Human Senses Experiment
How do we make sense of our surroundings? Obviously, we use our senses!
Our nervous system is critical in controlling and regulating all of our body functions. The central nervous system organizer is the brain. In order for the brain to complete its coordination, the nervous system must be able to detect things that are occurring both inside and outside of the body. The key to this detection is the presence of numerous and extremely sensitive sensors throughout the entire body.
The nervous system sensors can be roughly thought of as being of two types. The first group are those that are widely distributed throughout the skin and underlying tissues. These surface receptors are relatively simple in structure and are associated with the senses of touch, pressure, temperature and pain. There is also a second group of more complicated sensors that are parts of sensory organs and are responsible for the specialized senses of smell, taste, equilibrium, hearing and vision.
Although there are many different-appearing sensory receptors, they all have some features in common and are all tied to nerves in the nervous system. Different receptors are sensitive to different changes in the environment and may be completely insensitive to other forms of environmental change. Sensors are often categorized based upon their environmental sensitivity. Five general groups have been identified and will be used in this activity. The five groups sense the following: (1) chemical concentrations (chemoreceptors), (2) tissue damage (pain receptors), (3) temperature (thermoreceptors), (4) mechanical forces (mechanoreceptors) or (5) light intensity (photoreceptors). Various sensory tissues and specialized sense organs can respond to combinations of stimuli because they contain various combinations of the five different kinds of receptors.
Not all senses are easily studied in the laboratory. Safety considerations will limit experimentation. A variety of senses, however, will be discussed and some will be studied in your laboratory activities.
The skin is the largest organ of the body. It usually weighs about six to ten pounds. Three types of touch receptors sense pressure, pain and temperature. The concentration of these receptors varies over the surface of the body. The fingertips, tongue and lips are the most sensitive areas of the body because they contain the greatest concentration of nerve endings and sensors. Our sense of touch allows us to explore textures in our environment, alerts us to danger and is critical for our well-being.
The sense of smell is associated with sensory structures in the upper region of the nasal cavity. Smell is perhaps the most direct of our senses. Molecules “float” back into the nasal cavity behind the nose where they are detected by an estimated five million receptor cells. The receptor cells are unique to the nasal cavity region. If neurons in the eyes or ears are damaged, they are not replaced. Neurons in the nasal cavity, however, are replaced, but they are not completely replaced and over time the number of receptors decreases. Thus, as people age they experience a progressive diminishing of olfactory sense (about 1% per year). “Smells” are stored in the brain in long-term memory areas. Smell molecules stimulate these areas of the brain and can be recognized for years and even a lifetime. Smells can stimulate long-term memories of places or things associated with smells from previous times.
The sense of hearing centers around the specialized organ of hearing, the ear. The ear has a series of complex parts often described as the external, middle and inner ear. Each part of the ear contributes its own specialized function to the perception of sound. The key to sound reception is vibrating objects. The specialized receptors in the ear pick up the vibrations and send the impulses to the brain, which “hears” the sound. The quality of a sound (its pitch, quality and loudness) is what distinguishes one sound from another. The characteristics of sounds allow your brain to distinguish a train from a bicycle. Sound is caused by vibrating objects. When an object vibrates, it sends out waves that ripple out in all directions. The waves of sound travel to our ears where they make the eardrum vibrate. This moves the three tiniest bones in our body (the hammer, the anvil and stirrup). The cavity these bones are in is about ⅓" wide and ⅙" deep. The three bones press fluid in the inner ear against membranes, which brush tiny hairs that trigger nearby nerve cells. The nerve cells send the message to the brain, which then interprets the vibration as a specific sound.
We think of our eyes as “seeing” and think of our eyes as our sense of sight. But the truth is that our eye is really a specialized organ for gathering and focusing light. The interpretation of the light patterns in our brain is what is really the “seeing.” The rubbery, crystalline lens of the eye changes its shape as it focuses the light entering the eye. Lining the rear of the eyeball is a thin sheet, the retina, which has specialized photosensor cells (rods and cones) that report colors and light patterns to nerves leading to the brain. The brain ultimately interprets the signals and “sees” things. Where the optic nerve and optic blood vessels leave the back of the eyeball, there is an area of the retina with no receptors. Since there are no receptors in this area, it is essentially a “blind spot” in the back of the eye. You can experience the blind spot phenomenon in this activity.
Taste buds are the special organs of taste. They occur primarily on the surface of the tongue where they are associated with tiny elevations called papillae. Just as we can smell something only when it begins to evaporate (release molecules), we can taste something only when it begins to dissolve. As with our olfactory nerves, we slowly lose taste buds as we get older and our sense of taste deteriorates with age.
Scientists generally describe human taste perception in terms of five qualities: saltiness, sourness, sweetness, bitterness and unami. Some scientists have suggested that there might be additional basic tastes. Separate biochemical pathways seem to underlie each taste quality since we can distinguish these sensations when placed upon our tongue. Are there separate pathways for each type of chemical or are there different taste buds for each chemical taste? You can experiment with your own sense of taste to try to answer these questions.
Bitter solution, 1–3 mL
Salt solution, 1–3 mL
Smelling solution A, vial
Smelling solution B, vial
Sour solution, 1–3 mL
Sweet solution, 1–3 mL
Apple, fresh pieces without peel, ½" cubes
Cotton-tipped swabs, 4
Plastic cups, 4
Plastic spoons, 4
Potato, raw pieces without peel, ½ cubes
Extreme caution should be exercised when students are used as experimental subjects. Sense organs are some of the most sensitive and fragile parts of the body. If something doesn’t “feel” right, stop the activity. Follow all other normal laboratory safety rules. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron
Part I. Taste
- Secure a small sample of one of the four taste solutions (salty, bitter, sour, sweet) in a clean plastic cup. Use only enough to barely cover the bottom of the cup.
- Dip a new cotton-tipped swab into the test liquid. Drain the excess liquid from the tip by pressing it against the side of the cup. Once a swab is used, do not dip it in the test solution again.
- Touch the applicator to the subject’s tongue as shown in the diagrams of the tongue on the top of the Senses Worksheet. Place a plus (+) sign on the areas where the subject can taste the solution and a minus (–) where taste is not detected.
- When the areas have been tested for one solution, snap the applicator swab in half and discard it as directed by your instructor.
- Exchange roles with your partner and repeat the test using a new swab. Record the results on the appropriate person’s worksheet.
- Rinse your mouth with water and dispose of the cup as directed by your instructor.
- Repeat the procedure with each of the other three test solutions.
- Complete Part I of the Human Senses Worksheet.
Part II. Smell
- Open the vial containing Smelling Solution A. Slowly waft the air above the vial with your hand. Move your wafting hand closer to the vial until you can identify the smell (see Figure 1). Recap the vial.
- Answer the questions for vial A on the Human Senses Worksheet.
- Open the vial containing Smelling Solution B. This vial contains oil of cloves. Use a finger to close one nostril. With the other nostril, smell the oil of cloves by holding the vial about 1.5 cm from your nose. Exhale through your mouth and inhale through your nose. Have your partner note the time and determine how long it takes before you can no longer smell the cloves. (This phenomenon is calledolfactory fatigue.)
- After one minute, and at increasing one-minute increments, take the cover off the vial and determine the length of your recovery time (i.e., how long before you can smell cloves again in the same fatigued nostril).
- Record your times on the worksheet and complete Part II of the worksheet.
- Repeat steps 1–5 for your partner.
Part III. Smell and Taste
- Take turns being the test subject. The test subject should hold her nose, close her eyes and stick out her tongue.
- Use a clean plastic spoon to place a piece of either apple or potato on the subject’s tongue. The test subject should not close his mouth or chew. Determine if the test subject can distinguish between the food types by taste alone. Try at least three different trials using unknown samples for each trial.
- Complete Part III of the Human Senses Worksheet.
Part IV. Touch/Two-Point Threshold
- Obtain nine toothpicks. Tape two toothpicks together so that their tips are 5 mm apart (see Figure 2). Use the metric ruler to make sure the distance is accurate.
Using the same procedure, make a set of taped toothpicks that are 10, 15 and 20 mm apart. Allow the last toothpick to remain as a single toothpick probe.
- One person will act as the subject. Eyes of the subject must be closed at all times (blindfolds might be necessary). Gently touch the subject’s fingertip with any of the sets of toothpicks. Have the subject indicate whether you are using one or two toothpicks. Record their reaction on the worksheet. Use the sets of toothpicks in random order so that the subject will not know your pattern of testing. Do enough random tests to establish an “average” sensitivity rating for the skin area being tested (i.e., the average distance that is felt as one point for the test area).
- Repeat the same random testing procedure on the palm, back of the hand, inside of the forearm and the back of the calf.
- Switch roles and repeat steps 2–3 with the other person being the subject.
- Complete Part IV of the Human Senses Worksheet.
Part V. Sight
- Roll up a piece of notebook paper into a tube.
- Hold up the tube and look through the tube (with both eyes) at an object across the room. What happens to the object?
- Without moving the tube, close one eye. Answer the question on the worksheet. What happens to the object?
- Now close the other eye and open the closed eye. What happens to the object?
- Alternate opening and closing your eyes. Can you determine which of your eyes is dominant?
- Look at the images of the individuals at the top of the Visuals Worksheet. Follow the directions and complete the measurements indicated.
- Follow the directions on the Blind Spot Visual Chart on the bottom of the Visuals Worksheet and experience the blind spot phenomenon.
Part VI. Hearing
- Your teacher will do one or more demonstrations relating to hearing and sound.
- Consult your instructor for appropriate disposal procedures for all activities.