 | OUR SENSE OF TOUCH: TWO-POINT DISCRIMINATIONDeveloped by Marjorie A. Murray, Ph.D.; Neuroscience for Kids Staff WriterTEACHER RESOURCEFEATURING: A "LESSON-PLAN EXPERIMENT"PLUS: "TRY YOUR OWN EXPERIMENT!" To view the Teacher Guide and Student Guide, you must have the free |  |
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| Students learn how to investigate the touchsensory system and discover how to plan and carry out their ownexperiments. In the "CLASS EXPERIMENT," students find that the abilityto tell that two points rather than just one are pressing on the skindepends on two things: the density of skin sensory receptors and theconnections that the sensory nerve cells make in the brain. Theylearn basic facts about sensory receptors and nerve connections, anduse their estimates of receptor density to predict the size of the brainareas devoted to input from different skin regions. In "TRY YOUR OWN EXPERIMENT!," students design their own experiments,investigating, for example, how touch information is important in motortasks, or whether additional sensory input can interfere with two-pointdiscrimination. | SUGGESTED TIMES for these activities:30-45 minutes for introducing and discussing the activity, 45 minutes for"Explore Time" the "Class Experiment;" and 45 minutes for the "Try YourOwn Experiment!. (Total time may be reduced if the number of skin areastested are limited and if materials for "Try Your Own Experiment!" arerestricted.
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| By reaching Project 2061 Benchmarks for Science Literacy, students willalso fulfill many of the National Science Education Standards andindividual state standards for understanding the content and applying themethods of science. Because the Benchmarks most clearly state what isexpected of students, they are used here. Below is a list of Benchmarksthat can be met while teaching the Two-Point Discrimination activities. The Benchmarks are now on-line at: http://project2061.aaas.org/tools/benchol/bolframe.html The Benchmarks are listed by chapter, grade level, and item number; forinstance, 1A, 6-8, #1 indicates Chapter 1, section A, grades 6-8,benchmark 1. The process of inquiry used in the Two-PointDiscrimination activities will help students reach the followingsummarized Benchmarks:
The neuroscience content in theTwo-Point Discrimination activities and Background material will help tomeet the following Benchmarks:
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| TABLE 1 Characteristics of sensory receptors in theskin | |||
| Receptor | Stimulus | Sensation | Adaptation |
| Merkel's disk | Steadyindentation | Pressure | Slow |
| Meissner'scorpuscle | Low frequency vibration | Gentlefluttering | Rapid |
| Ruffini'scorpuscle | Rapid indentation | Stretch | Slow |
| Paciniancorpuscle | Vibration | Vibration | Rapid |
| Hair receptor | Hairdeflection | Brushing | Rapid orSlow |
4. Sensory input is "mapped" onto specificbrain areas
Information from each skin receptor is carried along a pathway formed byseveral neuronal axons to a strip on the top of the brain surface calledthe somatosensory cortex. The cortex or "rind"is the cell body-containing outer layer of the brain and is about sixmillimeters, or one-quarter inch, thick. The somatosensory cortex ispacked with the cell bodies of CNS neurons, which receive "skin input"from all parts of the body via the "touch-neuron pathway."
Sensory input pours into the CNS neurons in a topographically faithfulmanner. This means, for instance, that the CNS neurons receiving inputfrom sensory receptors in the right thumb will have neighbor cells thatreceive input from the right index finger. These, in turn, will haveneighbors receiving input from the next finger, and so on. In this way, asensory "map" of the body surface is created on a section of the brainsurface. Neurologists discovered this years ago when they found that theycould produce the illusion of sensation in, say, a finger, by electricallystimulating the appropriate spot on the somatosensory cortex: the CNSneurons interpreted the artificial electrical stimulus as input comingfrom the finger that usually sent it information.
From the somatosensory cortex, messages about sensory input are sent toother brain areas; for example, to motor areas for use in performingactions, and to higher processing areas, for making decisions or enjoyingsensations or reflecting on them.
5. Sensory maps in the cortex are "distorted"
While tactile sensory maps in the cerebral cortex are faithful to thelocations of the sensory receptors, they do not reflect the correctproportions of the skin areas. Rather, the cortical area devoted toreceiving information from a spot on the skin reflects the density ofsensory receptors there, and this number in turn reflects the importanceof that body area for gathering information. The fingertips, for example,contain about 100 times more receptors per square centimeter than the skinon the back. Because of this, more CNS neurons must be devoted toreceiving fingertip sensations, and consequently the cortical area thatreceives input from the fingertips is huge compared to the area thatreceives input from skin on the back.
If pictures of the parts of the body are drawn next to their correspondingbrain areas, the fingers are very large and the arms and back are small.This type of picture is called a homunculus, literally,"little man" or person.
All sensory systems feed information into the cerebral cortex in orderlymaps, even though the other peripheral sensory receptors, unlike those ofthe touch or tactile system, are concentrated in small organs: eyes, ears,nose, and tongue. Information from each of these senses is mapped onto adifferent brain area.
6. Receptor density and thesizes of receptive fields of central neurons determinetwo-point discrimination ability
What properties of the touch sensory system allow us to discriminate twopoints pushing on our skin even when they are only 2 or 3 mm apart? Oneof the necessary properties is high receptor density, and the class shoulddiscuss this after students find that the two-point threshold distance onthe fingertips is two to three millimeters (mm). In other words, thereceptors must be packed closely enough so that a probe does not missthem when it touches the skin-this would mean that some skin areas wouldactually be numb. High receptor density alone, however, cannot explainwhy the fingertip can distinguish points so close together while the armsenses two points only when they are 35 to 40 mm apart. The secondproperty necessary for fine two-point discrimination is that neighboringreceptors must connect to different CNS neurons, which in turn means thatthese CNS neurons must have small receptive fields, as explained below.
Each sensory receptor connects through a series of relay neurons witha CNS neuron. A given central neuron responds to all information from itsinput area (the skin area that is the gathering field for onlythat CNS cell) as if it were coming from one point. This skin area iscalled the receptive field of the central neuron. Onthe arm, each sensory receptor gathers information from a much larger skinarea than a receptor on the fingertip, and this receptor is also connectedto a defined central neuron. This central neuron, like the central"finger neuron", interprets all input as coming from one point, eventhough the skin in this case is much larger. In order for aperson to feel two points, two separate central neuronal populations mustbe activated by stimulation of their respective receptive fields. Whenthis happens, two points are reported.
To summarize, two-point discrimination depends onactivating two separate populations of neurons, and in order todiscriminate two closely placed points, the receptive fields of theneurons must be small. This in turn means that the receptors must bedensely packed in a sensitive area, so that two points very close togetheractivate different receptors.
7. Sensory information from different receptorsis combined at higher brain levels
While individual receptors respond to only one type of stimulus, such aspressure or vibration, a stimulus in the real world almost alwaysactivates several kinds of receptors simultaneously.To form a representative picture of this in our minds, the differentsensations must all "get together" somewhere in the brain, and one placethis happens is in cortical neurons called feature-detecting neurons.These neurons each receive several different types of information fromneurons in the primary somatosensory cortex (which received theirinformationfrom receptors). This integration of sensations allows us to experience anice cube as both smooth and cold, or to feel that sand at the beachcontains different sized grains and may be hot or cool. As thisinformation is sent to higher brain centers, sensations also take onmeaning because of past experiences.
8. Neurologists use two-pointdiscrimination tests to check for nerveabnormalities
Neurologists, doctors who specialize in diseases of the central (brain andspinal cord) and peripheral (nerves to all the organs and muscles) nervoussystems, sometimes test patients for two-point discrimination. They may dothis if they suspect a problem with sensory information entry to the skin,the pathways to the brain, or the interpretation of sensory information.
| Give students "Explore Time" to motivate themto think critically and to ask for information. Science experiments lend themselves to a "let's see what happens"atmosphere, and a good way to take advantage of this is to provide"ExploreTime." Because of their curiosity, students usually "play" with labmaterials first even in a more traditional lab, so taking advantage ofthis natural behavior is usually successful. First, prepare students for lab activities by giving some backgroundinformation in the form of a lecture and discussion. For "Explore Time,"setthe lab supplies out on a bench before giving any instructions for theactivities. Ask the students how these materials could be used toinvestigate the sense of touch, then give them 10 or 15 minutes to workwith the materials. Give some basic safety precautions, then circulateamong students to answer questions and encourage hypotheses.After students begin to gain an interest in the materials and subject,lead the class to the Two-Point activity with the Teacher Demonstration,and help them to formulate the Lab Question. Determine the level of information appropriatefor your students Because students have no way of discovering sensory receptors or nervepathways for themselves, they need some basic anatomical and physiologicalinformation. Teachers may choose the degree of detail and the methods ofpresenting the touch system, based on grade level and time available. One approach that fosters student investigation is to give an overview ofreceptors, information flow to the brain, and specialized brain areas onthe day before "Explore Time" and experiments. After students havecollected and plotted their data, they will be more responsive to detailedinformation, such as concepts of receptor density, maps in the brain, andreceptive fields.
Discuss with students the fact that scientists have discovered the partsof the skin sensory system over many decades and that a combination ofanatomical and physiological investigations was used to uncoverways in which the senses work. Some researchers looked extensively atthin pieces of skinwith electron microscopes to find Pacinian corpuscles, Merkel's receptors,and other types of receptors. Other investigators recorded electricalimpulses from nerves with miniature electrodes, and still others examinedpatients and material from autopsies of people with known diseasesymptoms. From many pieces, the puzzle was assembled, and more detailis added constantly from contemporary studies. Get students involved by discussing everydayevents To get students actively involved in talking and thinking about thesenses, discuss whether one sense can substitute for another. Ask, forexample, what happens when we are watching TV and the sound goes off. Weuse the sense of sight to figure out what is happening, by trying to readlips and by imagining, for example, the sound we would hear if we saw carsracing around a track. On the other hand, if we close our eyes and hearan airplane overhead, we can imagine how it looks. Another short activity, this time illustrating the use of the tactilesense to replace sight and sound, would be to read a few excerpts fromHelen Keller's autobiography. Her description of realizing thatinformation from the outside world could be conveyed by tracings of wordson her hand by her teacher, and later by Braille, isstriking. |
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SUMMARY
What isit that allows us to make these distinctions? First, human skin containsdifferent kinds of sensory receptors (cells) that respond preferentiallyto various mechanical, thermal, or chemical stimuli. (The word "receptor"can mean a receptor cell or a membrane receptor in a cell. Here, itrefers to a cell.) Next, these receptors convey this information to thebrain and spinal cord, also known as the central nervous system(CNS), to areas where we perceive the stimuli. To accomplishthis,the nerve endings of the sensory receptors transduce, or convert,mechanical, thermal, or chemical energy into electrical signals. Theseelectrical signals then travel along neuronal extensions calledaxons, to the CNS. Finally, the way weinterpret or understand sensations is shaped not only by the properties ofreceptors and neurons, but also by previous experiences that are stored inour brains.
Scientists are still exploring thesomatosensory system![[email]](./gif/menue.gif)