Introduction to Synapses
In the previous chapters of Topic 8: Grey Matter, we looked at how an electrical impulse (an action potential) travels down a single neurone. But our nervous system isn't just one long wire! It is made of billions of individual neurones that need to "talk" to each other.
A synapse is the junction where two neurones meet. Because the electrical impulse cannot "jump" across the physical gap between cells, the body uses chemical messengers called neurotransmitters to pass the message on. In this chapter, we will focus on the structure of these junctions and how the most common neurotransmitter, acetylcholine, does its job.
The Structure of a Synapse
To understand how a synapse works, you need to be able to identify its three main parts:
1. The Presynaptic Neurone: This is the "sending" neurone. It ends in a swelling called the synaptic knob, which contains mitochondria (for energy) and synaptic vesicles filled with neurotransmitters.
2. The Synaptic Cleft: This is the tiny physical gap between the two neurones. It is usually about \(20-30\) nanometres wide.
3. The Postsynaptic Neurone: This is the "receiving" neurone. Its membrane contains specific receptor proteins that are complementary in shape to the neurotransmitter molecules.
Did you know? Synapses ensure that nerve impulses only travel in one direction. This is because neurotransmitter receptors are only found on the postsynaptic membrane, and vesicles are only found in the presynaptic knob!
Step-by-Step: How Synaptic Transmission Works
Don't worry if this seems like a lot of steps at first. Think of it as a chemical relay race. Here is the sequence of events using acetylcholine as our example:
1. The Arrival of the Impulse
An action potential arrives at the end of the presynaptic neurone. This causes the depolarisation of the presynaptic membrane.
2. Calcium Ions Enter
The depolarisation causes voltage-gated calcium ion channels to open. Calcium ions (\(Ca^{2+}\)) rush into the synaptic knob from the surrounding tissue fluid by facilitated diffusion.
3. Vesicle Fusion (Exocytosis)
The sudden rise in \(Ca^{2+}\) concentration causes the synaptic vesicles to move towards the presynaptic membrane. They fuse with the membrane and release the neurotransmitter (acetylcholine) into the synaptic cleft.
4. Diffusion Across the Cleft
Acetylcholine molecules diffuse across the narrow synaptic cleft. This is a short distance, so it happens very quickly.
5. Binding to Receptors
The acetylcholine molecules bind to specific receptor proteins on the postsynaptic membrane. This causes sodium ion channels in the postsynaptic membrane to open.
6. Postsynaptic Depolarisation
Sodium ions (\(Na^{+}\)) rush into the postsynaptic neurone. If enough sodium enters to reach the threshold potential, a new action potential is triggered in the next neurone.
7. Recycling the Neurotransmitter
We don't want the synapse to keep firing forever! An enzyme called acetylcholinesterase breaks down the acetylcholine in the cleft. The products (choline and ethanoic acid) are reabsorbed into the presynaptic neurone to be recycled into more neurotransmitter using energy from ATP.
Quick Summary Table:
Ion: \(Ca^{2+}\) | Location: Presynaptic | Role: Causes vesicles to fuse.
Ion: \(Na^{+}\) | Location: Postsynaptic | Role: Causes depolarisation/new impulse.
Key Concepts to Remember
Unidirectionality
As mentioned before, synapses act like one-way valves. Because vesicles are only in the presynaptic knob and receptors are only on the postsynaptic membrane, the signal can never go backwards.
Summation
Sometimes, a single impulse arriving at the synapse isn't enough to trigger a new action potential in the next neurone. Summation is where multiple impulses "add up" to reach the threshold. This allows the nervous system to process information and "decide" whether a stimulus is important enough to pass on.
Comparison: Nervous vs. Hormonal Coordination
While synapses allow for fast, localized communication, the body also uses hormones (Topic 8.7). Synaptic transmission is much faster than hormonal transport but the effects are usually shorter-lived.
Common Mistakes to Avoid
- The "Electric Spark" Myth: Students often think electricity jumps the gap. It doesn't! The signal is electrical in the neurone but chemical in the synapse.
- Confusing Ions: Remember that Calcium is for the "sending" side (vesicles) and Sodium is for the "receiving" side (new impulse).
- Direction of Diffusion: Neurotransmitters always diffuse from the presynaptic to the postsynaptic membrane.
Key Takeaways
- A synapse is the junction between two neurones.
- Neurotransmitters (like acetylcholine) carry the signal across the gap.
- \(Ca^{2+}\) entry into the presynaptic knob triggers neurotransmitter release.
- Binding of neurotransmitters to postsynaptic receptors opens \(Na^{+}\) channels.
- Enzymes like acetylcholinesterase stop the signal by breaking down the neurotransmitter.
Note: For more information on how specific drugs affect these synapses (like MDMA or L-Dopa), or how brain chemicals like dopamine and serotonin work, see the chapter "Brain chemicals, drugs and learning".