1. Chapter 8 The Special Senses
Biology 112
Tri-County Technical College
Pendleton, SC

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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”

8. 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

9. Human Eye Visual

10. Human Eye Visual II

11. 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

12. 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

13. 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

14. 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)

15. Eye Tunics Visual

16. 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

17. 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

18. 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

19. Rods and Cones Visual

20. 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

21. 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

22. Pathway of Light
Corneaaqueous humorthrough pupil through pupillensvitreous humor retina (rods and cones)

23. 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

24. 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

25. 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

26. 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

27. 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

28. Pathway Visual

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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 itvibration to anvilvibration to stirrup presses on oval window of inner ear

35. 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

36. 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

37. Ear Structures Visual I

38. Ear Structures Visual II

39. 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

40. 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

41. 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

42. 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

43. 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)

44. 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)

45. 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

46. 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

47. 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

48. 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

49. 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)

50. 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

51. 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

52. 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

53. 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)

54. 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)

55. 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