ScienceMakers Toolkit Manual - The History Makers

Experiment - Mapping the Homonculus Developed by Yash Shah
Target Audience
Secondary school students, grades 9-12
Objectives
Using two-point discrimination on the skin, students will determine the relative number of nerve endings located
in the skin. By calculating the reciprocal of these measurements, the students will have the appropriate data
for predicting the relative size and drawing the homunculus found on the cerebral cortex. Comparisons will be
made among the homunculi of other mammals. Additional activities will show the correlation between behavior
and the shape of the homunculus.
Student Prior Knowlege
1. Some background information in the structure and function of the nervous system is necessary.
2. History of phrenology as a pseudoscience.
Teacher Background
Located on the cerebral cortex is a representation of the human body called the homunculus or the’ little man;’
this is found on the somatosensory cortex or primary sensory cortex, immediately behind the primary motor
cortex. The central sulcus, a deep groove running across the cerebrum, is located just in front of the somatosensory
cortex. Neurons in this location can identify the area of the body being stimulated by the information they
receive from somatic receptors in the skin and from proprioceptors in skeletal muscles. The muscle receptors
transmit information about locomotion and position.
A particular body region is represented on the cortex with an area that is proportional to the number of touch
receptors in the body part, not by its size. Therefore, the neurons form a geometrically distorted projection of
the body surface. It should be noted that the neurons on the primary cortex are not laid down in the shape of a
human, but the relative size of the areas is indicative of how sensitive the skin of that area is to touch. It can be
useful for students to be able to put these parts together in a way that would represent a human. The sensory
homunculus in humans has a very large face, tongue, and fi ngertips, therefore the student drawings will have
large faces, tongues, and fi ngertips. Each hemisphere of the cortex receives information from the opposite side
of the body.
The maps in animals and humans have been made by tactile stimulation of the skin and recording of the resultant
electrical activity in the neurons of the sensory cortex. Facsimiles of these maps can be made in the
classroom by using two-point discrimination to fi nd the relative density of touch receptors in the skin. Maps are
available for a number of mammals, including humans, rats, rabbits and cats.
MATERIALS
Any items with which to produce two fi ne points of touch
• Tooth picks
• Large paper clip bent into a horseshoe shape
• Straight pins
• Student compass (clip off the sharp point used as the pivot for the compass)
• Calipers
• Graph paper-Large squares can help guide students in estimating sizes in their drawings
• Metric rulers
• Alcohol or other disinfectant
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Life Science