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Co-ordination and Response » Synapses and Neurotransmitters

What you'll learn this session

Study time: 30 minutes

Edexcel spec: 2.89

What a synapse is and where you find one
How a neurotransmitter carries a message across the gap
How the message starts a new impulse in the next neurone
Why impulses can only travel one way across a synapse

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The gap between neurones

In the lesson The Central Nervous System and Nerve Impulses you saw that impulses travel along sensory neurones into the CNS and out along motor neurones. But a pathway is made of many neurones joined one after another. Here is the problem: neurones do not actually touch each other.

Between one neurone and the next there is a tiny gap. This gap, and the place where the two neurones meet, is called a synapse. An electrical impulse cannot jump across the gap by itself. So the message has to be passed on in a different way: by a chemical.

Key terms:

Synapse: a tiny gap between two neurones where the impulse is passed from one to the next.
Neurotransmitter: a chemical that is released at a synapse, diffuses across the gap and carries the message to the next neurone.
Receptor molecule: a molecule on the surface of the next neurone that a neurotransmitter can bind to.

Step by step: crossing a synapse

Think of the first neurone as the sender and the second neurone as the receiver. The message crosses the gap in four steps.

⚡ Step 1: impulse arrives

An electrical impulse travels along the first neurone and reaches the end of it, next to the synapse.

💧 Step 2: neurotransmitter released

The arrival of the impulse makes the end of the first neurone release neurotransmitter into the gap.

🔄 Step 3: diffusion and binding

The neurotransmitter diffuses across the gap. It binds to receptor molecules on the surface of the next neurone.

💡 Step 4: new impulse

Binding starts a new electrical impulse in the next neurone, which carries on along it.

So the message changes form. It is electrical along a neurone, chemical across the gap, and electrical again in the next neurone. The gap is very small, so the neurotransmitter crosses it in a tiny fraction of a second. This is one reason nervous responses are so rapid.

Worked example: a message through two neurones

A sensory neurone carries an impulse to the end of its fibre, next to the synapse.
It releases neurotransmitter, which diffuses across the gap.
The neurotransmitter binds to receptor molecules on the next neurone.
A new impulse starts in that neurone and travels on towards its end.

Why impulses only go one way

Synapses act like one-way gates. Impulses can cross a synapse in one direction only, never backwards. There are two parts to the reason:

Neurotransmitter is released from only one side of the gap, the end of the first neurone.
The receptor molecules are on only the other side, the surface of the next neurone.

The chemical can only travel from sender to receiver, so the message can only travel that way. This keeps impulses moving in the correct direction through the nervous system, for example from a sensory neurone towards the CNS and from the CNS out along a motor neurone to an effector.

✅ The sender side

The end of the first neurone releases the neurotransmitter. There are no receptors here for it to bind to, so it cannot start an impulse backwards.

🎯 The receiver side

The surface of the next neurone has the receptor molecules. The neurotransmitter binds here and a new impulse starts.

Worked example: tracing a message through a pathway

A message goes from a sensory neurone to a second neurone in the CNS, and then on to a motor neurone. Each time it moves from one neurone to the next it must cross a synapse. At each synapse the same four steps happen: impulse arrives, neurotransmitter released, neurotransmitter diffuses and binds, new impulse starts. Because every synapse is a one-way gate, the message always moves in the same direction: in along the sensory neurone, then through the CNS, then out along the motor neurone.

Summing it up

A good way to remember it is to describe the message in three parts. Along the neurone, the message is an electrical impulse. Across the synapse, the message is a chemical, the neurotransmitter. In the next neurone, the chemical starts a new electrical impulse. If you can say those three parts in order, you have the main idea of the role of neurotransmitters at synapses.

Common mistakes

Saying the electrical impulse jumps across the synapse. It does not. A chemical, the neurotransmitter, carries the message across the gap.
Saying the neurotransmitter is an electrical signal. It is a chemical.
Saying the neurotransmitter is carried in the blood. It diffuses across the tiny gap.
Forgetting that binding to receptors on the next neurone is what starts the new impulse. Just arriving at the gap is not enough.
Writing that impulses go both ways. A synapse allows one direction only.

Exam-style question

(a) What is a synapse? [1 mark]

(b) Describe how an impulse is passed across a synapse from one neurone to the next. [3 marks]

Model answer

(a) A tiny gap between two neurones. (1)

(b) Neurotransmitter is released from the first neurone when the impulse arrives (1). It diffuses across the gap (1) and binds to receptor molecules on the next neurone, which starts a new impulse (1).

Exam tip

For "describe how" questions about a synapse, write the steps in order and use the word "diffuses" and the word "binds". Say it is a chemical that crosses the gap, never the impulse itself.

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