Course 5.1Lesson 1 of 3
Animals respond to changes around them through the nervous system. Receptors pick up a change, nerves carry the message as an electrical impulse, and effectors, such as muscles and glands, respond. A neuron has dendrites, a cell body, an axon and nerve endings. A chemical carries the message across the gap, the synapse, to the next neuron.
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A ball flies towards your face, and you duck. A smell of food reaches you, and your mouth waters. In each case something changed around you, and your body did something about it.
A change that the body responds to is called a stimulus (plural stimuli). Light, sound, heat, touch, smell and taste are all stimuli. Responding to stimuli needs control and coordination: many parts of the body must work together, in the right order, at the right time.
In animals, two systems do this work.
The nervous system has three jobs.
| Receptor | Where | Stimulus |
|---|---|---|
| Photoreceptors | eye | light |
| Phonoreceptors | ear | sound |
| Olfactory receptors | nose | smell |
| Gustatory receptors | tongue | taste |
| Thermoreceptors | skin | heat and cold |
The nervous system is built of nerve cells, called neurons. A neuron is made for one job: carrying messages over long distances, fast.
A nerve is a bundle of many axons, wrapped together like the wires in a cable.
Inside a neuron, the message travels as an electrical impulse. It starts at the dendrite, passes through the cell body, and runs along the axon to the nerve endings.
Between two neurons there is a tiny gap, the synapse. The electrical impulse cannot jump across it. Instead, the nerve endings release a chemical. The chemical crosses the gap and starts a new electrical impulse in the dendrite of the next neuron.
The chemical is released only at the nerve endings. So a message can cross a synapse in one direction only, from the axon of one neuron to the dendrite of the next.
Impulses are fast. In the fastest nerves of the body they travel at up to about m/s, faster than a racing car on a highway. Even so, a long nerve takes a few milliseconds:
You should find that m of nerve at m/s takes ms. A nerve twice as long takes twice as long.
The program works out the time for an impulse along three nerves of the body.
You should see about ms from the fingertip, ms from the big toe and ms from the eye. Halving the speed doubles every time.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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