Transcript Structure of the Eye

Chapter 8
Special Senses
The Senses
General senses of
touch
Special senses
• Smell
 Temperature
• Taste
 Pressure
• Sight
 Pain
• Hearing
• Equilibrium
The Eye and Vision
 70% of sensory
receptors are in eyes
 Each eye has over a
million nerve fibers
 Protection for the eye
- bony orbit
- surrounding fat
Accessory Structures of the Eye
 Eyelids
 Eyelashes
 Meibomian glands
- modified sebaceous glands
- oily secretion to lubricate
 Ciliary glands Modified
sweat glands between eyelashes
 Conjunctiva
- Membrane lining eyelids; connects to eye
surface; secretes mucus to lubricate
Accessory Structures of the Eye
Lacrimal apparatus
 Lacrimal gland – produces
lacrimal fluid
 Lacrimal canals – drains
lacrimal fluid from eyes
 Lacrimal sac – provides
passage of lacrimal fluid
towards nasal cavity
 Nasolacrimal duct – empties lacrimal
fluid into the nasal cavity
Function of the Lacrimal Apparatus
 Properties of lacrimal
fluid
- Dilute salt solution (tears)
- Contains antibodies and
lysozymes
 Protects, moistens, &
lubricates the eye
 Empties into the nasal
cavity
Extrinsic Eye Muscles
 Muscles attach to the outer surface of
the eye
 Produce eye movements
Figure 8.2
Structure of the Eye
The wall is composed of three tunics
 Fibrous tunic –
outside layer
 Choroid –
middle layer
 Sensory tunic –
inside layer
Figure 8.3a
The Fibrous Tunic
 Sclera
 White connective tissue layer
 Seen anteriorly as the “white of the eye”
 Cornea
 Transparent, central anterior portion
 Allows for light to pass through
 Repairs itself easily
 The only human tissue that can be
transplanted without fear of rejection
Choroid Layer
 Blood-rich nutritive tunic
 Pigment prevents light from scattering
 Modified interiorly into two structures
 Cilliary body – smooth muscle
 Iris
 Pigmented layer that gives eye color
 Pupil – rounded opening in the iris
Sensory Tunic (Retina)
 Contains receptor cells (photoreceptors)
 Rods
 Cones
 Signals pass from photoreceptors via a
two-neuron chain
 Bipolar neurons
 Ganglion cells
 Signals leave the retina toward the brain
through the optic nerve
Retina
Neurons of the Retina
Figure 8.4
Neurons of the Retina and Vision
Rods
 Most are found towards the edges of the retina
 Allow dim light vision and peripheral vision
 Perception is all in gray tones
Cones
 Allow for detailed color vision
 Densest in the center of the retina
 Fovea centralis – area of the retina with only
cones
Optic disk (Blind spot) - No photoreceptor cells
Cone Sensitivity
 There are three
types of cones
 Different cones are
sensitive to
different
wavelengths
 Color blindness is
the result of lack of
one cone type
Figure 8.6
Lens
 Biconvex crystal-like structure
 Held in place by a suspensory ligament attached
to the ciliary body
Internal Eye Chamber Fluids
Aqueous humor - Watery fluid in chamber
between lens & cornea
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Reabsorbed into blood by the canal of Schlemm
Vitreous humor - Gel-like substance behind lens
 Keeps the eye from collapsing
 Lasts a lifetime and is not replaced
Lens Accommodation
 Light must be
focused to a point
on the retina for
optimal vision
 eye is set for
distance vision
(over 20 ft away)
 lens must change
shape to focus for
closer objects
Images Formed on the Retina
Figure 8.10
Visual Pathway
 Photoreceptors of retina
 Optic nerve
 Optic nerve crosses at the
optic chiasma
 Optic tracts
 Thalamus (axons form
optic radiation)
 Visula cortex of the
occipital lobe
Eye Reflexes
 Internal muscles controlled by
autonomic nervous system
- Bright light causes pupils to constrict (radial
and ciliary muscles)
 Viewing close objects causes
accommodation
 External muscles control eye movement
to follow objects
 Viewing close objects causes
convergence (eyes moving medially)
Cataracts
• lens which has become opaque or clouded
Causes? Diabetes, old age, pollution?
STRABISMUS
• visual defect in which the eyes are
misaligned and point in different
directions
• misalignment of the eyes
GLAUCOMA
• disease of the eye in which damage occurs to the optic
nerve, typically as a result of an elevated pressure within
the eye.
• . Damage to the optic nerve causes progressive loss in
peripheral vision and can eventually lead to blindness.
PTERYGIUM
• wedge-shaped fibrovascular growth of
conjunctiva that extends onto the cornea
• benign lesions that can be found on either
side of the cornea.
Astigmatism
• irregularity in the shape of the cornea or the
lens. Instead of being shaped round, the
cornea is shaped oval, causing a blurred
image at all distances.
• Patients may notice blurred or ghost images
close up or far away.
• present in various degrees
Hyperopia
• (Farsightedness) -unable to see near objects
without extreme focusing.
• images are formed behind the retina
• eye too short, or the refractive powers of
cornea & lens are too weak
Myopia
• (Nearsightedness)-Distant objects are unclear in
cases of myopia.
• condition of the eye in which images are formed in
front of the retina
• the eye is relatively too long or refractive powers of the
cornea & lens are too strong.
LASIK Surgery
The process:
• After your eye has been numbed with
"eye drop" anesthesia, an instrument
known as an eyelid speculum will be
positioned to hold your eyelids open.
You will remain awake and comfortable
throughout the procedure.
• A small suction ring will be placed
around the cornea and serves as a
platform for the microkeratome.
• The microkeratome separates the
surface layers of the cornea, and the
corneal flap is folded back.
• You will be asked to look at a target
light while the Excimer laser reshapes
the corneal tissue. A clicking sound can
be heard as each microscopic layer of
tissue is vaporized. This process will
last from seconds to minutes,
depending on the amount of correction
necessary.
• The corneal flap is then placed back
into its original position and allowed to
dry for a few minutes.
Keratoconus
• Corneal degenerative disorder
• Cornea becomes progressively thin and steep
• The front of the eye bulges.
Conjunctivitis
• “Pink eye”
• Infection of conjuctiva
• Caused by bacteria or virus
• Highly contagious
Color Blindness
Complimentary Colors
Stare at the flag for 30 seconds. Then look at a white
surface. What happens & why?
Brain interprets
Fooling your brain
Fooling your brain
http://www.michaelbach.de/ot/
Human Hearing
sound wave
vibrates ear drum
amplified by bones
converted to nerve
impulses in
cochlea
Organs of Hearing
 Organ of Corti
 Located within the cochlea
 Receptors = hair cells on the basilar
membrane
 Gel-like tectorial membrane is capable of
bending hair cells
 Cochlear nerve attached to hair cells
transmits nerve impulses to auditory cortex
on temporal lobe
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Slide 8.27a
Organs of Hearing
Figure 8.13
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Slide 8.27b
Mechanisms of Hearing
 Vibrations from sound waves move
tectorial membrane
 Hair cells are bent by the membrane
 An action potential starts in the cochlear
nerve
 Continued stimulation can lead to
adaptation
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Slide 8.28
Mechanisms of Hearing
Figure 8.14
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Slide 8.29
Organs of Equilibrium
 Receptor cells are in two structures
 Vestibule
 Semicircular canals
Figure 8.16a, b
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Slide 8.30a
Organs of Equilibrium
 Equilibrium has two functional parts
 Static equilibrium
 Dynamic equilibrium
Figure 8.16a, b
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Slide 8.30b
Static Equilibrium
 Maculae – receptors in the vestibule
 Report on the position of the head
 Send information via the vestibular nerve
 Anatomy of the maculae
 Hair cells are embedded in the otolithic
membrane
 Otoliths (tiny stones) float in a gel around
the hair cells
 Movements cause otoliths to bend the hair
cells
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Slide 8.31
Function of Maculae
Figure 8.15
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Slide 8.32
Dynamic Equilibrium
 Crista ampullaris –
receptors in the
semicircular canals
 Tuft of hair cells
 Cupula (gelatinous cap)
covers the hair cells
Figure 8.16c
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Slide 8.33a
Dynamic Equilibrium
 Action of angular head
movements
 The cupula stimulates the
hair cells
 An impulse is sent via the
vestibular nerve to the
cerebellum
Figure 8.16c
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Slide 8.33b
Basic Types of Hearing Loss
• Conductive Hearing Loss
– sound is not conducted efficiently through
the outer ear canal to the eardrum and the
ossicles of the middle ear.
• Sensorineural Hearing Loss
– damage to the inner ear (cochlea), or to the nerve
pathways from the inner ear to the brain.
Causes of Hearing Loss
Conductive:
• External Ear
– congenital malformation where pinna and ear canal fail to
form
– blockage in ear canal - foreign body or accumulated
cerumen (ear wax)
• Middle ear
– perforation in tympanic membrane (ear drum) from
trauma or disease
– otitis media (ear infection)
– broken ossicular chain due to head trauma or trauma to
the ear
Causes of Hearing Loss
Sensorineural:
•
•
•
•
•
•
•
•
•
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•
genetic disorders
hearing loss from aging
ototoxic drugs such as some antibiotics
cancer treatments - chemotherapy and radiation therapy
head trauma - fractured temporal bone
excessive noise exposure
diseases of the vascular system such as sickle cell anemia
kidney disease
Ménière's disease
acquired infections such as influenza, meningitis, mumps, and syphilis
Tumors of or near the nerve of hearing and balance
Prevention
• Beware of loud or dangerous recreational activities
– Limit exposure to things such as:
• Firecrackers
• Loud Concerts
• Shooting a gun
• Avoid situations where you have to shout to someone
less than an arm’s length away
• Keep volume of tvs, radios, stereos, and ipods low
• Avoid medications that are dangerous to your hearing
• Try and keep noise levels reasonable
Treatment
• Hearing Aids:
– Behind The Ear (BTE)
– In The Ear (ITE)
• In the Canal (ITC)
• Completely in The Canal (CIC)
Tinnitus
• Causes ringing, humming, buzzing, and
cricket-like sounds
• Can be in one ear, both ears, or the head
• No actual cure for it
Other Hearing Disorders
• Sudden Sensorineural Hearing Loss (SSHL), or
sudden deafness
– Rapid loss of hearing
• Ménière’s disease
– Causes dizziness, tinnitus, and hearing loss
• Usher Syndrome
– affects both hearing and sight
– Is inherited
Test Your Hearing
http://www.phys.unsw.edu.au/jw/hearing.html
http://www.noiseaddicts.com/2009/03/can-youhear-this-hearing-test/
http://www.betterhearing.org/hearing_loss/heari
ng_loss_simulator/index.cfm
Chemical Senses – Taste and
Smell
 Both senses use chemoreceptors
 Stimulated by chemicals in solution
 Taste has four types of receptors
 Smell can differentiate a large range of
chemicals
 Both senses complement each other
and respond to many of the same
stimuli
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Slide 8.34
Olfaction – The Sense of Smell
 Olfactory receptors are in the roof of the
nasal cavity
 Neurons with long cilia
 Chemicals must be dissolved in mucus for
detection
 Impulses are transmitted via the
olfactory nerve
 Interpretation of smells is made in the
cortex
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Slide 8.35
Olfactory Epithelium
Figure 8.17
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Slide 8.36
The Sense of Taste
 Taste buds
house the
receptor
organs
 Location of
taste buds
 Most are on
the tongue
 Soft palate
 Cheeks
Figure 8.18a, b
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Slide 8.37
The Tongue and Taste
 The tongue is covered with projections
called papillae
 Filiform papillae – sharp with no taste buds
 Fungifiorm papillae – rounded with taste
buds
 Circumvallate papillae – large papillae with
taste buds
 Taste buds are found on the sides of
papillae
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Slide 8.38
Structure of Taste Buds
 Gustatory cells are the receptors
 Have gustatory hairs (long microvilli)
 Hairs are stimulated by chemicals
dissolved in saliva
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Slide 8.39a
Structure of Taste Buds
 Impulses are carried to the gustatory
complex by several cranial nerves
because taste buds are found in
different areas
 Facial nerve
 Glossopharyngeal nerve
 Vagus nerve
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Slide 8.39b
Anatomy of Taste Buds
Figure 8.18
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Slide 8.40
Taste Sensations
 Sweet receptors
 Salty receptors
 Metal ions
 Sugars
 Saccharine
 Umami
 Some amino acids
 “beef taste”
 Sour receptors
 MSG
 Discovered by
 Acids
the Japanese
 Bitter receptors
 Alkaloids
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Slide 8.41
Taste Sensations
 Historically, the tip of the
tongue was believed to be
most sensitive to sweet and
salty substances, its sides to
sour, the back of the tongue to bitter
and the pharynx to umami.
 Actually there are only slight differences
in the locations of the taste receptors in
different regions of the tongue.
 Most taste buds respond to two, three,
four or even all five taste modalities.
Taste Sensations
Taste likes and dislikes
have a homeostatic value.
 Sugar & Salt – satisfies the body’s need for
carbohydrates and minerals
 Sour (acidic) – typically sources of Vitamin C
 Umami – guides the intake of proteins
 Bitter – many natural poisons and
spoiled foods
Developmental Aspects of the
Special Senses
 Formed early in embryonic development
 Eyes are outgrowths of the brain
 All special senses are functional at birth
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Slide 8.42