Download Accessory Structures of the Eye

.The Senses
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Special senses
o Smell
o Taste
o Sight
o Hearing
o Equilibrium
The Eye and Vision
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70 percent of all sensory receptors are in the eyes
Each eye has over 1 million nerve fibers
Protection for the eye
o Most of the eye is enclosed in a bony orbit
o A cushion of fat surrounds most of the eye
Accessory Structures of the Eye
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Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
Accessory Structures of the Eye
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Eyelids
o Meet at the medial and lateral commissure (canthus)
Eyelashes
o Tarsal glands produce an oily secretion that lubricates the eye
o Ciliary glands are located between the eyelashes
Accessory Structures of the Eye
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Conjunctiva
o Membrane that lines the eyelids
o Connects to the outer surface of the eye
o Secretes mucus to lubricate the eye and keep it moist
Accessory Structures of the Eye
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Lacrimal apparatus  lacrimal gland and ducts
o Lacrimal gland—produces lacrimal fluid; situated on lateral aspect of each eye
o Lacrimal canaliculi—drain lacrimal fluid from eyes medially
o Lacrimal sac—provides passage of lacrimal fluid towards nasal cavity
o Nasolacrimal duct—empties lacrimal fluid into the nasal cavity
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Accessory Structures of the Eye
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Function of the lacrimal apparatus
o Protects, moistens, and lubricates the eye
o Empties into the nasal cavity
 Lacrimal secretions (tears) contain:
 Dilute salt solution
 Mucus
 Antibodies
 Lysozyme (enzyme that destroys bacteria)
Accessory Structures of the Eye
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Extrinsic eye muscles
o Six muscles attach to the outer surface of the eye
o Produce eye movements
Structure of the Eye
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Layers forming the wall of the eyeball
o Fibrous layer: outside layer
o Vascular layer: middle layer
o Sensory layer: inside layer
Humors are fluids that fill the interior of the eyeball
Structure of the Eye: The Fibrous Layer
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Sclera
o White connective tissue layer
o Seen anteriorly as the “white of the eye”
Cornea
o Transparent, central anterior portion
o Allows for light to pass through
o Repairs itself easily
o The only human tissue that can be transplanted without fear of rejection
Structure of the Eye: Vascular Layer
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Choroid is a blood-rich nutritive layer in the posterior of the eye
o Pigment prevents light from scattering
Modified anteriorly into two structures:
1. Ciliary body—smooth muscle attached to lens by ciliary zonule (suspensory
ligament)
2. Iris—regulates amount of light entering eye
 Pigmented layer that gives eye color
 Pupil—rounded opening in the iris
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Structure of the Eye: Sensory Layer
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Retina contains two layers
1. Outer pigmented layer absorbs light and prevents it from scattering
2. Inner neural layer
 Contains receptor cells (photoreceptors)
o Rods
o Cones
Structure of the Eye: Sensory Layer
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Signals pass from photoreceptors via a two-neuron chain
o Bipolar neurons
o Ganglion cells
Signals leave the retina toward the brain through the optic nerve
Optic disc (blind spot) is where the optic nerve leaves the eyeball
o Cannot see images focused on the optic disc
Structure of the Eye: Sensory Layer
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Neurons of the retina and vision
o Rods
 Most are found toward the edges of the retina
 Allow vision in dim light and peripheral vision
 All perception is in gray tones
Structure of the Eye: Sensory Layer
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Neurons of the retina and vision
o Cones
 Allow for detailed color vision
 Densest in the center of the retina
 Fovea centralis–lateral to blind spot
o Area of the retina with only cones
o Visual acuity (sharpest vision) is here
No photoreceptor cells are at the optic disc, or blind spot
Structure of the Eye: Sensory Layer
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Cone sensitivity
o Three types of cones
o Different cones are sensitive to different wavelengths
o Color blindness is the result of the lack of one cone type
Lens
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Biconvex crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body
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Lens
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Cataracts result when the lens becomes hard and opaque with age
o Vision becomes hazy and distorted
o Eventually causes blindness in affected eye
Risk factors include:
o Diabetes mellitus
o Frequent exposure to intense sunlight
o Heavy smoking
Two Segments, or Chambers, of the Eye
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Lens divides the eye into two chambers:
1. Anterior (aqueous) segment
 Anterior to the lens
 Contains aqueous humor
2. Posterior (vitreous) segment
 Posterior to the lens
 Contains vitreous humor
Anterior Segment
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Aqueous humor
o Watery fluid found between lens and cornea
o Similar to blood plasma
o Helps maintain intraocular pressure
o Provides nutrients for the lens and cornea
o Reabsorbed into venous blood through the scleral venous sinus, or canal of
Schlemm
Posterior Segment
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Vitreous humor
o Gel-like substance posterior to the lens
o Prevents the eye from collapsing
o Helps maintain intraocular pressure
Ophthalmoscope
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Instrument used to illuminate the interior of the eyeball and fundus (posterior wall)
Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina
Pathway of Light Through the Eye
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Light must be focused to a point on the retina for optimal vision
Light is bent, or refracted, by the cornea, aqueous humor, lens, and vitreous humor
The eye is set for distance vision
(over 20 feet away)
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Accommodation—the lens must change shape to focus on closer objects (less than
20 feet away)
Pathway of Light Through the Eye
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Image formed on the retina is a real image
Real images are:
o Reversed from left to right
o Upside down
o Smaller than the object
Pathway of Light Through the Eye
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The pathway of light through the eye:
1. Cornea
2. Aqueous humor
3. Through pupil
4. Aqueous humor
5. Lens
6. Vitreous humor
7. Retina
Visual Fields and Visual Pathways
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Optic chiasma
o Location where the optic nerves cross
o Fibers from the medial side of each eye cross over to the opposite side of the
brain
Optic tracts
o Contain fibers from the lateral side of the eye on the same side and the medial
side of the opposite eye
Visual Fields and Visual Pathways
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Overlap of the visual fields, and inputs from both eyes to each optic cortex provide for
depth perception
Pathway of Nerve Impulses into Brain
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The pathway of nerve impulses from the retina of the eye into the brain:
1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Thalamus
5. Optic radiation
6. Visual cortex in occipital lobe of brain
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Eye Reflexes
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Internal muscles are controlled by the autonomic nervous system
o Photopupillary reflex: bright light causes pupils to constrict through action of
radial, circular, and ciliary muscles
o Accommodation pupillary reflex: viewing close objects causes accommodation
Viewing close objects causes convergence (eyes moving medially)
A Closer Look
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Emmetropia—eye focuses images correctly on the retina
Myopia (nearsightedness)
o Distant objects appear blurry
o Light from those objects fails to reach the retina and are focused in front of it
o Results from an eyeball that is too long
A Closer Look
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Hyperopia (farsightedness)
o Near objects are blurry, whereas distant objects are clear
o Distant objects are focused behind the retina
o Results from an eyeball that is too short or from a “lazy lens”
A Closer Look
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Astigmatism
o Images are blurry
o Results from light focusing as lines, not points, on the retina because of
unequal curvatures of the cornea or lens
Homeostatic Imbalances of the Eyes
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Night blindness—inhibited rod function that hinders the ability to see at night
Color blindness—genetic conditions that result in the inability to see certain colors
o Due to the lack of one type of cone (partial color blindness)
Homeostatic Imbalances of the Eyes
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Glaucoma—can cause blindness due to increasing pressure within the eye
Hemianopia—loss of the same side of the visual field of both eyes; results from
damage to the visual cortex on one side only
The Ear
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Houses two senses:
1. Hearing
2. Equilibrium (balance)
Receptors are mechanoreceptors
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Different organs house receptors for each sense
Anatomy of the Ear
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The ear is divided into three areas:
1. External (outer) ear
2. Middle ear (tympanic cavity)
3. Inner ear (bony labyrinth)
The External Ear
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Involved in hearing only
Structures of the external ear
o Auricle (pinna)
o External acoustic meatus (auditory canal)
 Narrow chamber in the temporal bone
 Lined with skin and ceruminous (wax) glands
 Ends at the tympanic membrane (eardrum)
The Middle Ear (Tympanic Cavity)
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Air-filled cavity within the temporal bone
Involved only in the sense of hearing
Located between tympanic membrane and oval window and round window
The Middle Ear (Tympanic Cavity)
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Two tubes are associated with the inner ear:
1. The opening from the auditory canal is covered by the tympanic membrane
2. The pharyngotympanic, or auditory, tube connects the middle ear with the
throat
 Allows for equalizing pressure during yawning or swallowing
 This tube is otherwise collapsed
Bones of the Middle Ear (Tympanic Cavity)
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Three bones (ossicles) span the cavity:
1. Malleus (hammer)
2. Incus (anvil)
3. Stapes (stirrup)
Function
o Vibrations from tympanic membrane move the hammer  anvil  stirrup 
oval window of inner ear
Inner Ear or Bony Labyrinth
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Includes sense organs for hearing and balance
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Filled with perilymph
Contains a maze of bony chambers within the temporal bone:
o Cochlea
o Vestibule
o Semicircular canals
Membranous labyrinth is suspended in perilymph and contains endolymph
Organs of Equilibrium
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Equilibrium receptors of the inner ear are called the vestibular apparatus
Vestibular apparatus has two functional parts:
1. Static equilibrium
2. Dynamic equilibrium
Static Equilibrium
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Maculae—receptors in the vestibule
o Report on the position of the head
o Send information via the vestibular nerve
Anatomy of the maculae
o Hair cells are embedded in the otolithic membrane
o Otoliths (tiny stones) float in a gel around the hair cells
o Movements cause otoliths to bend the hair cells
Dynamic Equilibrium
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These receptors respond to angular or rotary movements
Crista ampullaris (in the ampulla of each semicircular canal)—dynamic equilibrium
receptors are located in the semicircular canals
o Tuft of hair cells covered with cupula (gelatinous cap)
o If the head moves, the cupula drags against the endolymph
Dynamic Equilibrium
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Action of angular head movements
o The movement of the cupula stimulates the hair cells
o An impulse is sent via the vestibular nerve to the cerebellum
Organs of Hearing
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Spiral organ of Corti
o Located within the cochlear duct
o Receptors  hair cells on the basilar membrane
o Gel-like tectorial membrane is capable of bending hair cells
o Cochlear nerve attached to hair cells transmits nerve impulses to auditory
cortex on temporal lobe
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Mechanism of Hearing
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Vibrations from sound waves move tectorial membrane
Hair cells are bent by the membrane
An action potential starts in the cochlear nerve
o Impulse travels to the temporal lobe
Continued stimulation can lead to adaptation
Mechanism of Hearing
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High-pitched sounds disturb the short, stiff fibers of the basilar membrane
o Receptor cells close to the oval window are stimulated
Low-pitched sounds disturb the long, floppy fibers of the basilar membrane
o Specific hair cells further along the cochlea are affected
Hearing and Equilibrium Deficits
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Deafness is any degree of hearing loss
o Conduction deafness results when the transmission of sound vibrations
through the external and middle ears is hindered
o Sensorineural deafness results from damage to the nervous system structures
involved in hearing
o Ménière’s syndrome affects the inner ear and causes progressive deafness
and perhaps vertigo (sensation of spinning)
Chemical Senses: Taste and Smell
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Both senses use chemoreceptors
o Stimulated by chemicals in solution
o Taste has four types of receptors
o Smell can differentiate a large range of chemicals
Both senses complement each other and respond to many of the same stimuli
Olfaction—The Sense of Smell
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Olfactory receptors are in roof of nasal cavity
o Olfactory receptors cells (neurons) with long cilia known as olfactory hairs
detect chemicals
o Chemicals must be dissolved in mucus for detection by chemoreceptors called
olfactory receptors
Impulses are transmitted via the olfactory filaments to the olfactory nerve
Interpretation of smells is made in the cortex
Taste Buds and the Sense of Taste
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Taste buds house the receptor organs
Locations of taste buds
o Most are on the tongue
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o Soft palate
o Cheeks
Taste Buds and the Sense of Taste
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The tongue is covered with projections called papillae
o Filiform papillae—sharp with no taste buds
o Fungiform papillae—rounded with taste buds
o Circumvallate papillae—large papillae with taste buds
Taste buds are found on the sides of papillae
Structure of Taste Buds
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Gustatory cells are the receptors
o Possess gustatory hairs (long microvilli)
o Hairs are stimulated by chemicals dissolved in saliva
Structure of Taste Buds
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Impulses are carried to the gustatory complex by several cranial nerves because taste
buds are found in different areas
o Facial nerve (cranial nerve VII)
o Glossopharyngeal nerve (cranial nerve IX)
o Vagus nerve (cranial nerve X)
Taste buds are replaced frequently by basal cells
Taste Sensations
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Sweet receptors respond to sugars, saccharine, some amino acids
Sour receptors respond to H ions or acids
Bitter receptors respond to alkaloids
Salty receptors respond to metal ions
Umami receptors respond to the amino acid glutamate or the beefy taste of meat
Developmental Aspects of the Special Senses
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Special sense organs are formed early in embryonic development
Maternal infections during the first 5 or 6 weeks of pregnancy may cause visual
abnormalities as well as sensorineural deafness in the developing child
Congenital ear problems usually result from missing pinnas and closed or missing
external acoustic meatuses
Developmental Aspects of the Special Senses
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Vision requires the most learning
The infant has poor visual acuity (is farsighted) and lacks color vision and depth
perception at birth
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The eye continues to grow and mature until age 8 or 9
Developmental Aspects of the Special Senses
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Eye problems
o Strabismus—“crossed eyes”; results from unequal pulls by the external eye
muscles in babies
o Ophthalmia neonatorum—conjunctivitis resulting from gonorrhea in the
mother; baby’s eyelids are swollen, and pus is produced
Developmental Aspects of the Special Senses
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Problems of aging associated with vision include presbyopia, glaucoma, cataracts,
and arteriosclerosis of the eye’s blood vessels
o Presbyopia—“old vision” results from decreasing lens elasticity that
accompanies aging
Developmental Aspects of the Special Senses
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The newborn infant can hear sounds, but initial responses are reflexive
By the toddler stage, the child is listening critically and beginning to imitate sounds as
language development begins
Age-related ear problems:
o Presbycusis—type of sensorineural deafness that may result from otosclerosis
 Otosclerosis—ear ossicles fuse
Developmental Aspects of the Special Senses
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Taste and smell are most acute at birth and decrease in sensitivity after age 40 as the
number of olfactory and gustatory receptors decreases
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