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Special Senses

The Senses
Special senses
Smell
Taste
Sight
Hearing
Equilibrium

The Eye and Vision
70 percent of all sensory receptors are in the eyes
Each eye has over a million nerve fibers
Protection for the eye
Most of the eye is enclosed in a bony orbit
A cushion of fat surrounds most of the eye

Accessory Structures of the Eye
Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus 
Extrinsic eye muscles
Eyelids 
Meet at the medial and lateral commissure (canthus)
Eyelashes
Tarsal glands produce an oily secretion that lubricates the eye
Ciliary glands are located between the eyelashes 
Conjunctiva
Membrane that lines the eyelids
Connects to the outer surface of the eye
Secretes mucus to lubricate the eye and keep it moist
Lacrimal apparatus = lacrimal gland + ducts
Lacrimal gland—produces lacrimal fluid; situated on lateral aspect of each eye
Lacrimal canaliculi—drain lacrimal fluid from eyes medially
Lacrimal sac—provides passage of lacrimal fluid towards nasal cavity
Nasolacrimal duct—empties lacrimal fluid into the nasal cavity
Function of the lacrimal apparatus 
Protects, moistens, and lubricates the eye
Empties into the nasal cavity
Lacrimal secretions (tears) contain:
Dilute salt solution
Mucus
Antibodies
Lysozyme (enzyme that destroys bacteria)
Extrinsic eye muscles 
Six muscles attach to the outer surface of the eye
Produce eye movements

Structure of the Eye
Layers forming the wall of the eyeball
Fibrous layer
Outside layer 
Vascular layer
Middle layer 
Sensory layer
Inside layer

The Fibrous Layer

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

Vascular Layer
Choroid is a blood-rich nutritive layer in the posterior of the eye
Pigment prevents light from scattering
Modified anteriorly into two structures
Ciliary body—smooth muscle attached to lens
Iris—regulates amount of light entering eye
Pigmented layer that gives eye color
Pupil—rounded opening in the iris

Sensory Layer
Retina contains two layers
Outer pigmented layer
Inner neural layer
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
Optic disc (blind spot) is where the optic nerve leaves the eyeball
Cannot see images focused on the optic disc
Neurons of the retina and vision 
Rods
Most are found towards the edges of the retina
Allow dim light vision and peripheral vision
All perception is in gray tones
Neurons of the retina and vision 
Cones
Allow for detailed color vision
Densest in the center of the retina
Fovea centralis–lateral to blind spot
Area of the retina with only cones
Visual acuity (sharpest vision) is here
No photoreceptor cells are at the optic disc, or blind spot
Cone sensitivity 
Three types of cones
Different cones are sensitive to different wavelengths
Color blindness is the result of the lack of one cone type
Lens
Biconvex crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body
Cataracts result when the lens becomes hard and opaque with age
Vision becomes hazy and distorted
Eventually causes blindness in affected eye
Risk factors include:
Diabetes mellitus
Frequent exposure to intense sunlight
Heavy smoking

Two Segments, or Chambers, of the Eye

Anterior (aqueous) segment
Anterior to the lens
Contains aqueous humor
Posterior (vitreous) segment
Posterior to the lens
Contains vitreous humor

Anterior Segment
Aqueous humor
Watery fluid found between lens and cornea
Similar to blood plasma
Helps maintain intraocular pressure
Provides nutrients for the lens and cornea
Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm
Posterior Segment
Vitreous humor
Gel-like substance posterior to the lens
Prevents the eye from collapsing
Helps maintain intraocular pressure

Ophthalmoscope
Instrument used to illuminate the interior of the eyeball
Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

Pathway of Light Through the Eye
Light must be focused to a point on the retina for optimal vision
The eye is set for distance vision 
(over 20 feet away)
Accommodation—the lens must change shape to focus on closer objects (less than 20 feet away)
Image formed on the retina is a real image
Real images are:
Reversed from left to right 
Upside down
Smaller than the object

Visual Fields and Visual Pathways
Optic chiasma 
Location where the optic nerves cross
Fibers from the medial side of each eye cross over to the opposite side of the brain
Optic tracts 
Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

Eye Reflexes
Internal muscles are controlled by the autonomic nervous system
Bright light causes pupils to constrict through action of radial, circular, and ciliary muscles
Viewing close objects causes accommodation
External muscles control eye movement to follow objects
Viewing close objects causes convergence (eyes moving medially)

A Closer Look
Emmetropia—eye focuses images correctly on the retina
Myopia (nearsighted)

Distant objects appear blurry 
Light from those objects fails to reach the retina and are focused in front of it
Results from an eyeball that is too long
Hyperopia (farsighted)
Near objects are blurry while distant objects are clear
Distant objects are focused behind the retina
Results from an eyeball that is too short or from a “lazy lens”
Astigmatism
Images are blurry
Results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens

Homeostatic Imbalances of the Eyes
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 
Due to the lack of one type of cone (partial color blindness)
Cataracts—when lens becomes hard and opaque, our vision becomes hazy and distorted
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
Houses two senses
Hearing 
Equilibrium (balance)
Receptors are mechanoreceptors
Different organs house receptors for each sense

Anatomy of the Ear

The ear is divided into three areas
External (outer) ear
Middle ear (tympanic cavity)
Inner ear (bony labyrinth)

The External Ear
Involved in hearing only
Structures of the external ear
Auricle (pinna)
External acoustic meatus (auditory canal)
Narrow chamber in the temporal bone
Lined with skin and ceruminous (wax) glands
Ends at the tympanic membrane

The Middle Ear (Tympanic Cavity)
Air-filled cavity within the temporal bone
Only involved in the sense of hearing
Two tubes are associated with the inner ear
The opening from the auditory canal is covered by the tympanic membrane
The auditory tube connecting 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)
Three bones (ossicles) span the cavity
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
Function
Vibrations from eardrum move the hammer  anvil  stirrup  inner ear

Inner Ear or Bony Labyrinth
Includes sense organs for hearing and balance
Filled with perilymph
Contains a maze of bony chambers within the temporal bone
Cochlea
Vestibule
Semicircular canals

Organs of Equilibrium
Equilibrium receptors of the inner ear are called the vestibular apparatus
Vestibular apparatus has two functional parts
Static equilibrium
Dynamic equilibrium
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
Dynamic Equilibrium
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
Tuft of hair cells covered with cupula (gelatinous cap)
If the head moves, the cupula drags against the endolymph
Action of angular head movements
The movement of the cupula stimulates the hair cells
An impulse is sent via the vestibular nerve to the cerebellum

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

Mechanism of Hearing
Vibrations from sound waves move tectorial membrane
Hair cells are bent by the membrane
An action potential starts in the cochlear nerve
Impulse travels to the temporal lobe
Continued stimulation can lead to adaptation
High-pitched sounds disturb the short, stiff fibers of the basilar membrane
Receptor cells close to the oval window are stimulated
Low-pitched sounds disturb the long, floppy fibers of the basilar membrane
Specific hair cells further along the cochlea are affected

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

The Sense of Taste
Taste buds house the receptor organs
Location of taste buds
Most are on the tongue
Soft palate
Cheeks

The Tongue and Taste
The tongue is covered with projections called papillae
Filiform papillae—sharp with no taste buds
Fungiform papillae—rounded with taste buds
Circumvallate papillae—large papillae with taste buds
Taste buds are found on the sides of papillae

Structure of Taste Buds
Gustatory cells are the receptors
Have gustatory hairs (long microvilli)
Hairs are stimulated by chemicals dissolved in saliva
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

Taste Sensations
Sweet receptors (sugars)
Saccharine
Some amino acids
Sour receptors
Acids
Bitter receptors
Alkaloids
Salty receptors
Metal ions

Developmental Aspects of the Special Senses
Formed early in embryonic development
Eyes are outgrowths of the brain
All special senses are functional at birth

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

Developmental Aspects of the Special Senses
Eye problems
Strabismus—“crossed eyes” results from unequal pulls by the external eye muscles in babies
Ophthalmia neonatorum—conjunctivitis resulting from mother having gonorrhea.  Baby’s eyelids are swollen and pus is produced 
Presbyopia—“old vision” results from decreasing lens elasticity that accompanies aging
Ear problems
Presbycusis—type of sensorineural deafness
Otosclerosis—ear ossicles fuse