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Attempt 1
In Progress
NEXT UP: Submit Assignment
Unlimited Attempts Allowed
Purpose
The purpose of this experiment is to assess the ability to localize single-point touch stimulation. The test shows the functional condition of the neural pathway that conveys touch impulses from the stimulated area of the skin to its corresponding area in the cerebral cortex (where the sensation is interpreted).
Part A: Light Touch Sensations
Background
The neural pathway involves light touch receptors located in the epidermis or dermis, and three neurons (one sensory and two association neurons). The touch receptors that detect a light touch to the skin include Meissner’s corpuscles, (corpuscles of touch), Merkel’s discs (type I cutaneous echanoreceptors), and hair root plexuses. Touch receptors are either free nerve endings or encapsulated nerve endings. They produce generator potentials that elicit nerve impulses in 1st order neurons.
The corpuscles of touch and hair root plexuses are rapidly adapting touch receptors, whereas Merkel’s discs are slowly adapting touch receptors.
When a touch receptor is stimulated, a generator potential is produced. A strong generator potential is needed to elicit a nerve impulse (action potential) in the first-order sensory neuron. Depolarization at the axon hillock of the 1st order sensory neuron must reach threshold level to initiate an action potential in the axon. Weak generator potentials produce subthreshold depolarizations at the axon hillock and no action potential is produced in the axon.
Action potentials in 1st order neurons enter the spinal cord and travel along the posterior column (fasciculus cuneatus or fasciculus gracilis) to the medulla oblongata where they synapse with 2nd order neurons. In the medulla oblongata, the 2nd order neurons cross over to the contralateral side, form the medial lemniscus of the midbrain, and ascend to the thalamus where they synapse with 3rd order neurons. The 3rd order neurons then send axons to the to the primary sensory cortex (AKA: primary somatosensory area
The fasciculus gracilis of the posterior column consists of 1st order fibers that arise from the lower half of the body, and the fasciculus cuneatus consists of 1st order fibers that arise from the upper half of the body, excluding the head.
Patients with injuries along specific peripheral nerves that become part of the posterior white column or the medial lemniscus are unable to localize light touch stimulations. Identify the sensory tract (fasciculus cuneatus or f. gracilis) that conveys touch stimulation from the following areas. Include this information in your lab report.
arm chest
foot
forearm
hand
leg
pelvis
thigh
Materials needed
pen or pencil
willing subject
camera
Procedure
Display all materials used for this experiment on a table and take a composite picture of them.
The subject (acting as patient) must have his eyes closed during the entire experiment.
Ask the patient to sit or stand up
Touch his hand with the pencil or pen. The subject should tell you the part of his body that was stimulated. He should also touch the same area with the index finger of his other hand. A normal individual should be able to touch same area or close to it. Repeat this step two more times and record your observations. Take a picture of the patient showing him or her performing the test. Patients who cannot pinpoint the stimulated area have nerve damage along the pathway for light touch.
Perform this test on the areas listed below and record your observations:
Anterior side of forearm
Middle of forehead
Middle phalanx of finger
Posterior side of leg (calf) Identify the sensory tracts that convey nerve impulses from the areas specified in step 4. Include this information in your lab report.
Describe how you performed the tests on the subject, and the test results.
Part B: Two-point Discrimination
Purpose
The purpose of this experiment is to assess the sensitivity (two-point discrimination) of various areas of the skin to touch stimulation. The sensory impulses for this test travel via the tracts in the posterior white column.
Background
The density of touch receptors varies significantly in different areas of the body. In general, areas with the greatest density of touch receptors have a finer sense of FEEL or tactile discrimination.
The primary sensory (somatosensory) cortex of the cerebrum has a map of the skin. Notice the disproportionate image of the human body. The size of a body area correlates directly with the number of touch receptors per square area of skin. Thus, the higher the density of touch receptors per square area of skin, the more discriminating is the skin to touch stimulation.
The motor cortex also displays similar correlation with the voluntary muscles in the body. The size of the body region in the cortex indicates the degree of motor innervation to the skeletal muscles in the corresponding region of the body. Functionally, it can be said that areas of fine motor control receive the most motor innervation.
Based on past experience to touch stimulation, predict the degree of sensitivity (two-point discrimination) in the following areas of the body: (a) the fingertip; (b) the middle of the forehead; (c) the medial surface of the forearm, where hair is absent (d) the posterior surface of the leg (calf), where hair is absent. List these areas in decreasing order of sensitivity, as you have predicted.
Materials needed
Compass or 10 paper clips
Scotch or masking tape
Willing subject
Ruler with metric measurement
3 x 5″ index card
camera
Procedure
Display all materials used for this experiment on a table and take a composite picture of them.
Prepare the materials
If you use a compass, you will have two adjust the distance between the tips (in two millimeters increments) as you proceed with the test.
If you use paper clips, you will have to prepare the paper clips so they can be used as a compass. First, you need to straighten 10 of them. Next, bend the paper clips so they look like the letter U. Adjust the distance of the tips of the first paper clip to 2 mm then tape the “arms” (not the tips) with scotch tape or masking tape to prevent the tips from moving apart. You must keep the distance between the tips constant. Next, do the same with the rest of the paper clips but adjust the distances to 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm.
On a 3″ x 5″ index card draw 10 horizontal lines parallel to each other. Draw a vertical line near the left edge. All horizontal lines should begin from the vertical line. Draw a 2 mm horizontal line first; below it draw a 4 mm line; below it draw a 6 mm line. Continue to do so until you draw the 20 mm line. Tape it on to a blank page and label it as an exhibit in your lab report.
Create a five-column table. On top of column 1 type Distance, on column 2 Fingertip, on column 3 Middle of forehead, on column 4 Anterior surface of forearm, and on column 5 Posterior surface of leg. Next, type the distances (2 mm, 4 mm, 6 mm, 8, 10, 12, 14, 16, 18 and 20 mm) in the Distance column. Record your findings on this table and include it in your lab report.
To perform the two-point discrimination test, the subject must be seated and have his eyes closed.
If you use a compass, separate its tips to 2 mm. Press them onto the fingertip. Ask the subject if he can feel the tips of the compass as separate points. If he cannot, record “no” in the table. If he can, record “yes.” Once you’ve determined the minimal distance the skin can detect the tips of the compass as two separate points, the sensitivity of the stimulated area is said to be the distance between the tips of the compass. Take a picture of the subject showing the test on his forearm. Follow the same procedure to determine the two-point discrimination in other areas of the subject’s body.
Describe how you performed the tests on the subject, and the test results. Note: since you used either a compass or paper clips, do not include both in your methods.
Part C: Stereogenosis
Background
Stereognosis refers to the ability to recognize familiar objects by feel and without seeing them. Failure to recognize familiar objects by feel alone is called astereognosis. It indicates damage along the neural pathway that conveys sensory impulses to the primary sensory (somatosensory) cortex and its association area.
Materials needed
Several small familiar objects
Willing subject
Camera
Procedure
Display all materials used for this experiment on a table and take a composite picture of them.
The subject must perform this experiment with his eyes closed.
Obtain 5 small objects that the subject has seen and felt in the past, and that small enough to hold on one hand.
Give the subject an “unidentified” object and let him feel it with ONE HAND only. The subject should be able to tell you what it is, describe its texture and what it is made of (for example: cloth, plastic, rubber, word, metal).
Do the same with the rest of the objects. Take a picture of the subject trying to identify the “unidentified” object.
Describe how you performed the tests on the subject, and the test results.
Part D: Adaptation of Sensations
Background
Adaptation of a sensation refers to the inability to perceive a sensation even though the stimulus has not been removed from the stimulated region. Adaptation is a functional characteristic of many sensory receptors. When a sensory receptor adapts to a stimulus, it sends fewer action potentials per unit time (second) to the sensory cortex.
In the skin, light touch receptors (Merkel’s discs, corpuscles of touch, hair root plexuses, and lamellated corpuscles) adapt fairly rapidly whereas pain receptors and those located in joints and muscles (Golgi tendon organs and muscle spindles) adapt slowly. The type II cutaneous mechanoreceptors (end organ of Ruffini), located deep in the dermis, detect heavy and continuous touch sensations; these adapt slowly.
Materials needed
Willing subject
Clock or wristwatch with a second hand
Small coins (pennies or dimes)
Hair
Camera
Procedure
Display all materials used for this experiment on a table and take a composite picture of them.
The subject must have his eyes closed and his forearm extended on a table.
Get several pennies and hold them in your hand. Once the coins have warmed to body temperature, place a penny on the anterior surface of the subject’s forearm.
Ask the subject to let you know when he can no longer feel the weight of the penny. Record the time. This is the time it took him to adapt to the weight of the penny. Which touch receptors adapted to the weight of the penny?
Add two pennies on top of the first one. Ask the subject to tell you when he can no longer feel their weight. Record the time. Which touch receptors took longer to adapt to the weight?
Next, determine how rapidly sensory adaptation occurs in hair root plexuses. Ask the subject to close his eyes. With the point of a pencil bend one hair. A steady hand is essential for this test. Measure the time it takes for the touch stimulation to disappear from the hair. Record the results.
Describe how you performed the tests on the subject, and the test results.
Results: Generate a single word document for Parts A, B, C and D.
Submission: Create your word document using word-processing software, such as Microsoft Word and save it to your computer as a .doc, .docx, or .rtf file. To submit the report, choose the link titled, Lab 13: Touch Sensations (Somatic Sensations), above. Use the “Browse My Computer” button in the Attach File area to attach your document. Be sure to complete your submission by choosing the “Submit” button at the bottom of the screen.

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