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Two-Point Discrimination Lab (courtesy of Mr Thiel; modified by Mrs Winward) Background Information: The nervous system is responsible for sensing changes in both the internal and external environment and making adjustments to insure stability of the internal environment. It is composed of specialized cells called neurons that communicate information to and from the brain. Sensory neurons exist throughout the body to "sense" changes that may threaten proper functioning of the body's internal environment. Interneurons relay what is sensed by the sensory neurons to and from the brain through the spinal cord. Motor neurons carry the brain's response, mostly to muscles, to induce a change in tissue, organ, or body function. There are many different types of sensory neurons that are organized in specific regions of the body. Each sensory neuron is equipped with a specialized receptor that is sensitive to specific types of stimuli. Examples shown below: Stretch receptors in the stomach, intestines, bladder, skin, muscles, tendons, and ligaments Chemical receptors in the nose and tongue Vibration receptors in the cochlea of the ear Touch, temperature, and pain in the skin Baroreceptors, found in arteries, sense changes in blood pressure Variations in the concentration of sensory neurons will give a higher or lower sensitivity to the stimulus. The greater the number of sensory neurons firing, the greater the sensation in the brain. Sensory maps in the cortex are "distorted". Although tactile sensory maps in the cerebral cortex are faithful to the locations of the sensory receptors, they do not reflect the correct proportions of the skin areas. Rather, the cortical area devoted to receiving information from a spot on the skin reflects the density of sensory receptors there, and this number in turn reflects the importance of that body area for gathering information. The fingertips, for example, contain about 100 times more receptors per square centimeter than the skin on the back. Because of this, more CNS neurons must be devoted to receiving fingertip sensations, and consequently the cortical area that receives input from the fingertips is huge compared to the area that receives input from skin on the back. If pictures of the parts of the body are drawn next to their corresponding brain 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 orderly maps, even though the other peripheral sensory receptors, unlike those of the touch or tactile system, are concentrated in small organs: eyes, ears, nose, and tongue. Information from each of these senses is mapped onto a different brain area. The Two Point Discrimination Test: Neurologists, doctors who specialize in diseases of the nervous system, sometimes test patients for two-point discrimination. They may do this if they suspect a problem with sensory information entry to the skin, the pathways to the brain, or the interpretation of sensory information. For example, if a patient has cut a finger badly, a neurologist may test for two-point discrimination at the time of injury to see if the nerve was cut. After the original injury has healed for a number of weeks, the neurologist will again test two-point discrimination and compare it with the normal fingers. The two point discrimination test involves placing the tips of two objects a certain distance apart on the skin and asking the patient if they are able to sense the presence of one or two stimuli. Moving the points closer or further apart can identify the range of sensitivity in different regions of the body. SUPPLIES large paperclip, unbent data recording sheets metric rulers graph paper PROCEDURE 1. In your lab group, choose one person to act as data recorder, one to be the subject, and one to act as neurologist. 2. The subject must close his/her eyes. The subject may not watch the procedure— this would give away the answer! 3. Wipe the paperclip ends with an alcohol swab. The tester should start with two ends of the paperclip about 50 millimeters (mm) apart. Make sure that the two points touch the skin at the same time. (Do not poke the subject.) 4. The data recorder asks how many points the subject feels. If the person feels two, move the points closer together—about 40 mm apart, and check again. Continue the procedure until you find the smallest distance the points can be separated for the person to feel two points instead of one. When the person reports “one point” for the first time, move the two points apart only one or two millimeters at a time and try to make a very accurate measurement. 5. When the smallest distance is found, the data recorder can measure the distance in millimeters between the two points while the experimenter holds them on the subject. 6. Continue this process for the rest of the skin areas on the Data Sheet. 7. Wipe the paperclip ends with an alcohol swab if another person becomes a subject. DATA RECORDING TABLE Skin Area For Testing Minimum Distance (For 2 point Discrimination Test - in mm) Forehead Cheek Back of Forearm Palm of Hand Tip of Thumb Tip of Index Finger Back of Lower Leg DATA AND OBSERVATIONS After you have measured and recorded all distances on the Data Sheet, make a histogram of your results. (Body areas on the X-axis and minimum distance for two point discrimination on the Y-axis) Questions: 1. Identify the three different types of neurons and describe their functions. 2. If a neuron responds to stimuli completely or not at all, how can you explain different levels of pain (mild pain versus severe pain)? 3. Regarding your experiment, how do your results compare with other lab groups? 4. Which parts of the body enabled the subject to tell that there were two points even when the points were very close together? 5. Which skin areas do you think have more receptors, areas that have small twopoint distances, or large two-point distances? Why do you think so? 6. How does information from sensory receptors in the skin get to the brain? 7. The data table below shows results from a two-point discrimination threshold experiment (published in The Skin Senses, edited by D. R. Kenshalo, Springfield, IL, 1968). Site Threshold Distance Fingers 2-3 mm Upper Lip 5 mm Cheek 6 mm Nose 7 mm Palm 10 mm Forehead 15 mm Foot 20 mm Belly 30 mm Forearm 35 mm Upper arm 39 mm Back 39 mm Shoulder 41 mm Thigh 42 mm Calf 45 mm How did your measurements compare?