2.14: Lab Exercise 16- Tactile Sensation

Lab Summary: This lab will give you an opportunity to study sensory receptors, explore two-point discrimination testing, localization of tactile sensations, and to compare relative sensitivity of different body parts based on these experiments.

Your objectives for this lab are:

• Describe the function(s) and locations of nociceptors, free nerve endings, Paccinian corpuscles, Ruffini endings, Meissner’s corpuscles, Merkel discs, hair follicle receptors, muscle spindles, Golgi tendon organs, and proprioceptors.
• Explain the two-point discrimination test, and
  • state its anatomical basis
  • explain what a small vs large discrimination distance tells you about the distribution of cutaneous receptors
• Define tactile localization, and describe how this ability varies in different areas of the body
• Define adaptation, and describe how this phenomenon can be demonstrated
  • Explain which types of receptors do adapt
  • Explain which types of receptors do not adapt and why this is beneficial to homeostasis and survival
• Define referred pain, and give an example of it

Background

Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, nociceptors (pain receptors), and chemoreceptors. These categories are based on the nature of the stimuli that each receptor class transduces. You can think of transduction as the receptor “translating” the stimuli it is capable of receiving into electrochemical language that’s understood by neurons.

Somatosensation is considered a general sense, as opposed to the special senses. This means that its receptors (Figure \(\PageIndex{1}\)) are not associated with a specialized organ but are instead spread throughout the body in a variety of organs. On the other hand, special senses are vision, hearing, taste, and smell. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia (awareness of movement and position). Many of the somatosensory receptors are located in the skin, but these receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.

Receptors in the periphery of the body allow us to understand stimuli in the environment, proprioception, and protect ourselves from harm. Receptors that are internal (interoceptors) allow us to understand, consciously or unconsciously, the inner activities of the body and respond appropriately to maintain homeostasis. Areas of the skin that have more mechanoreceptors are referred to as being “more sensitive” than areas with fewer receptors. Touch receptors are more dense in glabrous skin (found on human fingertips and lips, for example), which is typically more sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm). Thus, the fingers, which require the ability to detect fine detail, have many, densely-packed (up to 500 per cubic cm) mechanoreceptors with small receptive fields (around 10 square mm); in contrast, the back and legs, have fewer receptors with large receptive fields. Receptors with large receptive fields usually have a “hot spot”: an area within the receptive field (often in the center, directly over the receptor) where stimulation produces the most intense response. Tactile-sense-related cortical neurons have receptive fields on the skin that can be modified by experience or by injury to sensory nerves, resulting in changes in the field’s size and position. In general, these neurons have relatively large receptive fields (much larger than those of dorsal root ganglion cells). However, the neurons are able to discriminate fine detail due to patterns of excitation and inhibition relative to the field, which leads to spatial resolution.

Figure \(\PageIndex{1}\): Summary of Tactile Receptors

Activity 16.1: Two-point Discrimination Test

The two-point discrimination test looks at the relative distribution of tactile receptors in particular areas of the skin. As you read above, sensitivity is a function of the number appropriate receptors present in an area. “Appropriate” means that there is the correct receptor for the stimulus applied during the test. The more appropriate receptors there are, the more likely it is that the subject will be able to discern two points even when the points are close together. As you test each area, consider the survival advantages to relatively high or low sensitivity.

Procedure for Activity 16.1: Two-point Discrimination Test

1. Obtain a two-point discrimination tool.
2. Have your subject close their eyes or look away, so that they cannot see where you are touching the skin or the two-point discrimination tool.
3. Place the two points of the discrimination tool as close together as possible. The distance between the points is ~ 0mm at this point. Starting at the wrist, touch the skin with the two-point discrimination tool. Ask the subject if they felt one or two points.
4. Now, continue to spread the tool’s points gradually apart and touch the subject in the same area. Each time, ask the subject if they felt one or two points. When the subject can feel two points, the trial is complete; this distance is the two-point discrimination threshold. Record this distance in Table A below.
5. Repeat Steps 2—4 for each area listed in Table A below and record the results. This must be done on the same subject.
6. After you complete the procedure for all areas,
   • Put a double circle around the name of the area that you deem most sensitive based on the two- point discrimination tests.
   • Put a single circle around the area that you deem least sensitive based on the results of the two- point discrimination tests.

Table \(\PageIndex{1}\): Results of Two-Point Discrimination Tests (mm)

Body Area	Subject 1 Threshold (mm)	Subject 2 Threshold (mm)
Thigh or calf (anterior or posterior)
Wrist (anterior)
Palm (anterior)
Upper arm
Tip of the thumb
Tip of the index finger
Cheek
Posterior neck
Lip
Optional:  If your subject has a tattoo, try testing the tattooed area vs its non-tattooed counterpart

Activity 16.2: Tactile Localization Test

As with the two-point discrimination test, the tactile localization test looks at the relative distribution of tactile receptors in particular areas of the skin. Remember that sensitivity is a function of the number appropriate receptors present in an area. The more appropriate receptors there are, the more likely it is that the subject will be able to accurately localize (point to) an area that was touched. As you test each area, consider the survival advantages to being able to localize touch accurately.

Procedure for Activity 16.2: Tactile Localization Test

1. Obtain two markers/pens of different colors and a small millimeter ruler.
2. Have your subject close their eyes or look away, so that they cannot see where you are touching the skin.
3. For each of the areas listed in Table B, follow these steps:
   • Use your marker to place an ink dot on the skin in that area.
   • Now, have the subject use a different colored marker and place their ink dot where they felt you place a dot. Try to do this quickly.
   • Repeat Steps 3a and 3b in the same general area
   • Have the subject measure the distance between the pairs of ink dots. Record the results in Table B and calculate the average.
4. After you complete the procedure for all areas,
   • Put a double circle around the name of the area that you deem most sensitive based on the tactile localization tests.
   • Put a single circle around the area that you deem least sensitive based on the results of the tactile localization tests.

Table \(\PageIndex{2}\): Results of Tactile Localization Test (mm)

Body Area	Trial 1 Distance Between Ink Dots (mm)	Trial 2 Distance Between Ink Dots (mm)	Trial 3 Distance Between Ink Dots (mm)	Average Distance Between Ink Dots (mm)
Thigh or calf (anterior or posterior)
Wrist (anterior)
Palm (anterior)
Upper arm
Tip of the thumb
Tip of the index finger
Cheek
Posterior neck
Lip

Procedure for Activity 16.3: Adaptation of Tactile Receptors

1. Obtain five metal washers and a timer/stopwatch. Warm the washers slightly in your hand if the lab is cold.
2. Have the subject sit comfortably and lay one arm on the lab bench. The anterior arm should be supine.
3. As you place one washer on the subject’s forearm, start your timer. When the subject can longer feel the weight of the washer, record this time in Table C in the Forearm Area #1, Trial 1 column. Leave the washer in place on the arm.
4. Repeat Step 3 at a different location on the arm. Record the time in the Forearm Area #2, Trial 1 column
5. Now, immediately stack three more washers on the first washer (there will now be 4 stacked on the first spot). awareness of the sensation has been lost at the second site, stack three more coins atop the first one and start the timer. When the subject can longer feel the weight of the washer, record this time in Table C in the Forearm Area #1, Trial 1 column.
6. Repeat Steps 2-5 on the opposite arm. Record all data in the Trial 2 columns.
7. In which trials did adaptation occur most quickly? Why do you think this occurred?
8. What do you think would happen if proprioceptors adapted within 3.5 minutes every time you stood up? Explain your hypothesis.

Table \(\PageIndex{3}\): Results of Tactile Receptor Adaptation Test (seconds)

Body Area	Adaptation Time for One Washer (seconds)		Adaptation Time for Four Washers (seconds)
	Trial 1	Trial 2	Trial 1	Trial 2
Forearm Area #1
Forearm Area #2			No data required	No data required

Activity 16.4: Role of Sensory Inputs in Maintaining Balance

Vestibulation, the ability to maintain balance, requires multiple sensory inputs. The cerebellum and somatosensory cortex of the parietal lobe constantly receive visual information from the eyes, proprioceptive information from the muscles and joints, auditory information from the ears (similar to echolocation performed by bats), and vestibulatory information about head position from semicircular canals of the ears. The motor cortices of the frontal lobe utilize this information to adjust muscle contraction and “right” body position. The descending motor information is further modified by the cerebellum. As you work through the procedures below, you may notice that your subject exhibits behaviors also seen in intoxicated people. Because alcohol (specifically ethanol) is a small, lipophilic molecule, it can easily cross the blood-brain barrier. Thus, it can cause aberrant signals from the frontal lobe, impairment of sensory input to the parietal lobe and cerebellum, and inhibits the ability of the cerebellum to interpret proprioceptive feedback, making it more difficult to coordinate body movements, such as walking a straight line, or guide the movement of the hand to touch the tip of the nose.

The components of the vestibulatory apparatus of the ear are highly complex. This is a short summary of how they work. A mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. Signals are then sent to the somatosensory cortex and cerebellum via the vestibulocochlear nerve (CN X).

As head position changes, otoliths (very small ear “stones”) in the utricle and saccule move over a gel-like membrane (Figure \(\PageIndex{2}\)). Signals about their movement are sent to the brain to help us interpret head position.

Figure \(\PageIndex{2}\): Linear Acceleration Coding by Maculae The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.

The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure \(\PageIndex{2}\)). At the base of each semicircular canal, hair cells respond to rotational movement, such as turning the head while saying “no” (Figure \(\PageIndex{3}\)). As the head rotates, fluid in the area lags, deflecting the cupula in the direction opposite to the head movement. By comparing the relative movements of the structures of the semicircular canals, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.

Figure \(\PageIndex{3}\): Rotational Coding by Semicircular Canals Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.

Procedure for Activity 16.4:  Role of Mechanoreceptors in Vestibulation

1. Before you begin, watch this quick video to learn about vestibular apparatus of the ear (mins 7:30 to end):  https://www.youtube.com/watch?v=Ie2j7GpC4JU
2. need a group of at least 3-4 people for this experiment.   One person will be the subject; two-three people will serve as spotters.  The spotter’s job is to spot the subject, making sure to protect the subject from injury, and to make observations throughout the experiment.
3. Have the subject stand still on both feet for 30 seconds.  This is your baseline reading.  Record your observations in the space below.  Observations could include how much the subject wobbles, how steadily they stand, if they change body position often or not at all to remain upright, how long can they hold the foot in the air (for steps 4—6).  You should also note if the person practices a skill or sport that requires a keen sense of balance (such as yoga or snowboarding).
4. Now, have the subject stand on one foot with the opposite knee and hip flexed at a 90-degree angle for as long as possible without tipping over.  If your subject is still steady at 3 minutes, end the trial.  Record your observations in the space below.  
5. Next, have the subject again stand on one foot with the opposite knee and hip flexed at a 90-degree angle AND close both eyes for as long as possible without tipping over.  If your subject is still steady at 3 minutes, end the trial.  Record your observations in the space below.
6. Lastly, have the subject stand on one foot with the opposite knee and hip flexed at a 90-degree angle AND close both eyes AND rapidly turn the head left and right (as if shaking the head to say no) for as long as possible without tipping over.  If your subject is still steady at 3 minutes, end the trial.  Record your observations in the space below.
7. Consider the observations gathered in these experiments.  Explain which sensory input (proprioception, vision, vestibular apparatus) appears to be most important in maintaining one’s balance?  If you’re not sure, talk to your instructor about how you could better isolate one particular sensory input, then try again.

Activity 16.5: Referred Pain Demonstration

You’ve probably heard that myocardial infarction pain is felt in the left arm and shoulder (common in men) and the left jaw (common in women); this is termed “referred pain” because the visceral sensations are felt in unexpected places, such as the arm, shoulder, or jaw. Depending on the organ system affected, the referred pain will project to different areas of the body. The location of referred pain is not random, but its exact cause is still the subject of contemporary research. Generally, it is thought that visceral sensory fibers (from the heart for example) enter into the same level of the spinal cord as the somatosensory fibers of

the referred pain location (arm, shoulder, and jaw). Thus, the brain misinterprets the sensations from the heart’s region as being from the brachial, axillary, or buccal regions.

Because the degree to which a person feels pain is subjective and influenced by psychosocial well-being, referred pain can be difficult to study objectively. Research methods include injection of 6% saline solution or capsaicin into muscles, but a significant drawback is sustained referred pain after the injections (Arendt- Nielsen & Svensson, 2001). Intramuscular electrical stimulation is another method used by scientists. This method is beneficial because instances of referred pain can be elicited quickly, and they dissipate quickly (Arendt-Nielsen & Svensson, 2001). However, in our laboratory, we’ll simplify the procedure considerably by using cold as a stimulus to induce referred pain.

Procedure for Activity 16.5: Referred Pain Demonstration

1. Obtain a container of ice water (deep enough to fit an elbow).
2. Make sure your timer is ready. Don’t keep the elbow submerged for more than 3 minutes.
3. At the same time, have the subject submerge their elbow in the ice water and start the timer.
4. In the space below, note your observations, including,

• a. how long it takes the subject to feel local pain?
• b. how long it takes the subject to feel referred pain and where is this pain felt?
• c. does the subject make any particular movements to tolerate or distract from the pain?

Activity 16.6: Make a Map of the Homunculus (OPTIONAL ACTIVITY—TRY THIS AT HOME)

In this optional activity, you can do this activity one of two ways:

1. Take your measurements from Activity 16.1 and plug these into the homunculus mapper. You’ll have to round up or down because the mapper used fixed measurements. –OR—
2. You can print the template and do the measurements, then plug these into the mapper.
3. Weblink:  https://brainmapper.org/experiment/

Additional Learning Resources: 

• Watch this video to learn about free nerve endings, Paccinian corpuscles, Ruffini endings, Meissner’s corpuscles, Merkel discs:  https://www.youtube.com/watch?v=KJB_WNR-DBw
• Watch this video to learn about proprioception and muscle spindles: https://www.youtube.com/watch?v=yKfpBGicqNQ
• Watch this video to learn about nociceptors, pain, and pain relievers: https://www.youtube.com/watch?v=9mcuIc5O-DE
• Watch this video to learn more about how balance is achieved:  https://www.youtube.com/watch?v=tUfnCqKt6-o