Chapter 8 The Special Senses

Biology 112

Tri-County Technical College

Pendleton, SC

You’re special…I’m special

• Usually told we have five senses: touch, taste, smell, sight, and hearing

• TOUCH actually mixture of general senses: temperature, pressure, and pain receptors of skin and proprioceptors of muscles and joints

• Smell, taste, sight, and hearing are called special senses

Special Senses, cont.

• Receptors for 5th sense, equilibrium, are housed in ear along with organ of hearing

• Special sense receptors are either large, complex sensory organs (such as eyes and ears) or localized clusters of receptors (such as taste buds and olfactory epithelium)

The Human Eye

• Eyelids meet at medial and lateral canthus (protection)

• Modified sebaceous glands at eyelid edges called meibomian glands produce oily secretion that lubricates eyes

• Ciliary glands (modified sweat glands) lie between eyelashes and help lubricate eyeball

• Sty is inflammation of one of these ciliary glands

Human Eye, cont.

• Conjunctiva is delicate membrane that lines eyeball and covers parts of outer surface of eyeball

• Ends at edge of cornea by fusing with corneal epithelium

• Secretes mucus to lubricate and keep eyeball moist

• Conjunctivitis (inflammation of conjunctive) results in reddened, irritated eyes

• Pinkeye (conjunctiva infection caused by bacteria/virus) is highly contagious

Human Eye III

• Lacrimal apparatus consists of number of glands and ducts that drain lacrimal secretions into nasal cavity

• Lacrimal glands located above lateral end of each eye release dilute salt solution (tears) onto anterior surface of eyeball through lacrimal gland ducts

Human Eye IV

• Tears flush across eyeball into lacrimal canals medially, then into lacrimal sac, and finally into nasolacrimal duct which empties into nasal cavity

• Tears contain antibodies and lysozyme• Cleanses and protects as moistens and lubricates• Sclera (thick, white connective tissue) is

outermost protective tunic (also called fibrous tunic– Seen anteriorly as “white of the eye”

Human Eye V

• Central anterior portion (sclera) modified to crystal clear

• This transparent window is called the cornea allows light to enter the eye

• Iris is pigmented part of eye and contains the pupil

• Six extrinsic (external) eye muscles attached to outer surface of each eye

• Produce gross eye movements and make it possible for eye to follow a moving object

Human Eye Visual

Human Eye Visual II

Eye Tunics• Eye (eyeball) is hollow sphere whose wall is

composed of three tunics (coats)• Its interior is filled with fluids called humors that

help maintain its shape• Outermost tunic (sclera or fibrous tunic) is thick,

white connective tissue seen anteriorly as the “white of the eye”

• Central anterior portion modified to crystal clear structure called the cornea through which light enters eye

Eye Tunics, cont.• Cornea well supplied with nerve endings,

has ability to repair itself• **Cornea has NO blood vessels and is

ONLY tissue that can be transplanted from one person to another without fear of rejection

• Choroid is middle coat of eyeball and is blood-rich nutritive tunic containing dark pigment

Eye Tunics III

• Anteriorly, choroid modified to form two smooth muscle structures called ciliary body to which lens is attached and the iris

• Pigmented iris has rounded opening (pupil) through which light passes

• Circularly/radially arranged smooth muscle fibers form iris which acts like diaphragm of camera regulating amount of light entering eye

Eye Tunics IV• Innermost sensory tunic of eye is delicate white

retina• Retina contains millions of receptors cells (rods

and cones)• Rods and cones called photoreceptors because

they respond to light• Photoreceptor cells distributed over entire retina

except where optic nerve leaves eyeball• This site is called the optic disk (blind spot)

Eye Tunics Visual

Rods and Cones• Rods most dense at periphery of retina and

decrease in number as center of retina approaches

• Rods allow vision in gray tones in dim light and provide for peripheral vision

• Night blindness caused by interference with rod function – Most common cause prolonged vitamin A

deficiency

Rods and Cones, cont.• Cones are discriminatory receptors that allow

color vision under bright conditions• Cones are densest in center of retina and < in

number toward retinal edge• Fovea centralis is tiny pit that contains only

cones and is area of greatest visual acuity (point of sharpest vision)

• Three varieties of cones and each type is more sensitive to particular wavelength of light

Rods and Cones III

• Blue, green and range including both green and red

• Impulses received at same time from more than one type of cone by visual cortex are interpreted as intermediate colors

• Lack of all three cone types = total color blindness whereas lack of one type = partial color blindness

• Color blindness is sex-linked disorder

Rods and Cones Visual

Image Formation

• Light passes from one substance to another substance of different density, its speed changes and its rays are bent (refracted)

• Light rays are bent in eye as they encounter cornea, aqueous humor, lens, and vitreous humor

• Refractive power of cornea and humors constant

Image Formation, cont.

• Refractive power of lens changed by changing its shape

• Greater lens convexity (bulge) the more light is bent

• Flatter the lens, less light is bent• Image formed on retina is a real image• Reversed from right to left, inverted (upside

down) and smaller than the object

Pathway of Light

• Corneaaqueous humorthrough pupil through pupillensvitreous humor retina (rods and cones)

Coming to term with terms…

• Accommodation: ability of eye to focus specifically for close objects (< 20 feet)

• Astigmatism: results from unequal curvatures in different parts of cornea or lens & causes blurry images because points of light are focused not points on retina but as lines

• Blind spot: (optic disc) where optic nerve exits eyeball—no rods or cones here so light from object focused on optic disc, the object disappears

Terms, cont.

• Cataract: Caused by lens becoming >ingly hard and opague– Vision becomes hazy and eventually blindness– Lens implant or special cataract glasses

• Emmetropia: term given eye that focuses images correctly on retina (harmonious vision)

• Glaucoma: Drainage of aqueous humor is blocked causing pressure within eye to increase—results in compression of delicate retina and optic nerve

Terms III• Causes pain and eventual blindness

– Tonometer used to measure intraocular pressure (> 40 should be annual exam)

– Eyedrops (miotics) or surgical enlargement of drainage channel

• Hyperopia: “farsightedness” occurs when light rays from distant object focused behind retina

• Myopia: “nearsightedness” occurs when light rays from distant object fail to reach retina and are focused in front of retina

Pathway to Optic Cortex

• Axons carrying impulses from retina bundled together as posterior aspect of eyeball and exit as optic nerve

• Fibers from medial side of each eye cross over to opposite sides at optic chiasma

• Fiber tracts that result are the optic tracts• Each tract contains fibers from lateral side

of eye on same side and medial side of eye of opposite eye

Optic Cortex, cont.

• Optic tract fibers synapse with neurons in thalamus whose axons form optic radiation which runs to occipital lobe of brain

• There they synapse with cortical cells and visual interpretation occurs

Pathway Visual

Visual Reflexes

• Convergence is reflexive movement of eye medially when viewing close objects– Convergence occurs both eyes aimed toward near

object being viewed

• Photopupillary reflex: occurs when eyes are suddenly exposed to bright light and pupils immediately constrict– Protective reflex prevents damage to photoreceptors

• Accommodation pupillary reflex: occurs when pupils constrict when close objects viewed– Provides for more acute vision

Ear Structures

• Outer and middle ear involved with hearing ONLY

• Inner ear functions in both equilibrium and hearing

• Outer ear composed of pinna and external auditory canal

• Pinna (auricle) what most would call “ear”– Shell shaped structure surrounding auditory

canal opening

Ear Structures, cont.

• External auditory canal is short, narrow chamber carved into temporal bone of skull

• Ceruminous glands (secrete earwax) located in skin lined walls of external auditory canal

• Sound waves entering canal eventually hit tympanic membrane (eardrum) and cause it to vibrate

Ear Structures III

• Tympanic membrane separates outer from middle ear

• Middle ear (tympanic cavity) is small, air-filled cavity within temporal bone

• Flanked laterally by eardrum and medially by bony wall with two openings: oval window and inferior, membrane-covered round window

Ear Structures IV• Auditory tube links middle ear with throat• Normally flattened and closed but

swallowing/yawning can open it briefly to equalize pressure in middle ear with external pressure

• **Important because eardrum does NOT vibrate freely unless pressure on both sides of its surfaces is the same

• Bulge outward or inward otherwise = hearing difficulty

Ear Structures V• Tympanic cavity spanned by three smallest

bones in body called ossicles which transmit vibratory motion of eardrum to fluids in inner ear

• Malleus (hammer), incus (anvil), and stapes (stirrup)

• When eardrum moves, hammer moves with itvibration to anvilvibration to stirrup presses on oval window of inner ear

Ear Structures VI• Movement of oval window sets fluids of

inner ear into motion…eventually exciting hearing receptors

• Inner ear is maze of bony chambers called osseous or bony labyrinth located deep in temporal bone just behind eye socket

• Three subdivisions called cochlea, vestibule, and semicircular canals

Ear Structures VII

• Bony labyrinth filled with plasma-like fluid called perilymph

• Suspended in perilymph is membranous labyrinth which is system of membrane sacs that more or less follow shape of bony labyrinth

• Membranous labyrinth contains thicker fluid called endolymph

Ear Structures Visual I

Ear Structures Visual II

Organ of Corti

• Within membranes of snail-like cochlea is Organ of Corti which contains hearing receptors called hair cells

• Sound waves reach cochlea through vibrations of eardrum, ossicles, and oval window & set cochlear fluids into motion

• Receptor cells on basilar membrane in organ of Corti are stimulated when their “hairs” are bent or “tweaked” by movement of gel-like tectorial membrane that lies over them

Organ of Corti, and more• Once stimulated, hair cells transmit impulses along

cochlear nerve (division of 8th cranial-vestibulocochlear) to the auditory cortex in temporal lobe = hearing

• Sensorineural deafness occurs when degeneration or damage to receptor cells in organ of Corti, to cochlear nerve, or to neurons of auditory cortex

• Often occurs from extending listening to excessively loud noises

• Is a problem of nervous system structures

More, cont.

• Conduction deafness results when something interferes with conduction of sound vibrations to fluids of inner ear

• Earwax buildup, fusion of ossicles, ruptured eardrum, or otitis media (inner ear infection)

• Is a problem of mechanical factors

Sound Location

• Sound usually reaches the two ears at different times so we have stereo hearing– Well, least ways, the lucky ones among us do…

• Functionally, this slight difference assists in helping determine where sounds are coming from in the environment

Equilibrium and Balance• Equilibrium sense “responds” to various

movements of head• Equilibrium receptors of inner ear (often called

vestibular apparatus) can be divided into two functional parts

• One is responsible for monitoring static equilibrium (report on position of head with respect to gravity when body is not moving)

• The other monitors dynamic equilibrium (respond to angular or rotatory movements of head rather than to straight-line movements)

E and B, cont.

• Within membrane sacs of vestibule are receptors called maculae that are essential to static equilibrium

• Maculae report on position of head with respect to pull of gravity when body NOT moving

• Provide info on which way is up or down and help keep head erect

• Each macula is patch of receptor cells with “hairs” embedded in otolithic membrane (gel or jellylike material containing otoliths (tiny stones of calcium salts)

E and B III• As head moves, otoliths roll in response to

changes in pull of gravity• This movement creates pull on the gel

which slides over hair cells, bending their hairs

• This activates hair cells which send impulses along vestibular nerve to cerebellum of brain, informing it of position of head in space

E and B IV• Dynamic equilibrium receptors found in

semicircular canals and respond to angular or rotatory movments of head rather than to straight-line movements

• Semicircular canals oriented in three planes of space

• Regardless of plane one moves in, there will be receptors to detect that movement

E and B V• Within each SC canal is receptor region called

crista ampullaris—turf of hair cells covered with gelatinous cap called capula

• Head moves in arclike/angular direction, endolymph in canal lags behind and moves in opposite direction pushing capula in direction opposite to body’s motion

• This stimulates hair cells and impulses transmitted up vestibular nerve to cerebellum

• When moving at constant rate, receptors stop sending and one no longer has sense of motion until speed or direction of movement changes

Taste and Olfaction• Receptors for taste and olfaction are

chemoreceptors because they respond to chemicals in solution

• Thousands of olfactory receptors occupy postage stamp size area in roof of each nasal cavity

• Olfactory receptor cells are neurons with olfactory hairs– Long cilia on nasal epithelium that are continually

bathed by layer of mucus secreted by underlying glands

Olfaction, cont.

• When receptors stimulated by chemicals dissolved in mucus, they transmit impulses along olfactory nerve (1st cranial nerve) to olfactory cortex of brain

• Interpretation of odor occurs and “odor snapshot” is made

• Olfactory pathways closely tied to limbic system (emotional-visceral part of brain)

Olfaction III

• Olfactory impressions are long-lasting and part of one’s memories and emotions

• OMG, Ms. Pennington and My Sin® perfume…Beam me up Scotty….I can’t Captain, I ain’t got the power

• Olfactory receptors are very sensitive—just a few molecules can activate them

• Olfactory neurons adapt very quickly

Taste• Taste buds (specific receptors for sense of

taste) are widely scattered in oral cavity– Most of 10,000 or so located on tongue– Few found on soft palate & inner surface of

cheeks

• Three kinds of papillae: filiform (sharp), fungiform (rounded), and circumvallate

• Taste buds found on sides of circumvallate and more numerous fungiform papillae

Taste, cont.• Specific cells that respond to chemicals dissolved

in saliva are epithelial cells called gustatory cells which are surrounded by supporting cells in taste bud

• Gustatory cell’s long microvilli (gustatory hairs) protrude through taste pores and when depolarized, they send impulses to the brain

• Three cranial nerves (7th, 9th, and 10th) carry impulses from various taste buds to gustatory cortex

Taste Sensations

• Four basic taste sensations, each corresponding to stimulation of one of four major types of taste buds

• Sweet (may be the OH- group)• Sour (H+)• Bitter (alkaloids)• Salty (metal ions in solution)• Taste heavily impacted by sense of olfaction• Dislike for bitterness is protective (many natural

poisons and spoiled foods are bitter)

Happens to all of us…SOL

• Presbyopia or “old vision” results from decreasing lens elasticity that accompanies aging

• Lacrimal glands become less active and eyes tend to be dry

• Lens loses it clarity, dilator muscles of iris become less efficient, and photoreceptors begin to die die to lack of oxygen and nutrients (poor circulation)

But considering the alternatives,..

• Presbycusis begins by age 60 and results from gradual deterioration and atrophy of organ of Corti

• Leads to loss of hearing in high tones/speec sounds

• By mid-40s, ability to taste and smell diminishes• By 80, half cannot smell at all and sense of taste is

very poor