   • The enzyme acetylcholinesterase (AChE), located in the synaptic cleft, rapidly hydrolyses ACh into choline and ethanoic acid (acetate).

   • This prevents continuous stimulation of the postsynaptic membrane.

7. Recycling of neurotransmitter components

   • Choline and acetate are reabsorbed into the presynaptic neurone, recombined to form ACh using ATP, and stored in vesicles for reuse.

This highly controlled process ensures one-way transmission, as only the presynaptic neurone has vesicles and the postsynaptic membrane possesses receptor sites.

Summation

When individual EPSPs are subthreshold, they can combine to trigger an action potential. This process is known as summation.

Graphical illustration of postsynaptic potential summation showing how multiple EPSPs can depolarise the membrane to threshold and how IPSPs counteract excitation. Highlights spatial and temporal summation mechanisms relevant to OCR-level understanding. Source.

Types of Summation

• Temporal summation – Several impulses arrive in quick succession from the same presynaptic neurone, allowing their combined EPSPs to reach threshold.

• Spatial summation – Simultaneous impulses from multiple presynaptic neurones combine their EPSPs on a single postsynaptic neurone.

These mechanisms enhance the flexibility and sensitivity of neural circuits, allowing integration of signals from various sources.

Inhibitory Synapses

Not all synapses are excitatory. Inhibitory synapses reduce the likelihood of an action potential being generated.

Illustration of IPSP generation showing opening of chloride channels that allow Cl⁻ influx, hyperpolarising the membrane and reducing the likelihood of an action potential. Depicts ligand-gated Cl⁻ channel action; extra page details on reversal potentials exceed OCR scope. Source.

Mechanism of Inhibition

• The neurotransmitter (for example GABA, gamma-aminobutyric acid) binds to receptors on the postsynaptic membrane.

• This opens chloride (Cl⁻) or potassium (K⁺) channels, causing either influx of Cl⁻ ions or efflux of K⁺ ions.

• The membrane becomes hyperpolarised (more negative inside), making it less likely to reach threshold potential.

The balance of EPSPs and IPSPs determines whether the postsynaptic neurone fires, allowing fine-tuned control of neural activity.

Comparison: Excitatory vs Inhibitory Synapses

• Excitatory synapses: Depolarise the postsynaptic membrane; neurotransmitters open sodium ion channels; may trigger action potentials.

• Inhibitory synapses: Hyperpolarise the postsynaptic membrane; neurotransmitters open chloride or potassium ion channels; prevent depolarisation.

• The integration of both types provides neural pathway modulation, enabling complex control of behaviour and movement.

Importance of Synapses in the Nervous System

Synapses are vital for information processing and coordination within the nervous system. They perform several essential functions:

• Ensure unidirectional transmission of impulses.

• Allow divergence, where one neurone influences several others, amplifying responses.

• Permit convergence, where multiple neurones combine inputs for a coordinated response.

• Enable filtering of weak stimuli through summation thresholds.

• Provide sites for synaptic plasticity, crucial for learning and memory.

The chemical and electrical events at synapses underpin every aspect of nervous system activity, from reflexes to conscious thought.