1. Taste, smell, touch, sight, hearing, & balance
THE SPECIAL SENSES
Taste, smell, touch, sight, hearing, & balance

2. How sensory structures gather information:
sensory receptors detect stimuli (chemicals, light, sound, cold, heat, touch, etc) and convert that signal to an electrical signal
the conversion is called sensory transduction, and it is accomplished as a change in the membrane potential of the receptor cell
unlike action potentials, receptor potentials vary in intensity (NOT all-or-none)

3. How sensory structures gather information:
the signal then enters the CNS and forms a synapse with a sensory neuron
the brain interprets the intensity of the stimulus by the rate at which it is receiving action potentials (more signals = more intense stimulus)
sensory adaptation = the tendency of some sensory receptors to become less sensitive when they are stimulated repeatedly. This keeps the body from continuously reacting to normal background stimuli & becoming overloaded

4. Human senses are made possible by 5 types of sensory receptors:
Mechanoreceptors = for touch, pressure, motion, sound
Thermoreceptors = temperature receptors
Nociceptors = pain receptors
Chemoreceptors = for chemicals
electromagnetic receptors = for energy such as light & electricity (ex. photoreceptors)

5. THE CHEMICAL SENSES: TASTE & SMELL
Taste = gustitation, smell = olfaction
Their receptors are classified as chemoreceptors because they are responding to chemicals dissolved in fluid
These receptors compliment each other & respond to many of the same stimuli

6. THE SENSE OF TASTE Taste buds = sensory receptors for taste
~10,000 in a young adult, lessen as we age
located primarily on the tongue, also on the soft palate, inner cheeks, pharanyx, epiglottis

7. One cannibal to another while eating a clown: "Does this taste funny to you?"

8. THE SENSE OF TASTE
2 main types of tongue papillae containing taste buds:
Fungiform papillae cover the tongue, especially the tip & sides
Circumvallate papillae are larger, form an inverted “V” at the back of the tongue
replaced every 7-10 days

10. The taste buds themselves are too small to see without a microscope, but papillae are visible by close inspection of the tongue's surface 

11. THE PHYSIOLOGY OF TASTE
Taste buds are composed of groups of between 50 and 150 columnar taste receptor cells bundled together like a cluster of bananas.
The taste receptor cells within a bud are arranged such that their tips form a small taste pore, and through this pore extend microvilli from the taste cells.
The microvilli of the taste cells bear taste receptors.

12. taste receptor cells:

13. THE PHYSIOLOGY OF TASTE
Interwoven among the taste cells in a taste bud is a network of dendrites of sensory nerves called "taste nerves".
When taste cells are stimulated by binding of chemicals to their receptors, they depolarize
this depolarization is transmitted to the taste nerve fibers resulting in an action potential that is ultimately transmitted to the brain.

15. Once taste signals are transmitted to the brain, several motor neural pathways are activated that are important to digestive function:
increased salivation
secretion of gastric juices in the stomach.

16. Among humans, there is substantial difference in taste sensitivity.
one in four people is a "supertaster" = several times more sensitive to bitter and other tastes than those that taste poorly.
such differences are heritable and reflect differences in the number of fungiform papillae and taste buds on the tongue.

17. five types of tastes are commonly recognized by humans:
receptors for a large number of specific chemicals have been identified that contribute to the reception of taste.
five types of tastes are commonly recognized by humans:
Sweet  sugars, alcohols, amino acids; usually indicates energy rich nutrients
Salty  metal ions, sodium chloride; allows modulating diet for electrolyte balance

18. Sour  acids such as oranges, tomatoes; typically foods rich in vitamin C
Bitter  alkaloids (nicotine, caffeine) + aspirin; allows sensing of diverse natural toxins therefore protective in nature
Umami = beefy; enhanced by monosodium glutamate (a food additive)

19. TASTE & SMELL
it is clear that the sense of smell profoundly affects the sensation of taste.
Taste may be up to 80% smell
Taste is also influenced by the temperature & texture of food, due to thermoreceptors and mechanoreceptors

21. THE SENSE OF SMELL
Specialized smell receptor cells are located in a small yellowish patch of mucus membrane lining the roof of the nose
Axons of these sensory cells pass through perforations in the overlying bone and enter 2 elongated olfactory bulbs lying in the top of the bone

22. The portion of the sensory cell that is exposed to odors possesses hair-like cilia
These cilia contain the receptor sites that are stimulated by odorants carried by airborne molecules
odorants dissolve in the mucus lining in order to start the smell response

23. Olfactory receptor cells are unique in that they are one of the few neurons to renew themselves (every 60 days)
An odorant acts on many receptors to different degrees
Similarly, a receptor interacts with many different odorants to varying degrees

24. substances that are volatile (easily turns into a gas), will give off molecules, or odorants
Temperature and humidity affect odor because they increase molecular volatility

25. The pattern of activity set up in the receptor cells is projected to the olfactory bulb, where it forms a spatial image of the odor
Impulses created by this stimulation pass to other smell regions, giving rise to conscious perceptions of odor in the frontal lobe and emotional responses in the limbic system of the brain

27. SMELL & MEMORY
A smell can bring on a flood of memories, influence people's moods and even affect their work performance. Because the olfactory bulb is part of the brain's limbic system, an area so closely associated with memory and feeling it's sometimes called the "emotional brain," smell can call up memories and powerful responses almost instantaneously.

28. Humans have seven primary odors that help them determine objects:
Example
Camphoric
Mothballs
Musky
Perfume/Aftershave
Roses
Floral
Pepperminty
Mint Gum
Etheral
Dry Cleaning Fluid
Pungent
Vinegar
Putrid
Rotten Eggs

30. Anosmia
Anosmia is the inability to smell. Just as the deaf cannot hear and the blind cannot see, anosmics cannot perceive odor and so can barely perceive taste. Sinus disease, growths in the nasal passage, viral infections and head trauma can all cause the disorder. Children born with anosmia often have difficulty recognizing and expressing the disability.

31. TOUCH & PAIN Our skin has 3 types of sensory receptors:
Mechanoreceptors = for touch, pressure, motion, sound
Thermoreceptors = temperature receptors
Nociceptors = pain receptors

32. Mechanoreceptors are those activated by mechanical stimuli.
The tactile senses of touch and pressure are served by a variety of these receptors. 
Meissner's corpuscles are abundant in areas with great sensitivity to touch such as finger tips and lips.  They conduct nerve impulses very quickly. 

33. Mechanoreceptors are those activated by mechanical stimuli.
Merkel’s discs adapt to stimuli slowly and give the sensation of continuous pressure against the body surface. 
Ruffini's end-organs adapt slowly.  They are thought to yield sensations of continuous touch and deep pressure on skin. 

34. Pacinian corpuscles adapt very rapidly and are stimulated by high frequency stimuli to give the sense of vibration. 
Free nerve endings associated with hair follicles respond to touch stimuli that move hairs. (ex. cat’s whiskers)

35. Cutaneous Receptors

37. In addition to mechanoreceptors, there are receptors for pain and the thermal sensations of hot and cold.  These sensations are also served by free nerve endings
Thermoreceptors:
Detect hot or cold in the skin
Other sensors deep in the body regulate temp of the blood, helping the hypothalamus regulate body temp

38. "nociceptors" (noci- as in noxious):
two kinds for temperature (hot or cold)
Two for pain (rapid pricking pain or slow burning pain)

39. "important for a number of reasons:
often indicates danger
can make us aware of injury or disease
pain receptors respond to excessive heat or pressure
pain receptors respond to chemicals released by damaged or inflamed tissues
prostaglandins increase pain by sensitizing pain receptors
aspirin & ibuprofen reduce pain by inhibiting prostaglandins

40. Summary of Sensory receptors in the skin:
Thermoreceptors = temperature receptors
Nociceptors = pain receptors
Mechanoreceptors:
Free nerve endings - heat, light pressure, pain
Pacinian corpuscles – vibration
Meissner corpuscles- onset and end of continuous light pressure
Ruffini endings - continuous touch & deep pressure
Merkels discs - continuous pressure

41. The pathway of communication to the CNS:
Touch receptors sensory nerves spinal cord thalamus sensory cortex of cerebrum
Some areas of the body are more sensitive than others:
Hands & lips are VERY sensitive
Chest & back are less sensitive

43. TOUCH & PAIN
Our sense of touch can be tested using calipers to test our two-point threshold
Acuity is greatest in the most densely nerve-packed areas of the body
The touch homunculus 

44. EFFECTS OF AGING: people lose sensory receptors as they age.
At age 10, most people have about 50 touch receptors per square millimeter of skin.
At 50, we have about 10. Nevertheless, there is little midlife loss of sensitivity (there are still more receptors than nerve fibers)
preliminary results show a 50 percent loss in touch acuity by age 70 or so.

45. EFFECTS OF AGING:
People also lose acuity in their sense of taste & smell
This contributes to malnutrition in the elderly, because food doesn’t taste as good

46. THE EAR: HEARING & BALANCE
The ear is divided into 3 major areas:
Outer ear
Middle ear
Inner ear
The outer & middle ear are for hearing only
The inner ear is for hearing & balance
The inner ear is the most complex

48. THE EAR: HEARING & BALANCE
Parts of the outer ear:
Pinna or auricle = shell-shaped flap that collects sound waves & channels them in
Helix (the rim) + lobe

49. THE EAR: HEARING & BALANCE
Parts of the outer ear:
Auditory canal = a curved tube that extends from the auricle to the eardrum (2.5 cm long)
Passes through the temporal bone via the external auditory meatus
Lined with ceruminous glands that make cerumen, or earwax which traps foreign bodies & repels bugs

50. THE EAR: HEARING & BALANCE
The boundary between the outer & middle ear is the eardrum or tympanic membrane
The middle ear is an air-filled cavity in the temporal bone
Parts of the middle ear: 3 bones or ossicles:
Hammer / malleus
Anvil / incus
Stirrup / stapes

51. THE EAR: HEARING & BALANCE
Eustachian tube / pharyngotympanic tube equalizes pressure on either side of the eardrum to allow it to vibrate freely
Swallowing or yawning helps to equalize the pressure & open the eustachian tubes
The eustachian tubes of children are short & nearly horizontal, which permits bacteria to spread from the pharynx to the middle ear = otitis media

52. THE EAR: HEARING & BALANCE
The boundary between the middle & inner ear is the oval window
The inner ear is a cavity called the bony labyrinth which is lined with a membrane called the membranous labyrinth
Perilymph is the fluid between the bone & membrane
Endolymph is the fluid within the membranes

54. THE EAR: HEARING & BALANCE
Parts of the inner ear / labyrinth:
Semicircular canals
Anterior, posterior, & lateral canals oriented in the 3 planes of space
Vestibule = central egg-shaped cavity
2 membranous sacs called the saccule & utricle house equilibrium receptor regions that respond to the pull of gravity & report on changes in head position

55. THE EAR: HEARING & BALANCE
Parts of the inner ear / labyrinth:
Cochlea = “snail”
Houses the organ of Corti, the actual organ for hearing
The organ of Corti consists of hair cells (actually specialized microvilli) embedded in the basilar membrane.
The hairs are the receptor cells of the ear
sensory neurons at the base of the hairs carry action potentials from here to the brain via the auditory / vestibulocochlear nerve
Filled with perilymph, fluid similar to and continuous with cerebrospinal fluid

57. MECHANISMS OF HEARING:
A vibrating object creates pressure waves in the surrounding air
These waves are picked up by the outer ear & channeled inward, causing the eardrum to vibrate
Vibrations pass through the hammer, anvil & stirrup in the middle ear
The stirrup transmits the vibrations to the inner ear through the oval window

58. MECHANISMS OF HEARING:
As the stirrup transmits the vibrations to the inner ear, pressure waves pass through the cochlea
This causes the basilar membrane to vibrate
These vibrations cause the hair-like projections on the hair cells to bend
A receptor potential results, and neurotransmitters are released

59. Neurotransmitters cross the synapse & cause an action potential in a sensory neuron
The sensory neuron sends the action potential through the auditory nerve to the brain
Eardrum malleus incus stapes oval window of inner ear perilymph & endolymph of the cochlea 8th cranial nerve brain (auditory area in the temporal lobe of the cerebral cortex)

60. HOW IS VOLUME DETERMINED?
The louder the sound, the greater the pressure wave it generates
greater amplitude of the wave = more vigorous vibrations of the fluid in the cochlea = more pronounced bending of the hair cells = more action potentials generated in the sensory neurons

61. LOSS OF HEARING: Caused by the inability to conduct sounds
Tinnitus = ringing or buzzing in ears after exposure to a very loud environment
Possible causes of hearing loss:
Middle ear infections
Ruptured eardrums
Stiffening of the middle ear bones (old age)
Damage to receptor cells or neurons
Damage or destruction of hair cells

63. BALANCE & EQUILIBRIUM:
The utricle & saccule are membranous sacs in the vestibule, between the cochlea & the semicircular canals
they provide information about the position of the body at rest
Within the utricle & saccule are hair cells embedded in a gelatinous membrane with tiny crystals of calcium carbonate called otoliths

64. BALANCE & EQUILIBRIUM:
Gravity pulls on the otoliths & bends the hair cells as the position of the head changes
The impulses generated by the hair cells are carried by the 8th cranial nerve to the cerebellum, the midbrain, & the temporal lobes of the cerebrum

66. BALANCE & EQUILIBRIUM:
The semicircular canals provide information about the body in motion
These fluid-filled membranous ovals are oriented in 3 different planes
At the base of each is an enlarged area called the ampulla which contains hair cells called crista

67. BALANCE & EQUILIBRIUM:
As the body moves, the hair cells are bent & then they straighten
The bending generates impulses carried by the 8th cranial nerve to the cerebellum, midbrain, and temporal lobes of the cerebrum
These impulses are interpreted as starting or stopping, or changing direction

68. EFFECTS OF AGING:
By age 60, the deterioration of the organ of Corti becomes noticeable
We are born with ~40,000 hair cells but they are lost due to loud noises, disease, or drugs & they do not regenerate
We lose the ability to hear high-pitched sounds first
Our sense of equilibrium diminishes too – we react more slowly to tilting & fall more

69. THE SENSE OF VISION: Our eyes are the sense organs of vision
Able to detect many colors
Able to form images of objects near & far
Able to respond to minute amounts of light energy

70. The eyeball is protected by 6 bones:
Lacrimal, maxilla, zygomatic, frontal, sphenoid, ethmoid
Movement of the eyeball is accomplished by 6 muscles:
4 rectus muscles
2 oblique muscles
These muscles are innervated by cranial nerves 3,4,& 6

71. THE ANATOMY OF VISION:
The outer surface of the eyeball is a tough whitish layer of ct called the sclera
At the front of the eye, the sclera becomes the transparent cornea
The sclera surrounds a pigmented layer called the choroid
At the front of the eye, the choroid forms the iris, which gives the eye its color

72. THE ANATOMY OF VISION:
The muscles of the iris regulate the size of the pupil, the opening that lets light into the interior of the eye
Light passes through the pupil to the lens which is held in position by ligaments

73. THE ANATOMY OF VISION:
The lens focuses images on the retina, which contains photoreceptor cells to transduce light energy
Photoreceptor cells are highly concentrated at the retina’s center of focus, the fovea
There are no photoreceptor cells at the area where the optic nerve passes through the back of the eye = “blind spot”

74. THE ANATOMY OF VISION:
Action potentials pass via sensory neurons in the optic nerve to the visual centers of the brain
2 fluid-filled chambers make up the bulk of the eye & helps maintain its shape:
Vitreous humor
Fills the large chamber behind the eye

75. THE ANATOMY OF VISION: Aqueous humor
Fills the small chamber in front of the lens
The fluid circulates supplying nutrients & O2; blockage of the ducts that drain this fluid can cause glaucoma

76. THE ANATOMY OF VISION:
Conjunctiva = a thin mucous membrane that keeps the outside of the eye moist
Conjunctivitis = “pink eye” = infection of the conjunctiva; can be bacterial or viral
Lacrimal gland = secretes tears to cleanse & moisten the eye surface

78. THE PHYSIOLOGY OF VISION:
The lens of the eye focuses light onto the retina by bending (refracting) light rays
Focusing is accomplished by changing the shape of the lens
The thicker the lens, the more sharply it bends light
The lens becomes thicker & rounder to focus on nearby objects
The lens becomes flatter to focus on distant objects

79. THE PHYSIOLOGY OF VISION: color vision
In the retina, there are 125 million rod cells & 6 million cone cells
These two photoreceptor cells are named for their shape:
Cones
Stimulated by bright light
Can distinguish color
Do not function in night vision
Found in a high concentration in the fovea
Contain visual pigments called photopsin
3 types of cones = blue, green, red (deficiency = color blindness)

80. THE PHYSIOLOGY OF VISION: color vision
Rods
Extremely sensitive to light
Enable us to see in dim light
Only give shades of gray
Found in the greatest density at the outer edges of the retina, completely absent from the fovea
Contain visual pigments called rhodopsins which absorb dim light

81. THE PHYSIOLOGY OF VISION:
Rods & cones are stimulus transducers
When rhodopsin & photopsin absorb light, they change chemically, which alters the permeability of the cell membrane
The resulting receptor potentials trigger a complex integration process that begins in the retina

82. Action potentials carry the partly integrated information into the brain via the optic nerve
3-D perceptions result from further integration in several processing centers of the cerebral cortex
We have stereoscopic vision. Each eye receives a slightly different view of the visual field. These views are fused to provide depth perception

83. VISION PROBLEMS & CORRECTIONS: Errors of refraction
Nearsightedness = myopia
Cannot focus well on distant objects, but can see well at short distances
The eyeball is longer than normal & the lens cannot flatten enough to compensate
Distant objects are focused in front of the retina instead of on it
Corrected by glasses that are thinner at the middle than at the edges

84. VISION PROBLEMS & CORRECTIONS: Errors of refraction
Farsightedness = hyperopia
Cannot focus well at short distances, but can see distant objects normally
The eyeball is shorter than normal
nearby objects are focused behind the retina
Corrected by glasses that are thicker at the middle than at the edges
Presbyopia = age-related farsightedness due to a less elastic lens.

86. VISION PROBLEMS & CORRECTIONS: Errors of refraction
Astigmatism = blurred vision caused by a misshapen lens or cornea
Makes light rays converge unevenly & not focus at any one point on the retina
Corrective lenses are asymmetrical in a way that compensates for the asymmetry in the eye

87. VISION PROBLEMS & CORRECTIONS: Errors of refraction
Surgical procedures to correct vision disorders:
RK = radial keratotomy = a knife is used to cut slits in the cornea to change its shape
PRK (photorefractive keratectomy) & LASIK (laser-assisted in situ keratomileusis) = a laser is used to reshape the cornea and change its focusing power

88. MORE VISION PROBLEMS: Night blindness Color blindness
The inability to see well in dim light or at night
Usually caused by a deficiency of vitamin A, which is necessary to synthesize rhodopsin
Color blindness
An X-linked genetic disorder in which one of the three sets of cones is lacking or nonfunctional
Total colorblindness is rare, the most common form is red-green colorblindness

89. MORE VISION PROBLEMS: Cataracts = cloudy or opaque lenses
Most common among the elderly
Proteins in the lens break down and lose their transparency
Contributing factors include smoking & long-term exposure to UV light
Small cataracts can be destroyed by lasers, artificial lenses can be surgically implanted

90. MORE VISION PROBLEMS: Glaucoma =
Normal = the presence of aqueous humor creates a pressure in the eye called intraocular pressure
When the intraocular pressure increases to levels that cause compression of the retina and optic nerve, it can lead to blindness
Can be caused by high blood pressure & diabetes
Can be controlled by medication if caught early