Function of Synapses (AQA A-Level Biology): Revision Notes
Function of Synapses
Synapses are specialised junctions that allow communication between neurons. They serve as critical connection points where information passes from one neurone to another, enabling the nervous system to coordinate complex responses throughout the body.
Primary functions of synapses
Synapses perform two main functions that make nervous coordination possible:
Understanding these two primary functions is essential for comprehending how the nervous system processes and responds to information effectively.
Signal amplification
A single nerve impulse travelling along one neurone can trigger new impulses in multiple different neurons at a synapse. This amplification allows one stimulus to generate several simultaneous responses across the nervous system.
Real-world Example: Hot Surface Response
When you touch something hot:
- One sensory signal is generated at the point of contact
- This single signal simultaneously triggers:
- Withdrawal reflexes - your hand pulls away automatically
- Pain responses - you feel the sensation of heat/pain
- Memory formation - you remember to avoid that surface
This demonstrates how one input creates multiple coordinated outputs.
Signal integration
Multiple nerve impulses from different receptor neurons can be combined at a single synapse. This summation allows various stimuli to contribute collectively to a single coordinated response. This integration is essential for processing complex information and making appropriate responses based on multiple inputs.
Mechanism of synaptic transmission
Synapses transmit information through a chemical process involving neurotransmitters:
Neurotransmitter production and storage
The neurotransmitter chemical is produced exclusively in the presynaptic neurone - the neurone sending the signal. These chemicals are stored in synaptic vesicles within the presynaptic terminal, ready for release when needed.
Critical Concept: Directional Communication
Neurotransmitters are produced ONLY in presynaptic neurons, while receptor proteins exist ONLY on postsynaptic neurons. This creates unidirectional signal flow and prevents signal confusion.
Signal transmission process
When an action potential reaches the synaptic terminal, it triggers the synaptic vesicles to fuse with the presynaptic membrane. This releases neurotransmitter molecules into the synaptic cleft - the small gap between neurons.
Signal reception and continuation
The neurotransmitter diffuses across the synaptic cleft and binds to specific receptor proteins located exclusively on the postsynaptic neurone - the neurone receiving the signal. This binding event triggers a new action potential in the postsynaptic neurone, continuing the signal transmission.
Types of synapses
Excitatory synapses are synapses that generate new action potentials in the postsynaptic neurone when neurotransmitter binding occurs. These synapses ensure that signals continue to propagate through neural pathways, allowing information to reach its intended destination.
There are also inhibitory synapses (not covered here) that prevent action potentials from forming, providing the nervous system with both "go" and "stop" signals for precise control.
Links to more detailed synaptic transmission mechanisms covered in transmission across synapses.
Key Points to Remember:
- Synapses enable both signal amplification (one input creating multiple outputs) and signal integration (multiple inputs creating one output)
- Neurotransmitters are produced only in presynaptic neurons and receptor proteins exist only on postsynaptic neurons
- Synaptic transmission requires chemical messengers - neurotransmitters - rather than direct electrical connection
- Action potentials trigger neurotransmitter release from synaptic vesicles through membrane fusion
- Excitatory synapses generate new action potentials in the receiving neurone to continue signal transmission