1. Special Senses
In this exercise you will conduct a brief review of eye and ear anatomy. You will also work with a partner to perform multiple special senses tests. The lab consists of three special senses; 1) Vision 2) Hearing 3) Equilibrium

2. Vision
The axons of the ganglion cells of the retina converge as the optic nerve.
There are no photoreceptors at the optic disc which is an area called the blind spot.
At the optic chiasm, some of the axons of the optic nerves cross over to the opposite side and extend as optic tracts.
The optic tract synapse with neurons in the thalamus and end in the visual cortex in the occipital lobe.
Axons from medial retina cross over at optic chiasm, lateral axons remain on the same side

3. Vision Each eye receives light from both left and right hemifields.
The yellow represents the left hemifield and the blue represents the right hemifield.

4. Experimental Procedures for Vision
Blind Spot Test
This is a test for the presence of the blind spot or optic disc.
To test the right eye, close the left eye and stare at the plus sign with the right eye. You should be aware of the dot in the periphery of your vision.

5. Vision
When light rays pass from one medium to another, their speed of transmission changes and the rays are bent, or refracted.
Therefore, the light rays are refracted when they encounter the cornea, lens, and vitreous humor of the eye.
The lens refractive index, or bending power, can be varied based on the lens shape. The greater the lens convexity, or bulge, the greater the light will refract.

6. Vision
Ciliary muscles
Suspensory ligaments

7. Vision
The lens becomes more convex when focusing light from a near object onto the retina.
This occurs when the ciliary muscle contract, decreasing the tension on the suspensory ligament attached to the lens and allowing the lens to “plump up”. Making these adjustments is referred to as, accommodation.

8. Accommodation

9. Eye Movement
The intrinsic muscles of the eyes include the ciliary muscles, and muscles of the iris.
The extrinsic muscles control eye movement and make it possible to keep moving objects focused on the fovea centralis. They are also responsible for convergence, or medial eye movement which is essential for near vision.

10. Experimental Procedures for Vision
Test For Near Point Accommodations
The closest distance at which one can see an object in sharp focus is called the near point. Move the page up to your eye until the first letter “T” becomes blurred; measure the distance from your eye to the page.
Eye Reflexes Test
Focusing for close vision requires that the eye make three adjustments. Look at a distant object, such as a wall across the room. Have your partner note the size of your pupils and the position of your eyeballs. Do the same with an object up close.

11. Vision Visual acuity is the sharpness of vision.
Astigmatism is a condition resulting is unequal curvature of either the cornea or lens. This condition prevents light rays from being focused with equal sharpness on the retina.

12. Experimental Procedures for Vision
Visual Acuity Test
Stand 20 feet away from the Snellen eye chart, cover one eye and attempt to read the line with the smallest letters that you can see.
Test for Astigmatism
Using the Astigmatism chart, cover one eye and focus on the circle in the center. If all the radiating lines appear equally dark and distinct, there’s no astigmatism.
20/20 is what most people can see clearly at 20ft away. 20/50 means you see at 20ft what most people can see from 50ft.

13. Color Vision
Color vision depends on impulses formed by three types of cones (receptors for red, green, and blue) in the retina.
Color blindness is more common in men than in women because the genes for the red and green color receptors are located on the X chromosome, of which males have only one and females have two.
The most common type of color blindness is red- green color blindness. A complete color blind person would see everything as shades of gray.

14. Experimental Procedures for Vision
Color Blindness Test
Have your lab partner hold up the color blindness test plates about 30 inches from your eyes.
Look at the color blindness test plate (Ishihara plates) for 3 seconds and state which number you see on the plate.

15. Hearing
Hearing begins as sound waves pass through the external auditory canal to the tympanic membrane through the middle ear into the inner ear, where the vibrations reach the spiral organ of Corti.

16. Hearing
Once the mechanoreceptors are stimulated they depolarize and begin the chain of neural impulses by way of the cochlear nerve to the auditory centers of the temporal lobe cortex.
Which cranial nerve is this? VIII

17. Hearing
Tone or pitch is determined by the frequency of the waves. Different frequencies stimulate receptors in different areas of the cochlea and brain.
Loudness is determined by the frequency of the nerve impulses that reach the brain which depends on the amplitude of the vibrations that produce these impulses.
High frequency waves result in high-pitch sounds detected close to the oval window. Low frequency waves result in low-pitch sounds detected near the apex of the cochlea.

18. Hearing
Conduction deafness is a result from blockage of sound waves from reaching the inner ear. This can be corrected by surgery or hearing aids and is detected with a Rinne test.
Nerve deafness is caused by damage to the sounds receptors or neurons that send the impulses to the brain. This usually is a result of loud noise and is not correctable.
A Weber test detects if there is a difference between the left and right ear simultaneously.
/Weber is the loss unilateral or bilateral
Rinne if AC > BC than normal. If BC > AC than conduction deafness.
Sensorineural. Sensory = cochlear; neural = nerve

19. Experimental Procedures for Hearing
Rinne Test
Produce vibrations in a tuning fork by holding it by the handle and striking it against the palm of your hand. Do not strike it against a hard object!
Place the handle of the tuning fork against the mastoid process of the temporal bone. When the sound is no longer audible, position the tines just outside of the external auditory meatus.
Compare air conduction (AC) to bone conduction (BC).
Rinne if AC > BC than normal. If BC > AC than conduction deafness.
Rinne test: A, Normal: sound is heard twice as long by air conduction (AC) as by bone conduction (BC); a “positive” Rinne, or AC > BC. B, Conductive loss: person hears as long by bone conduction (AC = BC) or even longer (AC < BC), a “negative” finding on the Rinne test. C, Sensorineural loss: normal ratio of AC > BC is intact but is reduced overall . That is, person hears poorly both ways. From Jarvis, 2000.

20. If AC > BC then hearing is normal
Rinne Test
If AC > BC then hearing is normal
If BC > AC then conduction deafness
If AC > BC but both are reduced then nerve deafness
Rinne if AC > BC than normal. If BC > AC than conduction deafness.
Rinne test: A, Normal: sound is heard twice as long by air conduction (AC) as by bone conduction (BC); a “positive” Rinne, or AC > BC. B, Conductive loss: person hears as long by bone conduction (AC = BC) or even longer (AC < BC), a “negative” finding on the Rinne test. C, Sensorineural loss: normal ratio of AC > BC is intact but is reduced overall . That is, person hears poorly both ways. From Jarvis, 2000.

21. Experimental Procedures for Hearing
Weber Test
Produce vibrations in the tuning fork and place the handle of the tuning fork against the top of the subject’s head.
If conduction deafness is present in one ear, the sound will be heard more strongly in the ear with hearing loss due to bone conduction of the skull.

22. Interpreting Results

23. Equilibrium
The equilibrium apparatus of the inner ear is in the vestibule and semicircular canals. The three semicircular ducts are involved in the mechanism of dynamic equilibrium.
When your head position changes in an angular direction, the endolymph in the canal lags behind, pushing the cupula in the opposite direction of the movement, bends the hair cells and creates an action potential.

24. Experimental Procedure for Equilibrium
Barany Test
Evaluates the semicircular canals and detects nystagmus, which is the trailing of the eyes slowly in one direction, followed by their rapid movement in the opposite direction.
If the semicircular canals are operating normally one will demonstrate nystagmus after rotation; abnormal otherwise.
Physiological nystagmus is a form of involuntary eye movement that is part of the vestibulo-ocular reflex (VOR), characterized by alternating smooth pursuit in one direction and saccadic movement in the other direction.

25. Experimental Procedures for Equilibrium
Romberg Test
Used for the clinical assessment of disequilibrium or ataxia from sensory and motor disorders
Equilibrium is maintained through proprioception, vestibular, and visual sensory input.
Should maintain position for 20 seconds with minimal swaying
Posterior white columns of the spinal cord.
Only two of the three systems are needed to maintain balance. When visual input is removed, instability due to lack of vision can be teased apart from other sensory impairments. A patient who has a problem with Proprioception (Somato sensory) can still maintain balance by compensating with vestibular function and vision.
A positive Romberg sign is present if the patient moves his/her feet away from the initial position, uncrosses his/her arms, or opens his/her eyes with the intention to maintain balance.A positive test is indicative of loss of the ascending proprioceptive function of the lower limbs. The sign may be observed in patients with peripheral neuropathy and proprioceptive changes as well as in patients with acute vestibular disorders.