A Laboratory Exercise in Somesthesis That Is

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265:S2-S9, 1993.Advan in Physiol EduJ. D. Greenspanclassesexpeditious, inexpensive, and suitable for largeA laboratory exercise in somesthesis that is

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http://www.the-aps.org/.Copyright 1993 by the American Physiological Society. ISSN: 1043-4046, ESSN: 1522-1229. Visit our website atSeptember and December by the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991.specialized courses and in the broader context of general biology education. It is published four times a year in March, June,

is dedicated to the improvement of teaching and learning physiology, both inAdvances in Physiology Education

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I N N 0 V A T I 0 N S A N D I D E A S

A LABORATORY EXERCISE IN SOMESTHESIS THAT ISEXPEDITIOUS, INEXPENSIVE, AND SUITABLE

FOR LARGE CLASSESJoel D. Greenspan

Departments of Neurosurgery and Physiology, State University of New York:Health Science Center at Syracuse, Syracuse, New York I.3210

A teaching laboratory is described that demonstrates two principles of thesomatosensory system: 1) the spot-like nature o f skin sens itiv ity and 2)tactile acui ty. This laboratory has been used for a large medical school class(140-160 students). One reason for the success of this laboratory is the ability toprovide a suff icient number of stimulating apparatuses. The two-point discriminationdevice used to measure tactile acuity can be made easily and cheaply. In addition, twoother demonstrations are described that are suitable for smaller classes or thatstudents can do on their own. Although this lab was designed for medical students,the content makes it suitable for undergraduate or high school students.AM. J. PHY SIOL. 265 (ADI? PHKS IOL. EDUC. 10): S2-S9, 1993.

teaching; temperature sense; touch; tactile acuityMost educators agree that a hands on laboratoryexperience will enhance learning of scientific sub-jects. However, when classesare large, there is toomuch demand for time, space, and supplies to allowall students such an opportunity. The present stu-dent laboratory exercise was designed with thesefactors in mind and was presented to first-yearmedical students during the sensory systems por-tion of their neuroscience course. The classsize wasapproximately 150 students. The laboratory hasbeen given each year since 1991 and was designedto demonstrate two properties of the somatosen-sory system: 1) the discrete nature of sensitive spotson the skin and 2) the concept of tactile acuity.Although this laboratory was designed for medicalstudents, the phenomena and basic principles caneasily be understood and appreciated bygraduate or high school students.PROCEDUREEquipmentOne key to having a reasonable classroomtory exercise is having a sufficient number of:

Jnder-

abora-ppara-

tuses for the students to use. This laboratory usestwo devices that are inexpensive enough to use invery large classes.The first is a ballpoint pen, whichthe students will provide themselves, and the sec-ond is a homemade two-point stimulator.The two-point stimulator is made from a 6-h. plasticruler and two paper clips (Fig. 1). One of the paperclips is cut to produce two straight pieces. One ofthese pieces is taped (or glued) to the 0 edge ofthe ruler. The other piece is taped (or glued) to theintact paper clip, which is then clipped to the plasticruler so that it can slide up and down the ruler. Theintact clip is positioned on the ruler so that the twostraight pieces can touch one another at the 0 endof the ruler. By sliding the paper clip up and downthe ruler, the two straight pieces can be separatedby any length indicated on the ruler.ProtocolThe students received a lecture on the somatosen-sory system before this laboratory and were as-signed appropriate readings from the course text-

1043 - 4046 / 93 - $2.00 - COP YRIG HT o 1993 THE AMERICAN PHYSIOL OGICAL SO CIE TY

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INNOVATIONS A N D I D E A S

FIG. 1.Thls two-point threshold device is inexpensively and quicklymade. Despite its unass uming appearance, it can providereasonably accurate data.

book (6; chapters 24-26). The students then met inthe lecture hall normally used for this course andwere instructed to review their handouts (repro-duced in APPENDIX). After reading the handout,the students were given an overview of how toproceed with the laboratory, and any questionswere answered. Next, 50 two-point devices werehanded out, and students were told to form groupsof three to carry out the exercise. After completingthe data collection, the students turned in their datasheets (see APPENDIX) and were told that theirgroup results would be presented at the next lec-ture period.Data AnalysisFor the cool spot mapping, the class distribution ofthe number of cool spots was determined. For thetwo-point thresholds, the class distribution of valueswas determined at each test site. Also, the mean andstandard deviation of two-point threshold werecalculated for each skin site.RESULTSThe group results and other relevant informationwere presented to the students at the next classmeeting.Cool Spot MappingAll students recognized the discrete nature of coolspots on the skin (See COOL SPOT MAPPING LAB

0 5 10 15 20Number of cool spotsFIG. 2.

Distribution of no. of cool spots mapped by 1st.yr medicalschool class (n = 146).

DATA SHEET in APPENDIX). The modal reportfrom one class was 7 cool spots out of 20. The classdistribution was unimodal and roughly normal (Fig.2).This type of temperature spot mapping has a longhistory as both a research tool and a laboratoryexercise. Examples of class data can be presented tothe students that show the discontinuous distribu-tion of both warm and cool spots (Fig. 3). It shouldbe pointed out that our senses of touch and painalso show a punctate nature. The difference is thatmechanoreceptors and nociceptors are more denselyconcentrated than warm or cool receptors, makingit more diff icult to find insensitive areas betweensensitive spots.Two-Point DiscriminationAll students observed an increase in two-pointthreshold as they progressed from the fingertip tothe palm (Fig. 4). The class distribution of two-pointthresholds at each site was unimodal and roughlynormally distributed (Fig. 5). The class mean valuescan be compared with published data on two-pointthresholds (Table 1 and Refs. 8 and 9).Furthermore, these thresholds can be related to thedensity of cutaneous mechanoreceptors in this partof the hand. The receptor density estimates re-ported by Johansson and Vallbo (5) were derived

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A FICL IV. Fro. V.

FIU. W.-Map of the heat-spots rrrld cold-s Jots on the back o f the lef t hand.Extent, 2 x 8 ems. Dots intlicnto co1a ; circles, heat ; the larger OIJCSrepresent the spots which give a strong rextion ; the smaller, those mhlclrive a weak one.9 The top of the map is peripileral, the bottom central.he right aide is the radial ; the left the ubr.FIQ. V.-Similar may for the symmetricd portion of the right hand of the mrne

individual. In thiy case, of cowye, the lef t side is tile radial, and the rightthe ulnar.

Bt 2X Cb ld 3

Cbnpositecold Cbmpitew rm

PI&Am IMAPS 01 COLD AND WARM SPOT m-t Dhea f aq.cm.

FIG. 3.Examples of clas sic temperature spot mapping. [A: from Donaldson (3) bypermission of Oxford Univ. Press. B: from Dallenbach (1) by permission of Univ. ofIllinois Press (Urbana, IL).]

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T

TIP DISTAL PROXIMAL THENARStimulus siteFIG. 4.

Two-point thresholds (means + SD) for each stimulu s site.

from two types of data. First, a single-unit electro-physiological survey of mechanoreceptors in thehuman median nerve produced a data set of > 300units. There are four types of mechanoreceptorsroutinely described in the human glabrous skin,Finger tip

TABLE 1.Two-point discrimina tion results from clas sic papersWeber, 1834(Translated in Ross and Murray, 1979)

Body site Two-point thresholdDistal phalanx of fingers 2.2 mmMiddle phalanx of fingers 4.5 mmPalm of hand 11.2 mm

Weinstein, 1968Body site Two-point thresholdFingers (not specified) 3-4 mmPalm of hand lo-11 mm

each with its unique electrophysiological propertiesand each associated with a specialized structuralending: I) rapidly adapting (Meissners corpuscle),2) slowly adapting type I (Merkel cell complex), 3)slowly adapting type II (Ruffini endings), and $)Pacinian corpuscle (9). The receptive fields of theseunits were precisely localized, and thus the relativedensity of each mechanoreceptor type could becalculated. Second, a section of human mediannerve was examined histologically and found to

40ul r Distal phalanx

o 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20Probe separation (mm) Probe separation (mm)

Proximal phalanx LUc Palm of hand

0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20Probe separation (mm) Probe separation (mm)

FIG. 5.Distribution of two-point thresholds for each site.

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160

RA-Meissner

m Tip of Digitj Distal Phalanxm Proximal Phalanxm Thenar

SAI-Merkel SAII-Ruffini PC-Pacinian

Mechanoreceptor typeFIG. 6.

Innervation density of cutaneou s mechano receptors in the human hand. [Based onJohansson and Vallbo (5).]

contain 27,300 large myelinated nerve fibers. Thesetwo pieces of information were combined to esti-mate the absolute innervation density of the fourtypes of mechanoreceptors for various parts of thehuman hand. One can see that there is an inverserelationship between two-point threshold andmechanoreceptor density that is most striking forthe rapidly adapting and slowly adapting type Imechanoreceptors (compare Fig. 4 and Fig. 6).DISCUSSIONThe experiential phenomena of these demonstra-tions are quite reliable and easily appreciated. De-spite the simplicity of the observations, they oftencome asa surprise to people.Part of the class discussion on the second daycenters on the reliability of the classdata. One thingthat most students discover by doing this lab is thatthey are sometimes uncertain whether they felt coolor not, or whether they felt one or two points. Theyshould realize that ambiguous sensations will occurin tests such as these. In this situation, because theyknew what the correct answer was supposed tobe (e.g., the probe was always cool, and there were

always two points), they would have a bias towardgiving that answer. The students should recognizethat proper testing of sensation, whether in anexperimental laboratory or in a clinic, should avoidgiving the idea that a particular answer is correct ordesired.At the conclusion of the class handout (see APPEN-DDE), there are some questions for the students toconsider. The first and the fourth questions areexplicitly addressed during the second class. Thesecond and third questions, having to do withsources of error and procedural modifications tocompensate or obviate these errors, should be apoint of some discussion after the students haveworked through the exercise. At least they shouldrecognize the biasing effect that foreknowledge ofthe stimulus sequence can have. With a longerperiod in which to do this laboratory, the studentscould also incorporate a less predictable sequenceof stimuli in the two-point threshold test.I also give the students two other demonstrations oftemperature sensitivity to try on their own. Thesedemonstrations could be done as class experiments



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with a smaller group. The first is an example of therelative vs. absolute nature of temperature sensa-tion. Start with three buckets ofwater: one cold, onehot (but not painful), and one close to skin tempera-ture (neutral). First, put one hand in the cold waterand the other hand in the hot water. Leave them infor - 1 min, during which time the water willgradually feel less cold and less hot. This is theprocess of adaptation. Then, place both hands in thethird bucket of water; one will experience a sense ofwarmth in the hand that was in the cold water, andcoolness in the hand that was in the hot water. Thisdemonstrates that thermoreceptors respond to thechange in temperature that the skin undergoesrather than the absolute temperature per se.The second demonstration shows how our sense oftouch influences our sense of temperature. Threecoins will be needed, two chilled by refrigeration,and the third near skin temperature. Place the threecoins in a row on a desk, with the cold coins oneither side of the warmer coin. Then place the tipsof three fingers on the three coins simultaneously.One will have a powerful illusion that all three coinsare cold. Our sense of touch can localize events onthe body very well. In contrast, our senses ofwarmth and coolness do not. Most of the time, wefeel warm and cool from objects that touch us, andwe localize the stimulated body site on the basis ofthe tactile information. In this artificial situation, thecool sensations are referred to all three tactile sites.The temperature spot mapping laboratory exercise is not new. InKarl Dallenbachs classic paper (1)) he remarks, The localizationand mapping of the cold and warm spots is usually the first

experiment performed in the qualitative laboratory course atCorne ll (p. 402).More recent scholars of soma tosensation who have used thislaboratory exercise include Drs. Dan R. Kensh alo, Sr., Glenn J.Giesler, and Ken D. Cliffer. My thanks to Dr. Cliffer for pointingout the usefuln ess of the inexpensive and readily available coolprobe. The phenomenon of the three-coin illusion was originally de-scrib ed by Dr. Barry G. Green (4).Received 8 March 1993; accepted in final form 9 July 1993.References

1.2.

3.4.

5.

6.7.8.9.

10.

Dallenbach , K. M. The temperature spots and end-organs.Am. J. Psychol. 39: 402-427, 1927.Dellon, A. L. Evaluation of Sens ibility and Re-Education ofSensa tion in the Hand. Baltimore: William s & Wilkins, 1981.Donaldson , H. H. On the temperature sense . Mind 10:399--416,1885.Green, B. G. Thermo -tactile interactions: effects of touch onthermal localization. In: Sensory Fun ctions of the Skin inHumans, edited by D. R. Kensh alo. NewYork: Plenum, 1979.Johans son, R. S., and A. B. Vallbo. Tac tile sensitivity in thehuman hand: relative and absolute dens ities of four types ofmechano receptive units in glabrous skin. J. Pbysiol. Land.286: 283-300, 1979.Kande l, E. R., J. H. Schwartz, and T. M. Jes sell. Principle sof Neural Scienc e. New York: Elsevier, 199 1.Rivers, W. H. R., and H. Head. A human experiment innerve division. Brain 31: 323-450, 1908.Ross, H. E., and D. J. Murray. E. H. Weber: 7&e Sense ofTouc h. New York: Acade mic, 1978.Vallbo, A. B., and R. S. Johan sson. Properties of cutaneou smechano receptors in the human hand related to touchsensa tion. Human Neurobiol. 3: 3-14, 1984.Weinste in, S. Intensive and extensive aspe cts of tactilesensitivity as a function of body part, sex, and lateral&y. In:Z%e Skin Sense s, edited by D. R. Kensh alo. Springfield, MA:Charles C. Thom as, 1968.

Teachers and their students may find the following articles fromNews in Physiological Sciences useful when exploring the physiol-ogy of the preceding paper:Cervero, F., and J. M. A. Laird. One pain or many pains? A newlook at pain mechanisms. NIPS 6: 268-273, 1991.Hammond, D. L. New insights regarding organization of spinalcord pain pathways. NIPS 4: 98-101, 1989.Tenney, S. M. Itching. In Notes, NIPS 8: 96, 1993.Verley, R. Reorganization of the somatosensory cerebral cortexafter peripheral damage. NIPS 1: 15-19, 1986.

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APPENDIXSTUDENT HANDOUT FOR SOMATOSENSORYPSYCHOPHYSICS LAB

MS1 Neuroscience Course, 1993Dr. Joel D. GreenspanFor this laboratory, you will work in groups ofthree. Each one of you will take turns being the experi-menter, the subject, and the record keeper. Each per-son, then, will have hidher own laboratory data sheetsto turn in at the end.

I. Thermal sensitivityWe will begin this laboratory with a demonstration

of the discrete nature of cutaneous sensitivity. If youuse a stimulating probe that is small enough, you candetermine spots on your skin that are particularly sensi-tive, and other spots that are less so. This phenomenonis most apparent with your sense of coolness, which wewill examine today.You will be using a bal lpoint pen to serve as yourstimulator. Since the pen tip is a good heat sink, and iscooler than your skin, it provides an adequate coolingstimulus. The subject should be seated, and not able tosee the stimulus applicat ion. Usually, the subjectwould be blindfolded, but in this case, well count onthe subject keeping his/her eyes closed. The subjectwill place his/her left hand on the table, palm down.The experimenter will envision a twenty grid array onthe middle phalange of digit three (see figure in labdata sheet). If dig it three is injured, use another digit orthe right hand. The experimenter will lightly touch thesubjects finger at each of these twenty spots. He/sheshould maintain contact with the skin for 2-3 seconds(enough contact to indent the skin a slight amount), andthen remove the pen. Then, the subject will saywhether he/she felt a cool sensation or not, and therecord keeper will write down the response. The exper-imenter should wait 5-6 seconds before the next stimu-lus. You may wish to practice at a few points firstbefore collecting the data. At the end, you will have amap of cool spots, as well as a count of how many ofthe 20 spots fe lt cool. We will look at the class distrib-ution of cool spot density at a later class.Consider what the anatomical and/or physiologicalbasis of your results are (Kandel, Schwartz, and Jessell,Chap. 24).

COOL SPOT MAPPING LAB DATA SHEET

Subject nameAge- Sex- Handedness

Indicate with a fil led circle which spots provided asensation of coolness, and use a hollow circle for spotsthat were otherwise.

Total # cool spots:- /20Experimenter nameRecord keeper name

II. Tact ile (two-point) discriminationThe class will perform a set of experiments to evalu-ate one measure of tact ile acuity: two-point discrimina-tion. As before, you will work in groups of three, eachtaking turns at being the subject, the experimenter, andthe record keeper.The basic question that you will be asking is, Howmuch of a separation between two points is needed inorder to perceive them as two distinct points? Theminimal separation that is needed for perceiving twodistinct points is called the two-point threshold. Youwill determine the two-point threshold on different sitesof your hand, in order to demonstrate the extent ofregional variability.The seminal work on tac tile sensitivity was doneover 100 years ago by the German experimental psy-chologist Ernst H. Weber. An English translation ofhis most influential works appears in E. H. Weber: The

Sense of Touch (translated by H. E Ross and D.J.Murray, Academic Press, 1978). Since that time, there

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have been some efinements n tools and protocols, butthe basic approach hasbeen used for various purposesin research and in diagnostic medicine. For instance,A. Lee Dellon describes he many sensory testing pro-cedures that he uses when evaluating recovery fromnerve damage Evaluation of Sensibility and Re-educa-tion of Sensation in the Hand, Williams and WilkinsPub., 1981). Also, the classic paper by W.H.R. Riversand Henry Head, A Human Experiment in NerveDivision (Brain, 3 1:323-450, 1908) describes n con-siderable detail the sensory changes hat occurred overthe course of several years following Dr. Heads volun-tary division of his radial nerve. This lab will not bequite as demanding.The protocol that you will be using is the method oflimits. For the sake of time, you will perform oneascending series of stimuli (e.g., progressively largerpoint separations),and one descendingseriesof stimulifor each skin site tested. Normally, one would repeattheseseriesseveral times and determine an average.The subject should be seated, with his/her eyesclosed. To begin an ascendingseries, he experimentersets he two-point device so that the two probes are asclose as possible. The experimenter applies the stimu-lus by pushing the probes down on the desired skin sitefor 2-3 sec. The subject then reports whether he/shefelt one or two points. As long as the subjectreports one, the experimenter increases he distancebetween the two points from stimulus to stimulus,being careful to use a consistent amount of force eachtime. When the subject gives a responseof two, theascending series is over, and the record keeper notesthe separation that was used to evoke the responseoftwo. Technically, the record keeper should use thevalue half-way between the separation that yielded aresponseof two, and the previous separation hat wasperceived as one. Thus, if you were increasing theseparation in l-mm steps (ascending series), and thefirst response of two was given at a separation of8mm, then the threshold value of that serieswould be7Smm (since the previous stimulus of 7mm yielded aresponseof one). To begin the descendingseriesofstimuli, adjust the two point device so that it has a sepa-ration several millimeters greater than the thresholddetermined by the ascendingseries. Then, the experi-menter repeats he stepsdescribedabove, only this timethe separation s reduced each time the subject reportstwo, until you reach a separation hat is perceived asone. You will then take the average of the ascendingand the descendingseries thresholds to determine thatsubjects wo-point threshold for that skin site.You will be testing four sites on each subjects ighthand (palmar surface). Site #l is the tip of digit 2; site#2 is the proximal half of the distal phalanx; site #3 is

the middle of the proximal phalanx of digit 2; site #4 isthe middle of the thenar eminence. If a person has aninjury on digit 2, another digit should be used. Groupresults will be presented o you later. After conductingthe experiments and recording the results on theattached ab sheet,consider the following questions:What were you able to determine about regional varia-tion of tactile acuity?What sources of error could affect the results of theseexperiments?What procedural modifications could reduce or elimi-nate sucherrors?What neuroanatomical and neurophysiological featuresdetermine two-point threshold? (Refer to chapter 26 inKandel, Schwartz, and Jessell)

TWO-POINT DISCRIMINATIONLAB DATA SHEETSubject nameAge - Sex ,-, HandednessSite #l: Tip of digit # - right hand.

Ascending series threshold: - mmDescending series threshold: - mmAverage two-point threshold: - mmSite #2: Proximal half of distal phalanx of digit # ,right hand.

Ascending series threshold: - mmDescending series threshold: mmAverage two-point threshold: -mmSite #3: Middle of proximal phalanx of digit # ,right hand.

Ascending series threshold: - mmDescending series threshold: -mmAverage two-point threshold: mmSite #4: Thenar eminence, ight hand.

Ascending series threshold: mmDescending series threshold: ,- mmAverage two-point threshold: - mmExperimenter nameRecord keeper name


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A Laboratory Exercise in Somesthesis That Is