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CHAPTER 8SPECIAL SENSES
Special Senses
Special Senses: smell, taste, sight, hearing and equilibrium
The special sense receptors are either large, complex sensory organs (eyes/ears) or localized clusters of receptors (tastebuds/olfactory epithelium)
Special Senses
Remember, these senses are overlapping!
What you sense of “feel” is a blending of stimulus effects
THE EYE AND VISION
Anatomy of the Eye
The adult eye is a sphere about 1 in. in diameter
Only 1/6 of the anterior surface can be seen
The rest of the eye is covered and protected by fat and the walls of the bony orbit
Anatomy of the Eye
The accessory structures include:◦Extrinsic eye muscles◦Eyelids◦Conjunctiva◦Lacrimal apparatus
Anatomy of the Eye
The eyelids offer anterior protection of the eye
Meet at the medial and lateral commissure (canthus)
Anatomy of the Eye
The palpebral fissure is the space between the eyelids in an open eye
The eyelashes protect the borders of each eyelid
The tarsal glands are modified sebaceous glands associated with the eyelids
Anatomy of the Eye
The conjunctiva is a delicate membrane that lines the eyelids and covers part of the outer surface of the eyeball
Anatomy of the Eye
Conjunctivitis is an irritation and inflammation of the conjunctiva
Pinkeye is a infectious form of Conjunctivitis caused by bacteria or viruses
Anatomy of the Eye
The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity
The lacrimal glands are located above the lateral end of each eye
Anatomy of the Eye
The lacrimal glands continually release a salt solution (your tears!) onto the anterior surface of the eye
Tears flush across the eyeball into the lacrimal canaliculi medially, then into the lacrimal sac and finally into the nasolacrimal duct which empties into the nasal cavity
Anatomy of the Eye
Anatomy of the Eye
Lacrimal secretions are high in antibodies and lysozyme to help cleanse and protect the eye from foreign substances
Anatomy of the Eye
There are six extrinsic or external eye muscles
Attached to the outer surface to each eyeProduce gross eye movementMake it possible to follow objectsControlled by the cranial nerves
(abducens, oculomotor, trochlear)
Anatomy of the Eye
Anatomy of the Eye
The eye itself, commonly called the eyeball, is a hollow sphere
The wall is composed of three layersThe interior is filled with fluids called
humors that help maintain its shape
Anatomy of the Eye
The lens is the main focusing apparatus of the eye and is supported upright within the eye cavity, separating it into two chambers
Anatomy of the Eye
Three layers of the eyeball wall:
1. Fibrous
2. Vascular
3. Sensory
Anatomy of the Eye
1. The Fibrous LayerOutermost layerConsists of the sclera (protective layer)
and cornea (transparent layer)
Anatomy of the Eye
1. Fibrous LayerSclera - the white of the eye
◦Thick, connective tissueCornea – the window of the eye, where
light enters the eye◦Many nerve endings (pain fibers)◦Most exposed part of the eye◦Self-repairing
Anatomy of the Eye
Fun Fact: the cornea is the only tissue in the body that can be transplanted from one person to another without the worry of rejection
Because the cornea has no blood vessels, it is beyond the reach of the immune system
Anatomy of the Eye
2. Vascular Layer3 distinguishable regions
◦1. Choroid (blood-rich, contains dark pigment)
◦Prevents light scattering inside the eye◦2. Ciliary body (smooth muscle to which the lens is attached by the ciliary zonule)
◦3. Iris (pigmented part of the eye)◦Includes the pupil, where light passes
Anatomy of the Eye
2. Vascular LayerCircularly and radially arranged smooth
muscle fibers form the irisRegulates the amount of light entering the
eye to see clearly in available lightIn bright light/close vision the pupil
contracts, in dim light/distant vision the pupil dilates
Anatomy of the Eye
3. Sensory LayerTwo-layered retinaExtends anteriorly to the ciliary body
Anatomy of the Eye
3. Sensory LayerThe outer pigmented layer of the retina is
full of pigmented cells that absorb light and prevent light scattering
Also act as phagocytes, and store vitamin A
Anatomy of the Eye
3. Sensory LayerThe inner neural layer of the retina
contains millions of photoreceptors – rods and cones
Anatomy of the Eye
3. Sensory LayerElectrical signals
pass from the photoreceptors via a 2 neuron chain, then leave via the optic nerve
This nerve impulse is transmitted to the optic cortex and results in vision
Anatomy of the Eye
3. Sensory LayerPhotoreceptors are over the entire retina
except at the optic disc, where the optic nerve leaves the eyeball
This is your blind spot!
Anatomy of the Eye
Rods allow us to see in gray tones and dim light
Peripheral vision
More dense at the edges or periphery of the retina
Anatomy of the Eye
Night Blindness (Nyctalopia)
Interferes with rod function and limits the ability to see at night
Usually results from prolonged vitamin A deficiency◦Causes neural retina
deterioration
Anatomy of the Eye
Cones allow for details and color to be seen in bright light
Densest in the center of the retina
Anatomy of the Eye
Lateral to each blind spot is the fovea centralis, a tiny pit that contains only cones
Greatest visual acuity (sharpest vision)
Anatomy of the Eye
There are three varieties of cones:“blue”“green”“red” (actually responds to green and red)
Anatomy of the Eye
Impulses received at the same time from more than one type of cone by the visual cortex are interpreted as intermediate colors
Ex. both blue and red result in purple
Anatomy of the Eye
When all three types of cones are stimulated we see white
“mixing” of colors occurs in the brain, not the retina
Anatomy of the Eye
Color blindnessLack of one type of
cone leads to partial color blindness
Lack of all three cones leads to total color blindness
Sex-linked trait, almost exclusively in males
Anatomy of the Eye
Lens: a flexible biconvex crystal-like structure
Light entering the eye is focused on the retina by the lens
Lens is held upright by the ciliary zonule
Anatomy of the Eye
The lens divides the eye into two segments:
1. anterior (aqueous) segment◦Contains aqueous humor◦Provides nutrients for lens and cornea
2. posterior (vitreous) segment◦Filled with vitreous humor◦Keeps eyeball from collapsing inward
Both provide intraocular pressure
Anatomy of the Eye
Aqueous humor is reabsorbed into the venous blood through the scleral venous sinus, or the canal of Schlemm
Anatomy of the Eye
CataractsIn youth, the lens is transparent and a
hardened jelly-like textureAs we age, it becomes increasingly hard
and opaque
Anatomy of the Eye
Cataracts cause vision to become hazy and distorted, and eventually cause blindness
Risk factors: Type II diabetes, exposure to intense sunlight, heavy smoking
Current treatments: surgical removal, replacement lens implants or specialized glasses
Anatomy of the Eye
GlaucomaOccurs when the drainage of the aqueous
humor is blocked, and fluids back upPressure on the eye increases,
compressing the retina and optic nerve
Anatomy of the Eye
Glaucoma causes pain and possible blindness
Progresses slowly and painlessly until the damage is done
Tonometer is used to test intraocular pressure
Anatomy of the Eye
Glaucoma is commonly treated with eyedrops that increase the rate of drainage
Laser or surgical enlargement of the drainage channel can also be used
Anatomy of the Eye
An ophthalmoscope is an instrument used to illuminate the interior of the eyeball
Conditions such as diabetes, arteriosclerosis, and degeneration of the optic nerve and retina, can be detected by examination with an ophthalmoscope
Anatomy of the Eye
Ophthalmascope
Anatomy of the Eye
Ophthalmascope
Pathway of Light
When light passes from one substance to another substance of a different density, it’s speed changes and the rays are bent or refracted
Light rays are refracted in the eye when they encounter the cornea, aqueous humor lens and vitreous humor
Pathway of Light
The refractive powers of the corneas and humors are constant
The refractive power of the lens changes by changing its shape
Pathway of Light
The greater the lens convexity (bulge) the more it bends the light
The flatter the lens, the less it bends light
Pathway of Light
The resting eye is set for distant visionLight from a distant source (+20ft)
approaches the eye in parallel raysThe lens does not need to change shape
to focus
Pathway of Light
Light from a near source tends to scatter or diverge
The lens must bulge to make clear vision possible
In order to bulge, the ciliary body contracts, and the lens becomes more convex
Pathway of Light
Accommodation: the ability of the eye to focus specifically on close objects
Pathway of Light
The image formed on the retina as a result of the light-bending activity of the lens is a real image – it is reversed from left to right, upside down, and smaller than the object
The farther away the object is, the smaller its image on the retina
Pathway of Light
The normal eye can accommodate properly
Vision problems occur when the lens is too strong or too weak, or from structural problems
Pathway of Light
The eye that focuses images correctly on the retina is said to have emmetropia
Pathway of Light
Myopia is nearsightednessIt occurs when the parallel light rays from
distant objects fail to reach the retina and instead are focused in front of it
Results from eyeball that is too long, a lens that is too strong, or a cornea that is too curved
Requires concave corrective lenses
Pathway of Light
Hyperopia is farsightednessIt occurs when the parallel light rays from
distant objects are focused behind the retina
Lens is usually short or “lazy”Often subject to eyestrainRequires convex corrective lenses
Pathway of Light
Unequal curvatures in different parts of the cornea or lens cause astigmatism
In this condition, images occur because points of light are focused not on points on the retina but as lines
Special cylindrically ground lenses or contacts are used to correct this problem
Pathway of Light
Visual Fields and Visual Pathways to the Brain
Visual Fields and Pathways
The optic nerve is located in back of the eye, and carries impulses form the retina to the brain
Visual Fields and Pathways
At the optic chiasma the fibers from the medial side of each eye cross over to the opposite site of the brain
Visual Fields and Pathways
The fiber tracts that result are called the optic tracts
Each optic tract contains fibers from the lateral side of the eye on the same side and the medial side of the opposite eye
Visual Fields and Pathways
The optic tract fibers synapse with neurons in the thalamus, whose axons form the optic radiation.
This runs to the occipital lobe of the brain
Visual Fields and Pathways
There they synapse with the cortical cells, and visual interpretation, or seeing, occurs.
Visual Fields and Pathways
Each side of the brain receives visual input from both eyes
Lateral of the same side eye, medial form the other eye
Visual Fields and Pathways
Each eye “sees” a slightly different viewThe visual fields of each eye overlapHumans have binocular vision, “two-eyed”
visionAllows for depth perception as the visual
cortex fuses the two images
Visual Fields and Pathways
Hemianopia: the loss of the same side of the visual field in both eyes
Results from damage to the visual cortex
Visual Fields and Pathways
People who suffer from hemianopia are not able to see things past the middle of the visual field on either the left or right side
Eye Reflexes
Both the internal and external eye muscles are needed for proper eye function
The external muscles, as mentioned earlier, are responsible for following moving objects
They are also responsible for convergence
Eye Reflexes
Convergence: the reflexive movement of the eyes medially when viewing close objects
Both eyes are aimed toward the near object being viewed
Eye Reflexes
When the eyes are suddenly exposed to bright light, the pupils instantly constrict
This is called the photopupillary reflex
Eye Reflexes
The photopupillary reflex prevents excessively bright light from damaging the photoreceptors
Eye Reflexes
The accomodation pupillary reflex also constricts the pupil, but occurs when viewing close objects
It allows for more acute vision
Eye Reflexes
Reading requires almost continuous work by both sets of muscles
Ciliary fibers cause the lens to bulge, the iris constricts the pupil, convergence occurs
This is why eyestrain often occurs
The Ear
Ears are key to the senses of hearing and balance
Sound vibrations move fluids to stimulate hearing receptors
Gross movements of the head move fluids in the balance organs
The Ear
Receptors that respond to physical forces are called mechanoreceptors
These two sense organs are housed in the ear
They respond to different stimuli and are activated independently of one another
Anatomy of the Ear
Anatomically, the ear is divided into 3 major sections:
1. external (outer) ear - hearing2. middle ear - hearing3. internal (inner) ear – hearing and
equilibrium
External Ear
The external (outer) ear is composed of the auricle and the external acoustic meatus
External Ear
The auricle (pinna) is “the ear,” the shell-shaped structure surrounding the auditory canal opening
It collects and directs sound waves into the auditory canal
Function is weak in humans
External Ear
The external acoustic meatus (auditory canal) is a short, narrow chamber (1x.25 in) carved into the temporal bone of the skull
External Ear
The auditory external acoustic meatus has skin-lined walls with ceruminous glands which secrete cerumen (earwax)
It provides a sticky trap for foreign bodies and repels insects
External Ear
Sound waves entering the auditory canal eventually hit the tympanic membrane (eardrum) and cause it to vibrate
The canal ends at the eardrum, which separate the external from the middle ear
Middle Ear
The tympanic cavity or middle ear is a small, air-filled, mucosa-lined cavity within the temporal bone
Middle Ear
It is flanked laterally by the eardrum and medially by a bony wall with two openings, the oval window and the inferior, membrane-covered round window
Middle Ear
The pharyngotympanic (auditory) tube also called the eustachian tube runs obliquely downward to link the middle ear cavity with the throat, and the mucosae lining the two regions are continuous
Middle Ear
Normally the auditory tube is flattened and closed
Swallowing and yawning opens this tube briefly to equalize the pressure in the middle ear cavity
The eardrum does not vibrate freely unless the pressure on both surfaces is the same
Middle Ear
When the pressures are unequal, the eardrum bulges inward or outward making hearing difficult or causes earaches
The sensation of ear-popping is the equalizing of pressures
Middle Ear
An inflammation of the otitis media is common in children with sore throats
The eardrum becomes inflamed, bulges and often the cavity fills with fluid or pus
A myringotomy is sometimes required to relieve the pressure
A tiny tube can also be used for drainage
Middle Ear
Middle Ear
The tympanic membrane is spanned by the three smallest bones in the body, the ossicles
The ossicles transmit the vibratory motion of the eardrum to the fluids of the inner ear
Middle Ear
The ossicles are named for their shape:Hammer (malleus)Anvil (incus)Stirrup (stapes)
Middle Ear
When the eardrum moves, the hammer moves as well, and transfers the vibration to the anvil
The anvil passes the vibration to the stirrup
Middle Ear
The stirrup which presses on the oval window of the inner ear
The movement at the oval window sets the fluids of the inner ear into motion, eventually exciting the hearing receptors
How Hearing Works
Inner Ear
The internal ear is a maze of bony chambers called the bony, or osseous, labyrinth, located deep within the temporal bone behind the eye socket
Inner Ear
The three subdivisions of the bony labyrinth are the:
1. cochlea (spiral, pea size)2. vestibule3. semicircular canals
Inner Ear
The inner ear is a cavity filled with a plasmalike fluid called perilymph
Suspended in the perilymph is a membraneous labyrinth, a system of membrane sacs that more or less follow the shape of the bony labyrinth
Inner Ear
Suspended in the perilymph is a membranous labyrinth, a system of membrane sacs that more or less follow the shape of the bony labyrinth
The membranous labyrinth itself contains a thicker fluid called endolymph
How Hearing Works
Mechanisms of Equilibrium
Equilibrium is not an easy sense to describe
It responds to various head movementsThe equilibrium receptors of the inner ear,
are collectively called the vestibular apparatus
Mechanisms of Equilibrium
The vestibular apparatus can be divided into two functional aspects:
1. static equilibrium2. dynamic equilibrium
Static Equilibrium
Within the membrane sacs of the vestibule are receptors called maculae
The maculae are essential to the sense of static equilibrium: balance concerned with changes in the position of the head
Static Equilibrium
The maculae report on changes in the position of the head in space with respect to the pull of gravity when the body is not moving
Give information on which way is up or down
Help keep the head erect
Static Equilibrium
Each macula is a patch of receptor (hair) cells with their “hairs” embedded in the otolithic hair membrane, a jellylike mass studded with otoliths or “earstones,” tiny stones made of calcium salts
Static Equilibrium
As the head moves, the otoliths roll in response to changes in gravitational pull
The surrounding gel pulls, slides over the hair cells, bending the hairs
Static Equilibrium
This movement activates the hair cells, which send impulses along the vestibular nerve to the cerebellum of the brain, giving information on head position in space
Dynamic Equilibrium
Dynamic equilibrium receptors, found in the semicircular canals, respond to angular or rotary movements of the head
Dynamic Equilibrium
Within the ampulla, a swollen region at the base of each membranous semicircular canal is a receptor region called a crista ampulliaris or crista
Dynamic Equilibrium
The crista consists of a tuft of hair cells covered with a gelatinous cap called cupula
When your head moves in arclike/angular motion, the endolymph lags behind
Dynamic Equilibrium
As the cupula drags against the stationary endolymph the cupula bends with the body’s motion
Dynamic Equilibrium
This motion stimulates the hair cells, and impulses are transmitted up the vestibular nerve to the cerebellum
Bending the cupula in the opposite direction reduces impulse generation
Dynamic Equilibrium
When you are moving at a constant rate, the receptors gradually stop sending impulses, and you no longer have the sensation of motion until your speed or direction of movement changes
Dynamic Equilibrium
The receptors of the semicircular canal and vestibule are responsible for dynamic and static equilibrium separately, they usually act together
Dynamic Equilibrium
Besides these equilibrium senses, sight and proprioceptors of the muscles and tendons are also important in providing information used to control balance to the cerebellum
Mechanism of Hearing
Within the cochlear duct, the endolymph-containing membranous labyrinth of the cochlea is the spiral organ of Corti
This contains the hearing receptors, or hair cells
Mechanism of Hearing
The scalae, chambers, above and below the cochlear duct contain perilymph
Sound waves that reach the cochlea through vibrations of the eardrum, ossicles and oval window set the cochlear fluids in motion
Mechanism of Hearing
As the sound waves are transmitted by the ossicles, their force is increased by the lever activity of the ossicles
The total force exerted in the large eardrum reaches the oval window, which sets the inner ear fluids in motion
Mechanism of Hearing
The pressure waves set up vibrations in the basilar membrane
The receptor cells are stimulated when the hairs are bent or tweaked by the movement of the tectorial membrane
Mechanism of Hearing
The length of the fibers spanning the basilar membrane “tunes” specific regions to vibrate at specific frequencies
Shorter fibers are disturbed by high-pitched sounds
Longer fibers are disturbed by low-pitched sounds
Mechanism of Hearing
Once stimulated, the hair cells transmit impulses along the cochlear nerve to the auditory cortex
Mechanism of Hearing
Sound usually reaches the two ears at different times, or “in stereo”
This helps us determine the location of sounds
Mechanism of Hearing
We are able to habituate, or adapt, to constant, repetitive sounds
Mechanism of Hearing
Hearing is also the last sense to leave us when we fall asleep and the first to return when we wake up
Hearing and Equilibrium Deficits
Deafness: hearing loss of any degreeTwo main kinds:1. conduction2. sensorineural
Hearing and Equilibrium Deficits
Temporary or permanent conduction deafness results when something interferes with the conduction of sound vibrations to the fluids of the inner ear
Hearing and Equilibrium Deficits
Conduction deafness can be caused by
EarwaxOtosclerosis, the
fusion of the ossicles
Ruptured eardrum
Hearing and Equilibrium Deficits
Sensorineural deafness occurs when there is degeneration or damage to the receptor cells in the spiral organ of Corti, cochlear nerve or neurons in the auditory cortex
Hearing and Equilibrium Deficits
Hearing aids use the skull bones to conduct sound vibrations to the inner ear
They are generally very successful in helping people with conduction deafness to hear
They are less helpful for people with sensorineural deafness
Hearing and Equilibrium Deficits
Equilibrium problems are usually obvious; nausea, dizziness, and problems in maintaining balance
There are often strange, jerky eye movements
Hearing and Equilibrium Deficits
Meniere’s Syndrome causes progressive deafness of the inner ear
Sufferers become nauseated and often have howling or ringing sounds in their ears and vertigo
Hearing and Equilibrium Deficits
Vertigo: a sense of spinning so severe that they cannot stand up without extreme discomfort
Chemical Senses - Taste and Smell
Taste and Smell
· Both senses use chemoreceptors·Stimulated by chemicals in solutions·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
Olfaction - Smell
· Olfactory receptors are in the roof of the nasal cavity·Neurons with long cilia
·Chemicals must be dissolved in mucus for detection
Olfaction - Smell
Olfaction - Smell
· Impulses are transmitted via the olfactory nerve
· Interpretation of smells is made in the cortex (olfactory area of temporal lobe)
http://asb.aecom.yu.edu/histology/labs/images/slides/A74_OlfactoryEpith_40X.jpg
Taste
· Taste buds house the receptor organs
· Location of taste buds·Most on the tongue·Soft palate·Cheeks
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
http://neuromedia.neurobio.ucla.edu/campbell/oral_cavity/wp_images/96_fungiform.gif
http://www.esg.montana.edu/esg/kla/ta/vallate.jpg
Structure of Taste Buds
· Gustatory cells are the receptors·Have gustatory hairs (long microvilli)
·Hairs are stimulated by chemicals dissolved in saliva
Structure of taste buds
Structure of taste buds
· Impulses are carried to the gustatory complex (parietal lobe) by several cranial nerves because taste buds are found in different areas·Facial nerve·Glossopharyngeal nerve·Vagus nerve
http://www.biosci.ohiou.edu/introbioslab/Bios171/images/lab6/Tastebuds.JPG
Taste Sensations
· Sweet receptors· Sugars· Saccharine· Some amino acids
· Sour receptors· Acids
· Bitter receptors· Alkaloids
· Salty receptors· Metal ions
· Umami· Glutamate, aspartate (MSG, meats)
Development 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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