Mapping the Homunculus
Peggy Walker
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ABSTRACT
Secondary school students often question how the human brain functions in interpreting environmental stimuli. Students will map the region of the cortex which receives tactile input from the body. This can give them a concrete representation of the topography of the somatosensory cortex. In addition they will have a relative idea of the importance of touch in survival behaviors of several mammalian species. This activity can also be used to reinforce the concept of localization of function in the brain.
This laboratory exercise was modified from an activity in the 1991 Woodrow Wilson Biology Module. To be published in the Laboratory Module fromNABT/Society for Neuroscience Workshop, WakeForestUniversity 1992
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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.
INTEGRATION INTO CURRICULUM
Suggestions for Implementation
Human Nervous System
Animal Behavior
Evolution
STUDENT PRIOR KNOWLEDGE
Some background information in the structure and function of the nervous system is necessary.
History of phrenology as a pseudoscience.
TEACHER BACKGROUND
Located on the cerebral cortex is a representation of the human body called the homunculus or " little man". It 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 fingertips, therefore the student drawings will have large faces, tongues and fingertips. 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 find 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 fine 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 inestimating sizes in their drawings
Metric rulers
Alcohol or other disinfectant
SAFETY PRECAUTIONS
The materials used in two-point discrimination are normal school materials but students should always be aware that everyday materials must be used carefully for personal safety. Points should be discarded or disinfected before using on another person.
PROCEDURE
EXPLORATION
In a prelab situation, demonstrate to the students a method of two-point discrimination on the skin. Gently touch two slightly pointed objects to the skin of your forearm. It is important to apply these at the same time and to have the subject looking away from the test site.
Pick up the points, move them closer together and gently touch the skin of the same area again. The objects should be moved closer together until the test subject can no longer distinguish two separate points.
Measure the distance between the points. Have students make the measurements for one side of the body and use as much area as is feasible for your classroom setting. For example, your class may map just face, neck, arm and hand. In other settings students may volunteer to change into shorts or swim suits and a more extensive map can be made. Suggest that students randomly test the reliability of subject responses by touching with only one point.
The students will need to take the reciprocal of the two-point discrimination measurements before making their drawings. The areas of the skin that have many touch receptors will have a small discrimination value. To demonstrate that the skin area would be represented by a proportionally larger area on the cortex, the reciprocal must be taken. The reciprocal can be calculated by dividing the measurements into 1. If the two-point discrimination on the knee is 2.0 mm, the reciprocal would be 0.5.
After all calculations are made, the students should use the reciprocals as estimates to predict the relative size of the body part representations and to draw the homunculus. The smaller the distance of the two-point discrimination, the larger the reciprocal and thus the larger the body part on the homunculus will be. Have the students use the large squares on the graph paper as a guide for the approximate sizes of body parts. If the calculated reciprocals for the index finger and the thumb are 1.0 and 2.0, the thumb should be drawn twice as large as the index finger. Since each hemisphere of the cortex receives information from only one side of the body, the student drawings should be in profile.
CONCEPT INTRODUCTION
When student drawings are complete, have the students compare their maps with those of students of different sex or ethnic background. If the maps have been made with care, they should be very similar.
Display the homunculi of other mammalian species on the overhead projector or with individual hand-outs and have the students discuss the differences among the species' representations. Representative mammalian homunculi can be found in Kandel, 1991
( see bibliography).
Introduce the anatomical and physiological terms for the nervous system which may be appropriate for the level of students in your class. A brain model could be used by the students to locate the somatosensory cortex.
CONCEPT APPLICATION
In the application phase of the activity, divide the students into cooperative learning groups of three or four to research the answers to the focus questions. The groups should report back to the class as a whole and they should be aware that their answers may vary.
FINGERTIP INFORMATION FOR EASIER TEACHING.
Lab preparation time-- 20 to 25 minutes
Difficulties
Some students may not wish to be the subjects to be mapped. They can record and help their group analyze data.
Time constraints
Time is not a problem as the activity can be stopped at any point and resumed the following class period.
STUDENT FOCUS QUESTIONS
How is the "pseudoscience" of phrenology similar to the idea of localized function in the brain? How is it different?
Compare the size and shape of the rat homunculus with a typical rat body. (Kandel, 1991)
Why does the cat have more touch receptors around its mouth and lips than in other parts of its body? Suggest some adaptive behaviors in which this characteristic would be important.
Using the principles of natural selection, explain why the areas of the somatosensory cortex might be different for different species of mammals.
Consider the changes that modern technology might bring to the life styles of humans and predict how the shape of the human homunculus might change in the next 10,000 years.
OPTIONAL QUESTION FOR ADVANCED PLACEMENT STUDENTS
Compare convergent and divergent circuits patterns in the nervous system. Suggest a mechanism to show how these patterns accomplish neural integration.
Research the concept of plasticity of the brain.Design an experiment to find if increased activation of receptors will change the representation on the cortex.
MODIFICATIONS
Students with disabilities should be assigned to activity groups in which there are students to assist them with the tasks. The teacher may how much of the body to map with different levels of students.
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REFERENCES
Crosby, E.C., Humphrey, T. and Lauer, E.W. ( 1962). Correlative Anatomy of the Nervous System. New York: The Macmillan Company.
Kandel, E.R., Schwartz, J.H. and Jessell, T.M. (1991). Principles of Neural Science. 3rd Edition. New York: Elsevier Science Publishing Company. Inc.
Marieb, E.N. Human Anatomy and Physiology. (1989). Redwood City, California: The Benjamin/Cummings Publishing Company Inc.
Tortora,G.J. and Anagnostakos, N.P.(1989). Principles of Anatomy and Physiology. 5th Edition. New York: Harper and Row Publishers.
Woodrow Wilson 1991 Biology Module. (1992). Princeton, New Jersey: Woodrow Wilson Foundation.
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MAPPING THE HOMUNCULUS: STUDENT FORMAT
EXPLORATION
INTRODUCTION
Humans learn a great deal about their immediate environment from the sense of touch. The brain is able to determined where the body has been touched and often can identify what the object is. Some areas of the skin have more touch receptors in a given area than others.
MATERIALS
Metric ruler
Graph paper
Student compass
Procedure
Gently place the two points of the compass on your lab partner's skin and determine if they can feel one or two points.
Pick up the compass and close the gap slightly and repeat the activity. Continue in the same area of skin until your partner can n longer identify two separate points. Record this distance on the data table.
Pick either the left or the right side of the body to test and make sure that your partner is not looking as the skin is touched. Test only those areas indicated by your teacher. Check the reliability of the responses by randomly touching with only one point.
SAFETY PRECAUTIONS.
Gently apply the pointed objects to the skin. You may be instructed by your teacher to use a compass, tooth picks or pins. Common household items or school supplies must be use with care to ensure personal safety. Disinfect the compass points as directed by your teacher before using on another individual.
Calculate the reciprocal value of each measurement and use these values to estimate the relative number of touch receptors in the skin of that area. For example, the thumb may have twice as many touch receptors as the index finger. This would mean that the area on the sensory cortex which receives signals for the thumb would be -- twice as large as the index finger. On your drawing, make the thumb -- twice as large as the index finger.
Draw the homunculus in profile. Make the area of the body large if it has more receptors or smaller if it has fewer receptors. Use the graph paper with large squares only as a guide to the relative sizes of the body structures. It is not necessary to measure exactly on the graph paper.
TERM INTRODUCTION
Compare your homunculus with those of other students in your class. Determine if there are more similarities or more differences.
Compare your homunculus with the diagrams from other mammalian species provided by your teacher. Discuss with your class how the shape of the animal representation indicates some of the behavioral adaptations in that animal.
Use a model brain or your textbook to find the somatosenory cortex where the homunculus is located.
APPLICATION
Working with your small group, answer the Focus Questions and be prepared to report your ideas to the class. Be aware that there may be more than one right answer as you analyze and predict future outcomes. The answers will vary from group to group.
Focus Questions
How is the "pseudo science" of phrenology similar to the idea of localized function in the brain? How is it different?
Compare the size and shape of the rat homunculus with a typical rat body.
Which part of the cats body has the most touch receptors? Suggest some adaptive behaviors in which this characteristic would be important.
Using the principles of natural selection, explain why the areas of the somatosensory cortex might be different for different species of mammals.
Consider the changes that modern technology might bring to the life styles of humans and predict how the shape of the human homunculus might change in the next 10,000 years.
Design an animal of your choice. Include a description of its physical characteristics and behaviors. Draw its homunculus.
OPTIONAL QUESTION FOR ADVANCED PLACEMENT STUDENTS
Compare convergent and divergent circuits patterns in the nervous system. Suggest a mechanism to show how these patterns accomplish neural integration.
Research the concept of plasticity of the brain. Design an experiment to determine if changes in activation of receptors will change the representation on the cortex.
Select 5 of the following body regions for your Team DATA TABLE:
Two-Point Discrimination: Reciprocal( 1/ measurement)
Measure to total square area in cm squared.
Table and plot Plot several discriminations from large to indeterminable ( Single Point)
Scalp
Forehead
Cheek
Nose
Upper Lip*
Lower Lip*
Tongue*
Chin
Front of Neck
Back of Neck
Upper Back
Shoulder
Upper Arm
Elbow
Forearm
Wrist
Back of Hand Palm of Hand
Tip of Thumb
Tip of Index Finger
Tip of Third Finger
Tip of Fourth Finger
Tip of Fifth Finger
Front of Thigh
Back of Thigh
Front of Knee
Back of Knee
Lower Leg
Back of Lower Leg
Top of Foot
Bottom of Foot
Big Toe
Second Toe
Third Toe
Fourth Toe
Fifth Toe
*change point or sterilize
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