Neurons, synapses and how nerves cause action
A neuron carries messages as electrical impulses from dendrite to axon. At the synapse, chemicals carry the message across to the next cell, and muscles respond by contracting.
Parts of a neuron
| Part | What it does |
|---|---|
| Dendrites | Short branches that receive information; a chemical reaction here starts an electrical impulse |
| Cell body | Contains the nucleus; the impulse passes through it |
| Axon | Long fibre that carries the impulse away from the cell body |
| Nerve ending | End of the axon; releases chemicals into the synapse |
How a message travels
- 1Receptors in a sense organ (tongue, nose, skin, eye, ear) detect a change, called a stimulus.
- 2At the tip of a dendrite, a chemical reaction creates an electrical impulse.
- 3The impulse travels from the dendrite to the cell body, and then along the axon to its end.
- 4At the end of the axon, the impulse makes the nerve ending release chemicals.
- 5★The chemicals cross the gap, the synapse, and start a new electrical impulse in a dendrite of the next neuron.
- 6A similar junction passes impulses from neurons to muscle cells or glands.
★ key step
How nervous tissue causes action
Muscles move by changing shape. Muscle cells contain special proteins that change both their shape and their arrangement when a nerve impulse arrives. The new arrangement makes the muscle cells shorter, so the muscle contracts. This is how a message from a nerve becomes an action, such as lifting your hand or kicking a ball.
How to draw a neuron
- Draw a rounded, star-shaped cell body and a round nucleus inside it.
- Draw short, branching dendrites spreading out from the cell body.
- From one side, draw one long fibre: the axon.
- At the far end of the axon, draw small branches: the nerve endings.
- Draw an arrow along the axon showing the direction of the impulse, from dendrite to nerve ending.
- To show a synapse, draw the nerve ending of one neuron close to the dendrite of the next, with a small gap between them.
When you smell jalebis frying at a sweet shop, olfactory receptors in your nose send impulses along neurons to your brain. Your mouth may water even before you taste one.
2 marks: “What is a synapse? How does a nerve impulse cross it?” The gap between the nerve ending of one neuron and the dendrite of the next. The impulse makes the nerve ending release chemicals, which cross the gap and start a new impulse.
3 marks: “Draw the structure of a neuron. Label the dendrite, cell body, axon and nerve ending, and show the direction of the impulse.” Neat diagram with all four labels and the nucleus; arrow from dendrite to nerve ending; one line on what each part does.
1 marks: “Name the receptors that detect taste.” Gustatory receptors.
Path of an impulse: Do Cats Always Nap? Dendrite, Cell body, Axon, Nerve ending.