Living organisms have an awareness of changes in their environment. This awareness is sensed through the services of specialized cell or groups of cells known as sensory receptors. Sensory receptors are transducers, changing one form of energy to another form which the organism can recognize and interpret quantitatively. Some sensory receptors are distributed generally over the body surfaces, and are therefore termed general senses, such as temperature, pressure, and pain receptors. Other receptors are localized in specific areas, and are termed special senses, such as sight, hearing, and the sense of equilibrium. Because we are so familiar with theses senses, we often do not fully appreciate their sensitivity or complexity. In the first portion of this laboratory session, we will investigate various properties of both general and special senses and our perception of their stimulation.
Stimulation of these sensory receptors can often trigger a neurally induced response without our conscious perception or involvement. These responses are termed reflexes. Reflex responses are often measured clinically to evaluate the status of specific portions of the central nervous system. For example, initial evaluation of the location and extent of damage to the brain following a stroke or head trauma can be performed by determining the response to specific stimuli. In the second portion of this laboratory session, we will be investigating some properties of a few types of reflexes.
A. Taste qualities and topography of the tongue
Work in pairs, with one person closing their eyes and sticking out their tongue, and the other person applying test solutions and recording results. Pour the solutions into vials and label them.
1. wash out your mouth with water so that no taste residue remains
2. dip a cotton tipped applicator in the 4%, 8%, or 16% sucrose solution
3. rest the applicator briefly on the tip, front center, back center, and side of the tongue
4. record where the sweet taste is the most intense (++), less intense (+), and absent (-)
5. wash out your mouth with water
6. repeat, using a fresh applicator for each solution listed below and washing out your mouth between each test:
a. NaCl (10%)
b. vinegar (1% acetic acid - sour taste)
c. quinine (0.1% - bitter taste)
Dry the subject's tongue with a clean tissue and place a few granules of sugar (sucrose) on it. Can the sugar be tasted immediately? Explain why there is a latent period.
B. Taste threshold
This test will compare the lowest concentration of several substances which can produce a noticeable sweet taste. The experimenter will place three drops of a solution on the subject's tongue, and the subject will state whether or not the solution tastes sweet. The solutions should be used in increasing concentration and the experimenter should use water occasionally to avoid the subject's anticipation of the sweet taste. The following solutions will be available: solutions of sucrose (0.25%, 1.0%, 4.0%, 8.0%, 16%), glucose (0.25%, 1.0%, 4.0%, 8.0%, 16%), and fructose (0.25%, 1.0%, 4.0%, 8.0%, 16%). At what concentration is the sugar perceived? Rank the types of sugar by the sensitivity you display to them, with #1 being the most sensitive. Sucrose, or table sugar, is a disaccharide made up of one molecule of glucose and one of fructose. Is the taste sensation dependent on one or the other of the monosaccharides, or are both capable of producing the sensation?
C. Adaptation and off response (after image)
Place 6 weights taped together on your forearm and let it remain for a couple of minutes. The temperature of the weights must be near skin temperature and the forearm must remain motionless. How long did the sensation last? Now, using the attached tape, remove the weights and compare the sensation with that perceived on application of the weights.
D. Discrimination
Several receptor terminals of one type (e.g., Pacinian corpuscle) may be connected to one afferent neuron. Thus, adequate stimulation of any of the receptor terminals would result in localization of the stimulus to the sensory field covered by all of the interconnected terminals, not just the sensory field of the stimulated terminal. Using the points of a scissors, touch various parts of the skin on the back of the hand, fingertip, arm, cheek, and back of the neck, and determine the distance apart the points must be before they are perceived as two distinct points. Also, while the subject's eyes are still closed, touch the skin of one arm with a point of the scissors. Have the subject try to place the point of a pencil on the place stimulated. Measure, in mm, how far the spot was missed. Repeat for the other areas listed above.
E. Shifting of physiological zero
Thermal receptors (Ruffini's organ for heat, Krause's bulb for cold) respond to changes in temperatures rather than a fixed value of temperature. Both types of thermal receptors adapt moderately to new levels of temperature. Place the index finger of the right hand in 40oC water and the index finger of the left hand in 20o C water for two minutes. Then, place both fingers simultaneously in 30o C water. What sensations are felt in each finger?
F. Weber-Fechner Law
When motor or secretory nerve fibers are stimulated, the result is a contraction or secretion which can be measured quantitatively. When a sensory terminal is stimulated, however, the result is a sensation which cannot be measured quantitatively with any degree of accuracy. Although the sensation cannot be quantified, the strength of the stimulus applied to the sensory terminal can be measured accurately. In discriminating the effect on sensation of increasing or decreasing the stimulus, we cannot say that a given sensation is two times as strong as or one-half as strong as another, but we can note the smallest appreciable increase or decrease (i.e., difference) of the sensation. The Weber-Fechner Law states that: "Gradations in stimulus strength are discriminated approximately in proportion to the logarithm of stimulus strength, K = delta I/I, where K is a constant and I is stimulus strength. Thus, the ratio of the change in stimulus strength required to produce perception of a change remains essentially constant: a person may barely detect a 5 gm change in weight when holding 50 gm, or a 50 gm change in weight when holding 500 gm." This law holds true over a rather narrow range of stimulus intensities. To test this law for the sense of touch or pressure, have the subject close their eyes and place their hand palm upward in a comfortable position on the table, making sure that the fingers are resting on the table and that they remain in that position. The experimenter will place a paper box containing 5 weights on the distal phalanges of the middle and index fingers. When the subject has formed a clear impression of this weight, this box is quickly exchanged for a second box containing either 1 more or 1 less weight. If the subject does not notice a distinct difference, try again with a a box containing 2 more or 2 less weights. Repeat this experiment several times and obtain a mean value for the number of weights representing the least perceptible difference in weight. Repeat the experiment with 10 and 20 weights, respectively, in the first box, and compare the delta I/I values.
A. Visual acuity
To measure visual acuity, one calculates the size of the retinal image by drawing lines from the extremities of the object through the nodal point of the eye to the retina. The angle formed at the nodal point is called the visual angle. As an object recedes farther from the eye, the visual angle become smaller. The average diameter of cones in the fovea centralis is approximately 1.5 mm. Since a spot of light has a bright center and shaded edges, a person can theoretically distinguish two separate points of light if their centers lie 2 mm apart on the retina. This distance is formed by a visual angle of 26 sec of arc. In practice, the smallest visual angle at which two objects can be discriminated is about 1 min of arc. The Snellen test letter chart used for measurement of visual acuity is based on the fact that the letters, at the distance of 20 ft, subtend a visual angle of 5 min, and the lines that form the letters subtend a visual angle of 1 min of arc. At a distance of 20 ft, cover one eye with an index card and read the letters, a row at a time, down the chart until you have 2 or more mistakes per line. The previous line will give your acuity: The visual acuity = d/D, where d = the maximum distance at which you can read the line of letters, and D = the maximum distance at which a "normal" person can read the letters. Thus, 20/15 indicates better than normal acuity, while 20/30 indicates poorer than normal acuity. Record your visual acuity for each eye separately, then using both eyes together. Does binocular vision enhance visual acuity?
B. Blind spot
The blind spot of the retina is the portion of the retina where the optic disc occurs, where nerve fibers from the retina converge and exit the eyeball via the optic nerve. There are no rods or cones present in the optic disc, therefore, no image can be formed in this area. To test for the blind spot, place the end of a meter stick at a right angle to your chin and position the index card with the dot and cross on it so that the dot is to your right and the cross is directly in front of your right eye at a distance of 90 cm. Cover your left eye, focus on the cross with your right eye, and slide the card slowly toward your face until a point is reached where the cross is seen but the dot fades from view. Record this distance. Calculate the retinal distance from the optic disc to the fovea centralis using the following equation:
A/B = C/D or C = A x D/B
where : A = cm distance between the dot and cross on the card
B = cm distance between the card and the eye
C = cm distance between the optic disc and the fovea centralis
D = cm distance from the lens to the retina (use 2 cm)
Repeat the procedure using your left eye, focusing on the dot, with the cross to your left.
C. Astigmatism
Light rays travel at a velocity of about 300,000 kilometers per second through air. The velocity through other media differs. The refractive index of a transparent substance is calculated as the ratio of the velocity of light in air to that in the substance. The refractive index for a specific type of glass through which light travels at 200,000 kilometers per second is 300,000/200,000, or, 1.5. If light passes from one medium to another of different refractive index at an angle, the light ray is bent, or refracted. The degree of refraction depends upon both the difference in refractive index between the two media and the degree of the angle between the light beam and the surface between the two media. A measure of the amount of refraction produced by a given angle between two media is represented in terms of diopter. A spherical lens having a refractive power of +1 diopter will focus parallel light rays to a focal point 1 meter beyond the lens, and another lens having a refractive power of +10 diopters will focus parallel light rays to a focal point 0.1 meters beyond the lens. With the lens accommodated for distant vision (more than 20 feet away, so that the light rays are approaching parallel to each other), the total refractive power of the eye (cornea plus lens) is about 59 diopters. Because the lens is situated between two fluid layers (aqueous and vitreous humors) having closely related refractive indices, the refractive power of the lens is about +15 diopters, or about 25% of the total refractive power of the eye. If the lens were suspended in air, the refractive power would be about 150 diopters. In young children, accommodating the lens for near vision ( by contracting the ciliary muscles and allowing the lens to elastically recoil to a more spherical shape) can increase the refractive power to about +29 diopters, for a range of change of +14 diopters.
Astigmatism is an abnormal refraction in which a variable degree of refraction exists in different meridians of the eyeball due to abnormal curvature of the cornea. Using the astigmatism chart, test each eye for astigmatism by standing 20 ft away and focusing on the center of the chart: if astigmatism is present, one or more of the lines will appear darker or more distinct than the others.
D. Near point of accommodation
As an object is brought closer to the eye, the object is held in focus by contraction of the ciliary muscles, allowing the lens to elastically recoil, and by convergence (crossing) of the eyes. Associated with this accommodation reflex is constriction of the pupil, which results in increasing the depth of field of focus at the closer focusing distance. It also accounts for the fact that you require more light when working at a close focusing distance.
***(The following exercise can only be done in a dark room and even then it is quite difficult. You may want to try this at home but you will not be able to do it here.)
To demonstrate that the change in lens curvature occurs primarily on the anterior surface of the lens, have an observer hold a bright object in front of the eyes of an individual whose ciliary muscles are relaxed (focusing at a distance). The observer will see 3 reflections of the object in the subject's eye: A clear, small, upright image is reflecting from the cornea; a larger, fainter, upright image is reflected from the anterior surface of the lens; and a small, inverted image is reflected from the posterior surface of the lens (because of the pigmented epithelium in the back of the retina in the human, no light is reflected back toward the front of the eye from the retina). If the subject then shifts their focus to close vision, the large, faint, upright image becomes smaller and moves toward the other upright image while the other two images change very little. The change is due to the increase in curvature of the anterior surface of the lens, with little change occurring in the curvature of the posterior surface of the lens.***
As an object is brought closer to the eye, a point is reached where accommodation of the lens becomes maximum, and the lens cannot focus at a closer distance. This is known as the near point of accommodation. Throughout life, cells are added to the lens at the outer surfaces, moving the older cells to the interior of the lens farther away from the nutrient fluid which bathes the lens. These central cells age and die, becoming stiff and limiting the overall elasticity of the lens. With advancing age, the lens of the eye gradually loses its elasticity and capacity for accommodation and the near point of accommodation recedes, a process resulting in presbyopia. Objects must now be held at farther distance for focusing, as when reading. At 20 years of age, the near point focusing distance is about 3.5 inches , at age 30, it is 4.5, at age 40, it is 7, and by age 50, it is 21 inches. To measure the near point, place a meter stick at a right angle to your chin. Slide a card with a sharply defined letter toward you at eye level on the stick, keeping one eye closed. Measure the shortest distance from the cornea of the eye (actually, from your chin) to where the letter can be clearly seen. Repeat the test for the other eye.
E. Accommodation reflex
The process of accommodation involves reflex adjustment of lens diameter and reflexes of convergence and divergence, as well as reflex adjustment of pupillary diameter. Have the subject focus on the point of a pencil and then move the pencil toward the subject's eyes. Notice the convergence of both eyeballs and change in the size of the pupils. Move the pencil away from the subject's eyes and note divergence of the eyeballs and change in pupillary diameter. The convergence and divergence reflexes are important in the judgment of distance. Hold the point of a pencil at arm's length from the eye. Close one eye and attempt to touch the pencil point with the index finger of the opposite hand. The arm movement should begin from the side of the body. Repeat the procedure with both eyes open.
F. Color vision
Use "Ishihara's test for color-blindness" and follow directions in the book or on the copied sheets. One fact to keep in mind is that color sensations (more properly called "hues") are not physical properties of lights and objects but rather private and personal sensory experiences. We can show a "colored stimulus" to another person, but we cannot show him what we see when we look at it. A color-blind person and a color-normal person ma look at the same stimulus and get utterly different color sensations from it. That stimulus then "has" two hues at once, which is no paradox since neither color is in the stimulus, but is in the subjective consciousness of the individual. Neither person is wrong, and the "normals" are normal only because they are in the majority. The completely red-green "color-blind" person sees only two hues - blue and yellow, where the normal person sees at least seven - red, orange, yellow, green, blue, violet, and purple. Individuals and protanopia seen to be missing the photopigment sensitive to red, or long wavelengths in their cones. A person with deuteranopia lack the pigment sensitive to green, or the middle wavelength. Because there is no loss of visual acuity, it is assumed that these individuals possess a full complement of cones. A more common, less severe type of red-green color-blindness occurs when the various photopigments are all present, but are said to be "weak. " A "weak" red pigment is further characterized as protanomally, a "weak" green pigment as deuteranomally. The basis for this test is that the differently hued dots which make up the circles will appear to have within them different shapes according to whether the person has color deficiencies or not.
G. Eye movements
Conjugate eye movements are those in which the axes of the two eyes move together in the same direction, and disjunctive eye movements are those in which they move in different directions. An individual eye is said to be adducting when it rotates towards the midline and abducting when it rotates away from the midline. Therefore, in conjugate horizontal eye movements, one eye will be abducting while the other is adducting. In convergence or divergent movements (disjunctive), both eyes will be either adducting together. Saccadic eye movements are those in which the eyes "flick" from one point to another during inspection of a stationary object. Conjugate saccadic eye movements are known clinically as "shifts of gaze," and they are under voluntary control as one chooses to look from one point to another. These abrupt shifts of the eyes have very high velocities (hundreds of degrees per second) and short duration (tens of milliseconds), during which visual perception is blocked out to prevent blurring. While facing the subject, hold your index fingers about a foot apart. Observe the subject's eyes as they shift their gaze from one fingertip to the other; notice the sharp, jerky, all-or-none character of the saccadic. Have the subject make a smooth, continuous eye movement between the fingertips and notice that the movement will actually be a series of small saccadic flicks. Saccadic movements are the only conjugate eye movements that can be generated voluntarily in the absence of a smoothly moving target. Also, notice the apparent motionlessness of your own visual field as you shift your gaze around the room, despite the fact that the optical image is moving swiftly across your retinae with each saccade and hence your occipital "map" of the visual field is in flux.
H. Retinal vasculature
The eye fundus (back) of the eye can be examined through the pupil with the ophthalmoscope. Retinal blood vessels are arranged so that the largest ones emerge from the optic-nerve head and curve over the retina giving off branches. The immediate neighborhood of the fovea centralis is left clear of vessels. All of these vessels are embedded in the retinal tissue, and lie between the incoming light and the light sensitive cells. Each vessel lets through less light than the tissue around it, so, on the sensory cells right behind the vessel, all along its length, the illumination is reduced. This shadowing of these sensory cells allows them to dark-adapt somewhat and increase their sensitivity, so that the reduced light reaching them looks just as bright to them as if the blood vessel were not there. Because of this, our whole pattern of retinal arteries and veins is normally invisible to us despite the fact that they lie between the sensory cells and the source of light. Obviously, if we would illuminate the retina with light that struck it from an unusual angle instead of coming straight in through the pupil, the shadow of every blood vessel would be shifted over a little, so as to fall upon a line of sensory cells which were not adapted to it. These cells would see the vessel as a dark line and simultaneously a bright line would appear alongside, where the displacement of the shadow had uncovered some dark-adapted sensory cells. This slantwise illumination of the retina is easy to accomplish: Sit facing a dark area and close both eyes. Put the bulb end of a penlight right against the upper lid of one eye, as high up toward the bone as you can, and wiggle the penlight slightly and continuously. It may help to keep your eyes turned downward in their orbits. The light is now shining through the lid and through the wall of the eyeball, not through the cornea and pupil. You should see, apparently standing out in space before you, the whole pattern of the retinal vessels, in great detail. As the penlight wiggles, each vessel will be seen as a dark line, a bright line, or a combination of both. After its discoverer, this appearance is known as the "Purkinje vessel figure."
I. Craik blindness
Cover one eye with the palm of your hand, keeping the other eye open. Place a fingertip against the open eye at the outer corner of the lid. Press on the eye steadily through the lid until you see a "phosphene," a visual sensation resulting from mechanical stimulation of the photoreceptors. Do not increase the pressure, but maintain it until your vision begins to fade. Within a few seconds your visual field should recede, even though the eye is wide open. As soon as the eye has become entirely blind, remove the pressure and note that completely normal vision quickly returns. The pressure on the eyeball compresses the retinal blood vessels and the neurons of the retina are quickly brought to an ischemic (low blood flow), functionless state. If this increase in intraocular pressure was maintained for long periods of time as in glaucoma, the ischemia would lead to the death of neurons and visual loss. The visual receptors are still receiving light and their photochemical activity is unimpaired, but they cannot activate optic nerve axons.
J. Dominant Eye Determination
Most individuals do not make equal use of both eyes. One eye is usually relied on more heavily than the other, making it the dominant eye. To determine this, roll a sheet of paper into a tube about 4 cm in diameter. Using both eyes simultaneously, view some object across the room through the tube. Holding the tube steady, close and reopen first one eye and then the other. The eye that is most closely aligned with the object being viewed through the tube is the dominant eye.
A. Weber's test
Strike a tuning fork with a rubber mallet. Place the tip of the fork handle in the median line of the skull or forehead of the subject. If hearing is normal, the subject will hear the sound equally well in both ears. Perform the test again, with the subject stopping one external auditory canal with a finger. The sound should be louder on the corresponding side. Similar effects occur in conduction impairments "conduction deafness" because the sound is transmitted via bone vibrations of the skull. In sensory-neural impairments "sensineural deafness", the sound is heard better in the unstopped ear.
B. Rinne's test
Strike a tuning fork with a rubber mallet. Place the tip of the fork handle on the mastoid process, being careful not to touch the prongs of the fork against the ear or hand. When the subject no longer hears the sound, move the fork to a position next to the open ear, and the subject should again be able to hear the sound. Normally, the sound conducted by air transmission is heard several seconds longer than by bone conduction because the threshold for air transmission is lower. When this relationship is noted, Rinne's test is positive. Rinne's test is negative when bone conduction shows a lower threshold than air conduction (e.g., in middle ear damage).
A. Plantar reflex
Have your partner assume a supine position on a laboratory table. Remove the sock and shoe from one foot. The subject should firmly brace their feet by pressing down on the heels. Using a probe, stroke lightly the outer border of the sole of the foot from the heel to the origin of the toes. If the reflex is normal (negative Babinski sign), the toes will adduct and plantar flex. If the response is abnormal (positive Babinski sign), the toes will abduct and dorsiflex. A positive Babinski reflex is normally obtained in infants up to the age of six months and sometimes upon to age four years. This response in an adult may occur temporarily during sleep or in epileptics immediately following a seizure, but usually signifies a lesion in the corticospinal tracts or else peripheral nerve damage.
B. Achilles tendon reflex
Kneel on a chair with your feet relaxed. Have your partner strike the Achilles tendon sharply with a patellar hammer. This will cause the tendon to stretch the gastrocnemius muscle, initiating a reflex contraction of the muscle and plantar flexion of the foot.
C. Patellar reflex
Sit on the edge of a table with the legs relaxed. Have your partner strike the patellar tendon sharply with a patellar hammer. This will cause the quadraceps femoris muscle to stretch, initiating a reflex contraction of the muscle and resulting in extension of the leg. Repeat the reflex while the subject tightly clasps their hands and attempts to pull them apart.
D. Ciliospinal reflex
Sit down and look straight ahead. Have your partner lightly scratch the skin of the side of your neck using a dissecting needle (stress), and observe your pupils. Which one responds? Also shine the light in one eye, and observe the pupils. Which one responds?
(Note that if you stroke the left side, the left eye dilates and the right does not change. Furthermore, the light will cause both pupils to constrict. Remember that the parasympathetic system causes the pupil to constrict (muscle relaxation) and the sympathetic system causes the pupil to dilate (muscle contraction). Thus, parasympathetics has good contralateral innervation but the sympathetic system has poor cross over innervation.)
A. Optical Illusions
Misrepresentations of what we see are called optical illusions. Every person with normal eye sight experiences them. Awareness of these deceptions is very important for the architect, draftsperson, artist, decorator, designer, scientist, or mathematician. It is important for all of us to understand what we see if we are to interpret out surroundings correctly. By becoming more familiar with some of the common optical illusions, we should be better able to cope with the tricks our eyes can play on us.
Study the drawings for a while and then use the explanations on the back to explain the optical illusion.
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