Physiology Lab: The Special Senses

In contrast to the small and widely distributed general receptors (touch, temperature, pressure and pain), the special sense receptors are large complex sensory organs (eyes and ears) or localized clusters of receptors (taste buds or olfactory epithelium). This lab focuses on the functional anatomy and physiology of each of the special sense organs, individually, but please keep in mind those sensory inputs in fact overlap.


Anatomy of the Eye

External Anatomy and Accessory Structures

The adult human eye is a sphere approximately one inch in diameter. Only about one-sixth of the eye's anterior surface is observable; a cushion of adipose tissue and the walls of the bony orbital protect the remainder.

Anteriorly each eye is protected by the eyelids. The medial and lateral junctions of the upper and lower eyelids are referred to as the medial and lateral canthus. A mucous membrane, the conjunctiva lines the internal surface of the eyelids and continues over the anterior surface of the eyeball to the outer edge of the cornea where it fuses with the corneal epithelium. The conjunctiva secretes mucus, which functions as a lubricant for the eyeball.

The eyelashes project from the edge of each eyelid. Modified sweat glands called ciliary glands lie between the eyelashes and also help in lubrication of the eyeball. The larger meibomian glands, locates posterior to the eyelashes secrete an oily substance.

The lacrimal apparatus consists of the lacrimal gland and a series of ducts. The lacrimal glands lie superior and lateral to each eye. They continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. The tears flush across the eyeball into the lacrimal canals medially, then into the lacrimal sac and finally into the nasoacrimal duct, which empties into the nasal cavity.

Six extrinsic eye muscles attached to the exterior surface of each eyeball control eye movement.

Name Controlling Cranial Nerve Action
Lateral retcus VI (abducens) Moves eye laterally
Medial rectus III (oculomotor) Moves eye medially
Superior rectus III (oculomotor) Elevates eye or rolls it superiorly
Inferior rectus III (oculomotor) Depresses eye or rolls it inferiorly
Inferior oblique III (oculomotor) Elevates eye and turns it laterally
Superior oblique IV (trochlear) Depresses eye and turns it laterally

Activity:

Observe the eyes of another student and identify as many structures as possible. Ask the student to look to the left. What extrinsic muscles produce this reaction for the:

Right eye:_____________________________

Left eye:______________________________

Now ask your subject to look superiorly. What two extrinsic muscles of each eye can bring about this motion?

Right eye:_____________________________

Left eye:______________________________

Activity:

Look closely into your lab partner's eye(s) and notice landmarks on the iris. Have him or her tilt the head from vertical to horizontal and notice the eyeball stays "upright" within limits. What are the limits? This would be mediated from the vestibula system to the oblique muscles.

Internal Anatomy of the Eye

The outermost fibrous tunic of the eye is a protective layer composed of dense connective tissue. This can be divided into two identifiable regions: the white sclera, also known as the white of the eye, makes up the majority of the fibrous tunic and the transparent cornea.

The middle tunic is referred to as the uvea or vascular tunic. The posterior region of the tunic, known as the choroid, is rich in blood and contains pigments that prevent the scattering of light within the eye. The choroid is modified to form the ciliary body to which the lens is attached. The iris is pigmented and the pupil is the rounded opening through which light passes.

The innermost sensory tunic of the eye is the retina. It contains the photoreceptors, rods and cones, which starts the electrical chain of events from photoreceptors to bipolar cells and then to ganglion cells. The photoreceptors are located all over the retina, except for where the optical nerve leaves the eyeball. This site is referred to as the optical disc or the blind spot.

The lens divides the eye into two segments. The anterior segment, found anterior to the lens, contains a clear watery fluid called the aqueous humor. The posterior segment behind the lens is filled with a more viscous fluid and is referred to as the vitreous humor.

Lateral to the each blind spot is the macula lutea, an area of high cone density. In its center is the fovea centralis, a minute pit approximately .5mm in diameter, which contains only cones and is the area of highest visual acuity.

Activity: Dissection of the Cow Eye

The cow eye is a typical mammalian eye, but it has specific adaptations characteristic to the grazing animals that have been domesticated. The eyes are located well to the side of the head, providing a total field of vision of approximately 350 degrees with a small binocular field of only 25 degrees. This arrangement allows for maximal alertness for monocular but not binocular vision. In humans, the frontal location of the eyes provides a total visual field of approximately 200 degrees with a binocular overlap of about 110 degrees. The eyes of the human are thus specialized for binocular depth perception of objects relatively close to the eyes.

The cow eye that has been provided for you has been fixed in Carosafe, a non-toxic version of formalin. Fixation of the specimen arrests decomposition and hardens the tissues by the coagulation of the protein material. The fixed eye is easier to handle and retains its form better, the unfixed eye being soft and partially collapsed owing to the loss of intraocular pressure. Fixation however, causes color changes in the tissues and causes loss of transparency of the cornea.

The dissection of the cow eye should be preformed by the guidelines in the accompanying handout.

Activity: Visual Tests and Experiments

The Blind Spot

A piece of paper is taped up. Here is what is on that paper. Have your right eye about 30 cm from the paper (you do the arithmetic). Keep your head horizontal. Fixate (point your fovea, Fig. 10-36, toward) the x. Move something easy to see outward until it disappears. Determine the visual angle (that is called trigonometry). The optic nerve exits the eye at about 15 degrees. It is blind because there are no receptors there (Fig. 10-35). If you want to do the left eye, tape the paper up the other way

Visual Acuity

Have your partner stand 20 feet from the posted Snellen eye chart and cover one eye. As your partner reads the lines, check for their accuracy. Record the line with the smallest-sized letters read. If itis 20/20 then the subject vision is normal. If the vision is recorded as any thing with a ratio less than one, 20/40 for example, then the vision acuity is recorded as less than normal. If the visual acuity ration is greater than one, then the subject has better than normal vision. Record your observations.


Right eye:_____________________________

Left eye:______________________________

Astigmatism

The astigmatism chart tests for defects in the refracting surfaced of the lens and/or cornea. View the chart first with one eye, then the other. Focus on the center of the chart. If all the radiating lines appear equally dark and distinct, your refracting surface surfaces are not distorted. If some of the lines are blurred and appear less dark than others, then some degree of astigmatism is present. Record your observations.


Right eye:_____________________________

Left eye:______________________________

Anatomy of the Ear

The ear contains the sensory receptors for hearing and equilibrium. It can be divided into three compartments: the outer ear, the middle ear and the inner ear. The outer and middle ear function in hearing only, while the inner ear functions in both equilibrium and hearing reception.

The outer ear is composed of the pinna and the external auditory canal. The pinna is the skin-covered cartilage region encircling the auditory opening. The sound waves that enter the external auditory canal eventually hit the tympanic membrane, commonly called the eardrum. The eardrum separates the outer ear from the middle ear.

The middle ear is a small fluid-filled chamber known as the tympanic cavity. Within the cavity are three small bones collectively known as the ossicles ( malleus, incus and stapes). These bones transmit the vibratory motion of the eardrum to the fluids of the inner ear via the oval window.

The inner ear is composed of many chambers referred to as the bony labyrinth. The three subdivisions of the bony labyrinth are the cochlea, vestibule and the semicircular canals.


Activity: Auditory Tests and Experiments

Acuity Test

Have your lab partner pack one ear with cotton and quietly sit with their eyes closed. Obtain a watch or ticking clock and hold it to your partners unpacked ear. Then slowly move the clock away from the ear until your partner indicates that he/she can no longer hear the ticking. Record the distance at which the ticking in inaudible.

Unpacked ear:_____________________________

Packed ear: ______________________________


Weber Test

Obtain a tuning fork and rubber mallet. (Do we have rubber mallets? How about those ones we used for the knee jerk reflex?) Strike the tuning fork with the mallet and place the handle of the tuning fork medially on your partner's head. Is the tone equally loud in both ears?

If it is equally loud in both ears then the subject has equal hearing or equal hearing loss. If sensineural deafness is heard in present in one ear, the tone will be heard in the unaffected ear but not in the ear with sensineural deafness. If conduction deafness is present, the sound will be heard more strongly in the ear in which there is hearing loss.


Rinne Test

Strike the tuning fork and place its handle on your partner's mastoid process, the bone behind the ear and slightly lower. Have your partner indicate when he/she can no longer hear the sound. At that point move the fork with the still vibrating prongs close to his/her auditory canal. If your partner can hear the fork again (by air conductance) then hearing is not impaired. Repeat for both ears. Record your observations.

Right ear:_____________________________

Left ear: ______________________________


Repeat this test, except do the air conductance first and the bone conductance second. If the subject hears the tone after hearing by air conductance is lost, then there is some conductive deafness. Repeat for both ears.

Right ear:_____________________________

Left ear: ______________________________


Does the subject hear better by bone or by air conductance?

Frequency discrimination

At about 1000 Hz, you can tell the difference of a few Hz. This is explained by Helmholtz's place theory as modified by lateral inhibition as described in Bekesy's (1961) Nobel Prize winning work. You can get the audio oscillator calibrated to be slightly different from a tuning fork by listening for beats. At low frequencies, frequency discrimination is better explained by Rutherford's telephone theory. Here, frequencies to both ears can cause neural impulses that stay true to the frequency so that beats can be from neural comparison from the two ears.

Notice the oscillator is HP, a company that still does well. Also that it is from 1964. Another physics discard I rescued from the dumpster.

For both sets of instructions, set the amplitude to a comfortable level

(1) Set the oscillator at about 1000 (10 x 100)
(2) Hit the 1024 tuning fork and put it between your ear and the speaker
(3) Listen for beats, physical interference in sound waves
(4) Alternate the fork and the speaker to one ear
(5) If there are a lot of beats, the pitch difference should be obvious
(6) By adjusting the knob, you should be able to get the beats down to a few per second
(7) Notice that you can still tell the difference down to a few Hz at 1000 Hz
(8) Hold the speaker to one ear and the fork to the other
(9) Notice there are no beats

(1) Set it to >130 (>13 x 10)
(2) With the 128 fork, between speaker and ear, set the dial it so there are beats
(3) Notice beats
(4) With the speaker close to one ear, press that ear closed to prove you cannot hear with the other
(5) Hold the speaker to one ear and the tuning fork to the other and hear binaural beats
(6) Alternate the fork and the speaker to one ear
(7) If there are a lot of beats, the pitch difference should be obvious
(8) By adjusting the knob, you should be able to get the beats down to a few per second
(9) Notice that you can still tell the difference down to a few Hz at 1000 Hz

G. Oster , Auditory Beats in the Brain, Scientific American, Vol 229, October 1973, pp. 94-102


Anatomy of the Tongue

The superior tongue surface is covered with small projections known as papillae. There are three major forms of papillae: filiform, fungiform and circumvallate. The taste buds, specific receptors for the sense of taste are distributed throughout the oral cavity; however, the majority are located on the tongue.

Each taste bud consists of a globular arrangement of two types of epithelial cells: the gustatory (taste cells) that are the actual receptors and the supporting cells.

Several types of papilla including the circumvallate papillae on the back of the tongue, shown in this picture from our histology course
Within each papilla are numerous clusters of cells called taste buds shown in this histology picture. (Support cells, sensory cells, and basal cells are shown.)


Activity: Stimulating the Taste Buds

With the paper towels provided, dry the superior surface of the tongue. Place a few sugar crystals on your dried tongue. Do NOT close your mouth. Time how long it takes for you to taste the sugar.

Time:_________________

Why couldn't you taste the sugar immediately?



Activity: Effects of Olfactory Stimulation

There is no question that what is commonly tasted depends heavily on the sense of smell, particularly in the case of heavy scented substances. These following experiments should illustrate that.

Obtain paper cups with labeled ingredients. Ask the subject to sit so that he/she cannot see which sample is being used. Have the subject dry the superior surface of their tongue and hold their nose closed. Apply one drop of the substance on the subject's tongue. Can the subject distinguish the flavor?


Now have the subject open their nose and record the change in sensation.


Have the subject rinse their mouth out and dry their tongue. Prepare two swabs of differing substances. Hold one swab under the subject's nose and hold the other on their tongue. Record your observations.



Most of this lab has been adapted from E. N. Marieb's Essential of HumanAnatomy and Physiology Seocnd Edition.

This lab was prepared and added to the General Physiology Lab curriculum for Fall 2004 mostly from the efforts of Christine Zelle, Lab coordinator for upper division biology labs.


This page was last updated 9/17/04

Return to syllabus

Return to Stark home page