6 min readPathway sheetPart I MCQ and Part II (2 marks); sometimes Part III (4 marks)

Nerve impulse and synapse

A nerve impulse is an electrical message that travels along a neuron. At the synapse, a chemical called a neurotransmitter carries it to the next neuron.

StimulusAny change around or inside the body that it responds to, e.g. heat, light, a pin prick

SynapseJunction between the axon end of one neuron and the dendrite of the next

NeurotransmitterChemical that carries the message across the synapse, e.g. acetylcholine

Direction in a neuronDendrite → cyton → axon → synapse → next neuron

Path of a message

Inside a neuron the message is electrical. Across the synapse it is chemical.

How a message crosses a synapse

1
The impulse reaches the synaptic knob at the end of the axon.
2
The knob releases a neurotransmitter, such as acetylcholine.
3
The neurotransmitter crosses the synaptic cleft, the tiny gap between the two neurons.
4
It joins onto the membrane of the next neuron's dendrite.
5★
A new electrical impulse starts in the next neuron.
6
The neurotransmitter is quickly broken down, so the signal stops.

★ key step

Why the message goes only one way

Only the axon end stores and releases the neurotransmitter. The dendrite of the next neuron only receives it. So at a synapse the message always moves one way, from axon to dendrite. This keeps signals from getting mixed up.

Key words

Stimulus: a change, like heat, sound, light or touch.
Receptor: a cell or sense organ that detects a stimulus, e.g. skin, eye, ear.
Nerve impulse: the electrical message carried along a neuron.
Synapse: the meeting point of two neurons. They come very close but do not touch.
Effector: the muscle or gland that finally acts on the message.

How to draw a synapse

1
On the left, draw the end of an axon swelling into a round knob. Label synaptic knob.
2
Inside the knob, draw small circles. Label vesicles with neurotransmitter.
3
Leave a narrow gap on the right. Label synaptic cleft.
4
On the right, draw the tip of a dendrite. Label dendrite of next neuron.
5
Draw dots crossing the gap and an arrow from left to right. Label direction of impulse.

In real life

When you smell sambar cooking and your mouth waters, the message has crossed several synapses in a fraction of a second. A dentist's local anaesthetic injection stops pain impulses travelling along the nerves, so you feel no pain.

Exam angle

1 marks: “The chemical released at a synapse is called a ______.” Neurotransmitter, e.g. acetylcholine.

2 marks: “What is a synapse?” The junction between the axon terminal of one neuron and the dendrite of the next. The message crosses it as a chemical.

4 marks: “How is a nerve impulse passed from one neuron to another?” Impulse reaches the synaptic knob; neurotransmitter (acetylcholine) is released; it crosses the synaptic cleft; it acts on the next dendrite; a new impulse starts; flow is one way.

Remember it

Electric inside, chemical between: a wire along the neuron, a messenger across the synapse.