Structure of Synapses (AQA A-Level Biology): Revision Notes

Structure of Synapses

What are synapses?

A synapse represents the communication junction between one neurone and another neurone, or between a neurone and an effector organ. These structures play a vital role in linking different neurones and coordinating nervous system activities throughout the body.

Unlike electrical impulses that travel along neurones, synapses transmit information through chemical messengers called neurotransmitters. This chemical transmission system allows precise control over neural communication.

Components of synaptic structure

The synaptic gap

Neurones do not physically connect at synapses. Instead, they are separated by a narrow space called the synaptic cleft, which measures approximately 20-30 nanometres in width.

Presynaptic neurone

The neurone that sends the signal is termed the presynaptic neurone. Its axon terminates in a swollen region called the synaptic knob, which contains several important structures:

• Mitochondria - provide energy for neurotransmitter synthesis and release
• Smooth endoplasmic reticulum - manufactures neurotransmitters
• Synaptic vesicles - store neurotransmitters ready for release
• Calcium ion protein channels - control neurotransmitter release when opened

Postsynaptic neurone

The receiving neurone is called the postsynaptic neurone. Its membrane contains:

• Receptor proteins - bind specifically to neurotransmitters
• Sodium ion protein channels - open when neurotransmitters bind to receptors
• These channels consist of five protein subunits that form the complete channel structure

Key features of synaptic function

Unidirectionality

Synapses operate as one-way valves for information flow. Signals can only pass from the presynaptic neurone to the postsynaptic neurone, never in reverse.

This occurs because:

• Only presynaptic terminals contain neurotransmitter-filled vesicles
• Only postsynaptic membranes possess the appropriate receptor proteins
• This directional control ensures organised neural pathways

Summation processes

Individual synaptic transmissions often release insufficient neurotransmitter to trigger new action potentials in postsynaptic neurones. Summation solves this problem by building up neurotransmitter concentrations through two distinct mechanisms:

Spatial summation occurs when multiple different presynaptic neurones simultaneously release neurotransmitter at the same postsynaptic neurone. The combined chemical release from several sources creates sufficient concentration to exceed the postsynaptic threshold value, generating a new action potential.

Temporal summation involves a single presynaptic neurone releasing neurotransmitter repeatedly over a short time period. Rapid, successive releases allow neurotransmitter concentration to accumulate before breakdown occurs, again reaching threshold levels in the postsynaptic neurone.

Both summation types ensure reliable signal transmission even when individual synaptic events produce sub-threshold responses.

Inhibitory synapses

Not all synapses promote action potential generation. Inhibitory synapses reduce the likelihood of postsynaptic action potentials through a different neurotransmitter mechanism:

• Presynaptic neurones release inhibitory neurotransmitters
• These bind to chloride ion protein channels on postsynaptic membranes
• Chloride ions (Cl⁻) enter the postsynaptic neurone through facilitated diffusion
• This binding also opens nearby potassium ion (K⁺) protein channels
• Potassium ions move out of the postsynaptic neurone into the synapse

The combined effect creates hyperpolarisation - the membrane potential becomes more negative (approximately $-80\text{mV}$ compared to the usual $-65\text{mV}$ resting potential). This increased negativity makes action potential generation much more difficult, as larger sodium ion influxes are needed to reach threshold.