Download Chapter 13b Special Senses

Chapter 13b
Special Senses
Lecture #35, 36
Objectives:
1. Describe the location, structure, and afferent pathways of taste and smell receptors,
and explain how these receptors are activated.
2. Trace the pathway of light through the eye to the retina, and explain how light is
focused for distant and close vision.
3. Describe the events involved in the stimulation of photoreceptors by light, and
compare and contrast the roles of rods and cones in vision.
4. Note the cause and consequences of astigmatism, cataract, glaucoma, hyperopia,
myopia, and color blindness.
5. Describe the sound conduction pathway to the fluids of the inner ear, and follow the
auditory pathway from the organ of Corti to the temporal cortex.
6. Explain how the balance organs of the semicircular canals and the vestibule help to
maintain dynamic and static equilibrium.
Taste: Tastebuds are the sense receptor organ. Found in papillae. Gustatory cells are the actual
receptors. Five tastes: sweet, soup, bitter, salty & umami. A chemical must be dissolved in saliva and
contact the gustatory hair in order to be tasted.
Smell: Olfactory epithelium in roof of nasal cavity. Olfactory receptor cells (bipolar neurons that are
replaced) have a specific protein for specific odorants. The odorant must also be volatile (a gas)
Structures of Eye
I. Conjunctiva: Transparent mucous membrane. Protects eye, produces a lubricating mucus
II. Lacrimal fluid: Contains mucus, antibodies, lysozyme. Cleanse, protects, moistens & lubricates.
III. Extrinsic eye muscles: Control the movement of the eye; strabismus (lazy eye), congenital
weakness of the muscles.
Pathway of Light (Refractive Media)
I. Cornea: Transparent anterior 1/6th of sclera, well supplied with nerve endings (most are pain
receptors) Very regenerative & transplantable.
II. Aqueous humor: Clear fluid similar to blood plasma, forms & drains continuously. Filters from
ciliary body and drains into canal of Schlemm. Maintains constant intraocular pressure.
III. Lens: Biconvex, transparent, flexible disc. Held in place by suspensory ligaments.
IV. Vitreous humor: A clear gel that also helps retina in place, forms embryonically & lasts a lifetime.
V. Fovea centralis/retina/rods and cones: Located directly behind the lens on the retina is the
fovea, which contains only cones which are for color vision & operate in bright light. Rods are dimlight &
peripheral vision receptors.
Distant vision: Ciliary muscles are relaxed & the lens is stretched thin & flat. 20ft plus
Close vision: Less than 20ft, the lens recoils & bulges as the ciliary muscles contract to pull ciliary body
up & in. Maximum bulge occurs @ 4”.
Eye Disorders
I. Astigmatism: Unequal curvatures of different parts of lens or cornea lead to blurred
vision.
II. Cataract: Clouding of lens due to age or excessive UV exposure.
III. Glaucoma: Intraocular pressure is too high (due to too much aqueous humor). Could damage
optic nerve
IV. Hyperopia: Farsightedness- eyball too short. Correct w/convex lens
V. Myopia: Nearsightedness- eyeball too long. Correct w/concave lens
VI. Color blindness: Lack of one or ore cone types. Sexlinked recessive trait.
Sound and mechanism of hearing



Sound is a pressure disturbance produced by a vibrating object and must travel through
a medium to be propagated.
Pitch is the frequency of a sound wave. The higher the frequency, the higher the
pitch of the sound we hear. Our range is from 1500 Hz – 4000 Hz.
Loudness is the amplitude or energy of the wave. It is measured in decibels (dB). Our
limit of loudness without pain is 120 dB. 130 dB causes pain. Prolonged exposure to
intensities greater than 90 dB will result in severe hearing loss, and employees
subjected to this must wear protection. Normal conversation is 50 dB, a noisy
restaurant is 70 dB and a rock concert is 120 dB.
Noise Limits
NOISE EXPOSURE TIME LIMITATIONS
Noise Level Exposure Limits
90 dB 8 hrs
95 dB 4 hrs
100 dB 2 hours
105 dB 1 hour
110 dB 30 minutes
115 dB 15 minutes
GUNFIRE NOISE LEVELS
156 dB .38 Special, 12 gauge 26" barrel
164 dB .44 Mag
170 dB .338 Rifle with muzzel brake
ENVIRONMENTAL NOISE LEVELS
140 dB Space rocket at blastoff
130 dB Jackhammer
120 dB Ambulance siren, Amplified rock band, Thunder clap
115 dB Sandblasting
110 dB Woodworking shop
100 dB Pneumatic drill, Chainsaw
90 dB Lawn mower, Disco dance music, Shop tools, Truck traffic, Noisy restaurant
80 dB City traffic, Loud music from radio
75 dB Kitchen appliances
70 dB Crowded restaurant
65 dB Conversation speech
60 dB Sewing machine, Typewriter
50 dB Average home interior
40 dB Quiet residential community
30 dB Whisper at five feet
20 dB Leaves rustling in a breeze
10 dB Normal breathing
0 dB Faintest sound heard by a human ear

Airborne sound enters external auditory canal, strikes tympanic membrane and vibrates
it with the same frequency.








The ear ossicles amplify the sound from the tympanic membrane and transfer it to the
oval window of the cochlea.
Perilymph, a fluid in the scala vestibuli, transmits the wave down toward the
helicotrema (the very end of the cochlea).
Low frequency or low pitched sounds (beyond our hearing) travel to the very end of the
cochlea, around the helicotrema, and back toward the round window through the scala
tympani (which also contains perilymph).
Higher pitched sounds take a shortcut through the cochlear duct and into the perilymph
of the scala tympani.
As the stapes pushes on the oval window, the fluid pushes out on the round window.
When waves pass through the cochlear duct, the basilar membrane (on the bottom, or
roof of scala tympani) vibrates. The highest pitched sounds set up this vibration near
the oval window. The lowest pitches we hear end up vibrating the opposite end of the
cochlea.
The Organ of Corti, which sits on top of the basilar membrane in the cochlear duct,
then receives the sound. The hair cells in the endolymph of the cochlear duct then
move up and down with the movement of the basilar membrane. They strike the tectorial
membrane above them, and as the cilia are displaced, the hair cells release a
neurotransmitter that stimulates the cochlear nerve endings to send the signal to the
brain.
Disorders of hearing include: conduction deafness, when something hampers sound
conduction to the fluids of the inner ear (like earwax, perforated eardrum, middle ear
infections and otosclerosis); sensorineural deafness, from damage to the neural
structures (hair cells, etc) that can occur from loud noises (can be somewhat treated
with a cochlear implant); tinnitus (ringing in ears), a symptom of a greater
pathology, such as cochlear nerve degeneration, or from inflammation or side effect of
medication; and Meniere’s syndrome, where both the semicircular canals and cochlear
are affected, causing vertigo, nausea ,vomiting, tinnitus, and hearing impairment.
Mechanism of equilibrium and orientation


Static equilibrium, or the sense of head position in space with respect to gravity (up
and down and side to side) is sensed by structures in the vestibule called the sacule
and utricle.
Vestibular nerve fibers innervate hair cells in each of these structures. The hair
cells themselves are covered by an otolithic membrane, studded with tiny stones called
otoliths.


As your head moves from side to side, the otoliths in the utricle move over the hair
cells and a stimulus is produced. As you move up and down, the otoliths move over the
hair cells in the saccule and a stimulus is produced.

The maculae in both the saccule and utricle respond only to changes in acceleration or
velocity of head movement and therefore help us maintain normal head position with
respect to gravity.

Dynamic equilibrium, or the sensing of angular or rotational movements of the head in
space, is detected by the crista ampullaris found at the base of each semicircular
canal.

Each crista has a conelike membrane called the cupula over hair cells. As we rotate,
the endolymph inside each of the semicircular canals flows in the direction opposite
to our body movement (because of inertia). The hair cells are bent and fibers of the
vestibular nerve are stimulated.

If we keep moving in the same direction, the endolymph finally stops moving and we
will no longer sense movement until we stop (and the endolymph keeps going).

Response to equilibrium signals is totally reflexive, because if we had to think about
it, we would fall down all the time!