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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Neurotransmitters at Synapses
Your nervous system is like a complex messaging network that helps your body respond to changes. Nerve cells (neurons) need to talk to each other and to other cells like muscles. But here's the thing - neurons don't actually touch each other! There's a tiny gap between them called a synapse and special chemicals called neurotransmitters help messages jump across this gap. Let's explore how this amazing process works!
Key Definitions:
A synapse has three main parts:
The pre-synaptic ending contains many small sacs called synaptic vesicles that store neurotransmitters.
Synapses are crucial because they:
When a nerve impulse (action potential) reaches the end of a pre-synaptic neuron, an amazing sequence of events happens in just a few milliseconds:
When an action potential arrives at the pre-synaptic terminal, it causes voltage-gated calcium channels to open. Calcium ions (Ca²⁺) rush into the neuron.
The calcium ions cause synaptic vesicles to move to the pre-synaptic membrane and fuse with it, releasing neurotransmitter molecules into the synaptic cleft.
Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the post-synaptic membrane, triggering a response.
Your body uses many different neurotransmitters, each with specific roles. Here are some important ones:
These make it more likely for the post-synaptic neuron to fire an action potential:
These make it less likely for the post-synaptic neuron to fire an action potential:
A special type of synapse is the neuromuscular junction (NMJ), where motor neurons connect to skeletal muscle fibres. This is where your brain's commands get turned into movement!
At the NMJ, the process follows the same basic steps as other synapses, but with some special features:
Myasthenia gravis is an autoimmune disease that affects the neuromuscular junction. The immune system mistakenly attacks and destroys ACh receptors at the NMJ. This means that even though the neuron releases normal amounts of ACh, there aren't enough receptors to receive the message. The result is muscle weakness that gets worse with repeated use. This condition shows how crucial properly functioning synapses are for normal body movement.
For your nervous system to work properly, synaptic transmission must be quickly switched off after the message is delivered. This happens in three main ways:
Special transport proteins in the pre-synaptic membrane pump neurotransmitter molecules back into the neuron for recycling.
Enzymes in the synaptic cleft break down neurotransmitters. For example, acetylcholinesterase breaks down ACh at the neuromuscular junction.
Neurotransmitters simply diffuse away from the synaptic cleft, reducing their concentration below the threshold needed for action.
Many substances can affect how synapses work, which explains their effects on the nervous system:
Many snake venoms contain neurotoxins that target synapses. For example, cobra venom contains α-bungarotoxin, which blocks ACh receptors at neuromuscular junctions. This causes paralysis that can affect breathing muscles, potentially leading to death. Understanding how these toxins work has helped scientists develop better antivenom treatments and even new medications.
Your ability to learn and form memories depends on changes at synapses. When you learn something new, certain synapses in your brain become stronger through a process called synaptic plasticity.
Two important processes in learning are:
The saying "neurons that fire together, wire together" describes how your brain forms stronger connections between neurons that are regularly activated at the same time.
Synapses are much more than simple connections - they're sophisticated processing units that:
Without the humble synapse and its neurotransmitters, your nervous system couldn't function and you wouldn't be able to think, feel, or move!
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