Activity 1.7 - Receptors, nerves and nerve impulses

The pupils are to learn to describe how the nervous system works, and how it reacts to both outer and inner stimuli. They are also to learn that certain disabilities are caused by neurological disorders. The pupils are to understand the difference between the central and the peripheral nervous systems. The pupils should also be able to plan an investigation where both sensory neurons and motor neurons are studied. The pupils should also learn to appreciate that different materials feel differently, even if they have the same temperature, because of the varying heat transferring capacity. The investigations are to include the formulation and testing of hypotheses, the collection of relevant data and the evaluation of its validity, and the drawing of conclusions from the investigation.

Biology, Physics

Time-piece (watch, mobile telephone), ice water, warm water, paper clips, ice cubes.

This Activity consists of four exercises. The two first exercises concern receptors and the sense of touch, but also the capacity of heat transfer in various materials. The picture of the conversation in the sauna can be used as an introduction to “How does the sense of touch work?”, about receptors in exercise 1. Judging temperature using touch is difficult. Exercise 2 describes an attempt to show that judging temperature is relative. Exercise 2b is included as an example of how one can integrate a task in physics to illustrate further why the feel of temperature is relative. There are advantages in doing this, as some neurological disorders can be compared to the capacity of different materials to transfer heat, ie that certain disorders cause nerve impulses to be transported at a slower rate (and with decreased accuracy). The two last exercises concern nerve impulses and how these and the central nervous system can cause phantom pain, amongst other things.

1. In the first exercise, the pupils plan an investigation where they study various types of sensory cells and if these are evenly distributed on the hand. In order to help the pupils initiate their work, the teacher can ask questions such as:

Are there different types of receptors to feel heat and cold?
Where are the sensory cells on the hand?
Are there areas where sensory cells are more densely situated?

To investigate the distribution of receptors that feel cold and heat, the following can be tried: use a sharp object that is warmed in hot water or chilled in iced water, and draw the object along a line on the hand. Mark where the receptors are found. It is important that the hot/cold object is dry when it is used.

2. The sauna - How does the sense of temperature work?

The picture above is an example of how difficult it is to judge temperature using touch. We have all been puzzled from time to time about how different it feels to touch objects with different heat transferring capacity. Wood has a lower heat transfer capacity than the metal in the nails. Another way of showing this is to compare the time it takes for ice to melt on a metal tray or on a piece of wood. The different construction of materials and the varying properties this gives them can affect our receptors in different ways.

3. In Activity 3, the pupils are to carry out an investigation where they calculate how fast a nerve impulse is. We suggest here that the groups are given various ways of how to go about their work. For example, groups with varying numbers of students can stand in a ring or in a row at arm’s length from each other, each with a hand on the shoulder of the pupil in front. A reaction chain is built by squeezing the shoulder of the person in front as soon as it is felt from behind. Ways of varying this are possible. The pupils carry out this investigation, and in order to practice evaluating their results and arguing for alternatives, they present their results to other groups, or similar. They can discuss reasons why the results differ and what might cause this. How much concentration is necessary? Is it better to close your eyes? Each group is to agree on how the investigation can be improved, considering the length of the chain, the number of investigations carried out, the calculations, concentration, etc.

Questions that can be asked of the pupils are:

Did you get similar results?
Have the groups carried out the investigation in the same way?
Which investigation worked best?
Why?

Allow the students to plan a new investigation where they take advantage of their experience of the first one.

4. In the fourth exercise, the pupils work with phantom pain. Phantom pain is pain that seems to come from an amputated limb. The pain is real and arises from stimulus to the nerves that have been severed. That the pain is incorrectly attributed to an absent limb is caused by the signals from the remaining fragments of the nerves being interpreted by the central nervous system – i.e. the spinal cord and finally the brain – as coming from the amputated limb. It is not always so that the pain is experienced as common pain, but can present as an irritating itch, heat or cold and can vary between individuals. Allow the students to investigate how phantom pain can occur and how it feels by asking them to put their elbow in a bucket of ice water for some minutes. Ask the pupils to read an article on the internet or from a journal where the latest research findings on phantom pain are described.