Special SensesChapter 8

Special Senses• Smell, taste, sight, and hearing• Equilibrium • Special sense receptors:

– large, complex sensory organs (eyes, ears)

– localized clusters of receptors (taste buds and olfactory epithelium)

• Senses blend to give us our sensations

The Eye & Vision

External Anatomy of the Eye• Only anterior 1/6 of eye’s surface can

normally be seen• Accessory structures:

– Extrinsic eye muscles– Eyelids– Conjunctiva– Lacrimal apparatus

Accessory Structures• Extrinsic eye muscles

– Six muscles are attached to outer surface of each eye

– Produce gross eye movements

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Accessory Structures• Eyelids

– 4 layers: skin, muscle, connective tissue, conjunctiva (mucous membrane)

– Moved by orbicularis oculi muscles (close) and levatator palpebrae superioris (open)

Accessory Structures• Lacrimal apparatus

– Consists of lacrimal gland & ducts that drain lacrimal secretions into nasal cavity

– Lacrimal glands: continuously secrete dilute salt solution (tears) onto anterior surface of eyeball

• Secretion contains mucus, antibodies, and lysozyme which is an enzyme that destroys bacteria

• Cleans and protects eye surface as it moistens & lubricates it

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Internal Structures• Eyeball is a hollow sphere• Wall composed of three layers:

– Fibrous layer• Sclera• Cornea

– Vascular layer• Choroid• Ciliary body• Iris

– Sensory layer• Retina

– Pigmented layer (prevent light from scattering)– Neural layer (photoreceptors)

Fibrous Layer• Sclera: thick, white connective tissue (“white of

the eye”)– Opaque due to large, disorganized collagenous and

elastic fibers– For protection & attachment

• Cornea: clear “window” through which light enters eye– anterior portion of eye– many nerve endings (pain fibers), easily repairs itself– avascular, so only tissue in the body that can be

transplanted without fear of rejection– helps focus entering light rays– Connective tissue with thin layer of epithelium;

unusually regular fiber pattern

Fibrous Layer: Sclera & Cornea

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Vascular Layer• Choroid: blood-rich nutritive tunic that contains a dark

pigment– Prevents light from scattering inside the eye– Modified anteriorly to form ciliary body & iris– Produces melanin to absorb light– Contains blood vessels

• Ciliary body: two smooth muscle structures to which the lens is attached with the ciliary zonule (ligament)

– Forms internal ring around front of eye (muscle fibers & ligaments)• Iris: pigmented, has an opening called the pupil through

which light passes– Thin diaphragm composed of muscle tissue and connective tissue– Circular and radial muscle fibers control size of pupil (stimulated by

photons of light)– Regulates the amount of light which enters the eye

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Choroid

Ciliary Body

Sensory Layer: Retina• Retina – two layers which extends

anteriorly only to the ciliary body– Pigmented layer

• composed of pigment cells that absorb light and prevent light from scattering inside the light

• act as phagocytes to remove dead or damaged receptor cells

• store vitamin A needed for vision

• Neural layer – contains millions of receptor cells – Photoreceptors (rods & cones): respond to light,

bipolar neurons• 70% of sensory receptors are in the eyes• Rods: more sensitive in low light, gives general

outline, seen as black and white• Cones: less sensitive in low light, sharp picture, color

– Electrical impulse leaves the retina via the optic nerve & nerve impulses are transmitted to the optic cortex which results in vision

– Fovea: all cones, sharpest vision (visual acuity)– Optic disc (“blind spot”) – where optic nerve

leaves eyeball

Sensory Layer: Retina

Retina

Sensory Layer: Retina• Night blindness: fewer working

rods (lack of vitamin A)• Day blindness: lack of working

cones

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Retinitis Pigmentosa• Genetic disorder• involves abnormalities in photoreceptors or retinal tissue

that leads to progressive vision loss• Bionic Eye (Argus II Retinal Prosthesis System)

– Mostly restores black & white vision– Did restore color vision in some

FDA approval

Sensory Layer: Retina• Rods: rhodopsin breaks down into opsin & retinal when

struck by photons; initiates chemical reaction (action potential) which is sent to visual cortex (occipital lobe)

– In bright light, nearly all rhodopsin I broken down, reducing rod sensitivity

• Cones: three different light sensitive proteins connected to retinal – each most sensitive to a particular wavelength of visible light

– Depending on which is stimulated, brain interprets that color– Erythrolabe: red– Chlorolabe: green– Cyanolabe: blue– Mixing & interpretation of color occurs in the brain, not the

retina! (i.e. red light in one light & green light in another eye will be seen as yellow)

Read pg. 286

Visual PigmentsIf all are stimulated at once, see white!

Color Blindness• Total color blindness: lack of all

three cones• Partial color blindness: lack of one

cone type (lack of red or green receptor is most common)

• Sex-linked condition (carried on X chromosome)

Color Blindness 4 Sex-Linked Traits:

 1. Normal Color Vision: A: 29,  B: 45,  C: --,  D: 26

 2. Red-Green Color-Blind: A: 70,  B: --,  C: 5,  D: --

 3. Red Color-blind: A: 70,  B: --,  C: 5,  D: 6

 4. Green Color-Blind: A: 70,  B: --,  C: 5,  D: 2

Internal Structures: Lens• Lies directly behind iris & pupil• Focuses light entering the eye on the retina

(changes shape to focus)• Held upright in the eye by a suspensory

ligament (ciliary zonule) attached to ciliary body• Epithelial cells (cytoplasm is transparent part)

Cataracts• With age, lens becomes

increasingly hard and opaque• Cataracts result from this process

and cause vision to become hazy and distorted– Can eventually cause blindness in

affected eye• Treatment: surgical removal of lens

and replacement with lens implant or special cataract glasses

Internal Structures: Humors• Aqueous humor

– Anterior to the lens, clear watery fluid

– Fills space between cornea and lens

– Nourishes, helps hold shape• Vitreous humor

– Reinforces eyeball internally– Posterior to lens

Glaucoma• aqueous humor made more quickly

than can be removed or drainage is blocked

• pressure builds, damage results from compression of retina and optic nerve

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Physiology of Vision• Light rays are bent (refracted) as light

encounters the cornea, aqueous humor, lens, and vitreous humor

• Refractory power of the lens can be changed by changing its shape (ciliary body controls shape of lens)

• Accommodation: ability of eye to focus for close objects (less than 20 ft. away)– Image formed on retina is a real image

(inverted)

Vision

Astigmatism

Visual Pathways• Axons carrying impulses from retina are bundled

together at posterior aspect of eyeball and issue from back of eye as optic nerve

• Optic chiasma: fibers from medial side of eye cross over to opposite side of brain

– Fiber tracts that result are 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– Optic tract fibers synapse with neurons in the thalamus,

whose axons form the optic radiation which runs to the occipital lobe of brain (visual interpretation occurs)

• Each side of brain receives visual input from both eyes• Visual fields overlap to give humans binocular vision

The Ear: Hearing & Balance

Anatomy of the Ear• Divided into three major areas

– External (outer) ear– Middle ear– Internal (inner) ear

External (Outer) Ear• Auricle (pinna) – “ear”

– Collects and directs sound waves into the auditory canal (function largely lost in humans)

• External acoustic meatus (auditory canal) – Short, narrow chamber carved into temporal

bone of skull– Ceruminous glands – secrete cerumen (earwax)– Tympanic membrane (eardrum) – vibrate when

sound waves strike it; separates external & middle ear

Swimmer’s EarOtitis Externus

Middle Ear• Small, air-filled, mucosa-lined cavity

within temporal bone• Transfers vibrations via the ossicles

(smallest bones in body)– malleus (hammer), incus (anvil), and stapes

(stirrup)– Stapes passes vibration to the oval window

of the inner ear• Pharyngotympanic tube (auditory

tube): pressure needs to be equalized to enable eardrum to vibrate

Middle Ear

Otitis Media• Inflammation of the middle ear• Pharyngotympanic tubes run

more horizontally in children• Ear tubes – implanted in ear

drum to allow pus to drain into external ear canal

• Infants with bottles “propped” or fed lying flat can get fluid in their ears through the pharyngotympanic tube

Internal (Inner) Ear• Bony (osseous) cavity located behind the eye

socket– Filled with perilymph (fluid)– Membranous labyrinth suspended in perilymph,

contains endolymph• Three subdivisions

– Cochlea– Vestibule– Semicircular canals

• Lined with hair cells (mechanoreceptors)

Equilibrium• Vestibular apparatus: equilibrium receptors of ear

– Static equilibrium: maculae receptors• Report on changes in position of head when body not moving

(keep head erect)• Otoliths: tiny stones that roll in response to changes in pull of

gravity– Dynamic equilibrium: bending of cupula indicates

rotation (gelatinous cap)• Report on changes when body moving (i.e. spinning)

• Receptors stimulate hair cells, which send impulses via the vestibular nerve to the cerebellum

• Work together with proprioceptors for control & balance

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Hearing• Spiral organ of corti – contains hair

cells (hearing receptors)• Vibrations set cochlear fluids in

motion, pressure waves cause vibrations

• impulses are sent via cochlear nerve to temporal lobe (auditory cortex)

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Chemical Senses: Smell & Taste

Olfaction• Olfactory receptors (chemoreceptors): receptors for

sense of smell, occupy a postage stamp-size area in roof of each nasal cavity– 10-100,000,000 receptors in nose

• Olfactory filaments (axons) make up olfactory nerve (cranial nerve I) which conducts impulses to olfactory cortex of brain– Olfactory lobes of brain (gray matter) – situated over

nose (bottom of frontal lobe)• Olfactory impulses closely tied to limbic system• Sensitive receptors, just a few molecule can activate

them (thousands of smells)• Adapt rather quickly when exposed to unchanging

stimulus

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Gustation: Taste• Taste buds: specific receptors for taste widely

scattered in the oral cavity– Live 7-10 days!

• protection (low threshold – bitter)• 10,000 taste buds (mostly on tongue)• Five tastes: sweet, salty, sour, bitter, and umami

(savory)– Flavors: combination of 5 tastes and olfactory and

touch sensations• 1st order neuron medulla hypothalamus or

thalamus (limbic) parietal lobe (conscious perception of taste)

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