GCSE · Biology · Edexcel · Spec 1BI0

Sensory, relay and motor neurones and synapses

The nerve cells carrying your body's messages don't join up. There's a tiny gap between each, and an electrical impulse can't just leap it. So how does the message cross?

Crossing a synapse

1Impulse arrives2345

Stage 1 of 5: Impulse arrives. paused

Impulse arrives · 1/5drag the signal across →

ELECTRICAL · A nerve impulse travels along the first neurone and arrives at the end of its axon. Ahead is the synapse, a very small gap.

Drag along the path. Watch the signal arrive as electricity, cross the gap as a chemical, then carry on as electricity again.

Inside one neurone

The parts every neurone shares
Myelin sheathDendronsCell bodyNucleusAxonEnd of axon

View: The parts. Showing 3 layers: Neurone, Myelin sheath, Names

View

Layers

Explore

tap a part, then switch the view

Every neurone has dendrons, a cell body with a nucleus, and an axon. Some also have a myelin sheath.

Now zoom out from the gap to one whole neurone. Tap each part to see its job, then switch to the route view.

Predict, then check

You've just seen the myelin sheath wrapped round the axon. Think about what a coating like that might be for.

Imagine an axon with its myelin sheath stripped away. What would you expect to happen to the impulses travelling along it?

Following one message

From the thing that notices to the thing that acts

Put the message's journey in order, from the moment a change is detected to the part that carries out the response.

1 · First5 · Last

  1. Motor neurone

  2. Receptor

  3. Effector (a muscle or a gland)

  4. Relay neurone

  5. Sensory neurone

Watch out: Sensory neurones carry impulses TOWARDS the central nervous system. Motor neurones carry them AWAY from it. Swap those two and the whole route runs backwards.

The one-way rule

?

Reason it through

Why can a signal only cross a synapse in one direction?

Link 1 of 3

First link · your turn

Start with the chemical. At this synapse, where are the vesicles of neurotransmitter?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Neurones carry messages round your body as electrical impulses. But between one neurone and the next there's a gap, and the message has to change form to get across it.

What you need to know

  • Neurones (nerve cells) are specialised cells that carry electrical nerve impulses between different parts of the body. They are very large and often very long.
  • There are three types. Sensory neurones carry impulses from a receptor towards the central nervous system. Relay neurones pass impulses between neurones and are found only in the central nervous system. Motor neurones carry impulses from the central nervous system to an effector.
  • All neurones have dendrons, a cell body containing the nucleus, and an axon. Some also have a myelin sheath.
  • Between two neurones is a synapse. The signal crosses it as a chemical, and it can only cross in one direction.

The big picture

Neurones are specialised cells that carry electrical impulses around the body. Sensory, relay and motor neurones pass a message from a receptor to an effector, and every neurone has dendrons, a cell body with a nucleus, and an axon; some also have a myelin sheath. Between neurones is a synapse, where the signal crosses as a chemical, neurotransmitter, and it can only cross one way.

Key points

1Route through a neurone: in at the dendrons → past the cell body → along the axon → out at the far end.
2The myelin sheath is a fatty coating that insulates the axon. It speeds up impulses (in some cases about 100 times faster) and stops interference from impulses in adjacent neurones.
3An effector is usually a muscle, but it can also be a gland.
4At a synapse: impulse arrives at the end of the axon → vesicles release neurotransmitter → it diffuses across the gap → it binds to receptors on the second neurone → an onward impulse starts.
5The signal goes electrical → chemical → electrical at every synapse.
6One way only: vesicles of neurotransmitter are only at the end of the first neurone's axon, and the receptors are only on the second neurone.

Worked example

Problem

Your finger detects a change and a muscle in your arm responds. Describe how the message gets from the receptor to the muscle. Name each type of neurone, and say what happens each time the message passes from one neurone to the next.

⚠ Watch out

Saying the electrical impulse "jumps" or "travels across" the synapse. It doesn't: neurotransmitter carries the signal across the gap as a chemical, and a new electrical impulse starts in the next neurone.

🧠

Memory hook

E-C-E: Electrical along the neurone, Chemical across the gap, Electrical again on the far side. And for the one-way rule: the Vesicles send, the Receptors receive, and they never swap sides.

✓

Check yourself

Cover the page. Name the three types of neurone and where each carries impulses from and to. Which part of the signal's journey is chemical, and why can it only go one way?

Flashcards

(14)
What is a neurone?
A nerve cell: a specialised cell that carries electrical nerve impulses between different parts of the body. Neurones are very large and often very long.
Sensory neurone: from where, to where?
From a sensory receptor (which senses a change in the environment) towards the central nervous system.
Relay neurone: what does it do, and where is it found?
It passes impulses between sensory and motor neurones, and between other relay neurones. It is found only in the central nervous system.
Motor neurone: from where, to where?
From the central nervous system to an effector, which is usually a muscle but can be a gland.
What do the dendrons do?
They receive the signal. The impulse enters the neurone here.
Where in a neurone is the nucleus found?
In the cell body.
What is the axon?
A long extension of the neurone that carries the signal from one end of the neurone to the other.
Trace an impulse through one neurone.
In at the dendrons, past the cell body, along the axon, to the other end.
What is the myelin sheath?
A fatty coating wrapped around the axon of some neurones. It insulates the axon.
Two effects of a myelin sheath?
It speeds up transmission of the impulse (in some cases about 100 times faster), and it stops the impulse being interrupted by impulses in adjacent neurones.
What is a synapse?
The junction between two neurones: a very small gap that nerve impulses pass across.
What form is the signal in, before, across and after a synapse?
Electrical, then chemical across the gap, then electrical again in the second neurone.
What is a neurotransmitter?
A chemical transmitter that crosses the synapse and triggers the onward response.
Why is a synapse one-way?
Vesicles of neurotransmitter are only at the end of the first neurone's axon, and the receptors are only on the second neurone.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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