THE SPECIAL SENSES

1. 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 • sensory transduction = conversion; is accomplished as a change in the membrane potential of the receptor cell • vary in intensity (NOT all-or-none)

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

4. 5 types of sensory receptors: • mechanoreceptors = touch, pressure, motion & sound • thermoreceptors = temperature • nociceptors = pain • chemoreceptors = chemicals • electromagneticreceptors = energy such as light & electricity (ex. photoreceptors)

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

6. taste buds = sensory receptors for taste • ~10,000 in a young adult, lessen as we age • located primarily on tongue, also on the soft palate, inner cheeks, pharynx & epiglottis

7. 2 main types of tongue papillae (contain 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

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

10. THE PHYSIOLOGY OF TASTE • Taste buds = groups of 50 -150 columnar taste receptor cells bundled like a cluster of bananas • arranged so tips of cells form a small taste pore, through which microvilli extend • microvilli have taste receptors.

11. when taste cells are stimulated by binding of chemicals to their receptors, they depolarize • depolarization transmitted to taste nerve fibers (interwoven among taste cells) results in an action potential that is ultimately transmitted to the brain

12. taste signals transmitted to brain  • increased salivation • secretion of gastric juices in the stomach

13. Among humans, there is substantial difference in taste sensitivity. • 1 in 4 people is a "supertaster" = much more sensitive to bitter and other tastes than most people • typically inherited and reflect differences in the number of fungiform papillae and taste buds

14. 5 tastes 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 • 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)

15. TASTE & SMELL • 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

16. 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

17. the portion of the sensory cell exposed to odors has hair-like cilia • cilia contain receptor sites that are stimulated by odorants carried by airborne molecules • odorants dissolve in the mucus lining in order to start the smell response • an odorant acts on many receptors to different degrees • similarly, a receptor interacts with many different odorants to varying degrees

18. substances that are volatile (easily turn into a gas), will give off odorants • temperature and humidity affect odor because they increase molecular volatility • Olfactory receptor cells are unique in that they are one of the few neurons to renew themselves (every 60 days) • olfaction gives rise to conscious perceptions of odor in the frontal lobe and emotional responses in the limbic system of the brain

19. SMELL & MEMORY A smell can: • bring on a flood of memories • influence people's moods • affect their work performance • since the olfactory bulb is part of the brain's limbic system (the “emotional brain”), smell can call up memories and powerful responses almost instantaneously

20. Humans have seven primary odors that help them determine objects:

22. Anosmia • anosmia = the inability to smell • 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

23. TOUCH & PAIN • Our skin has 3 types of sensory receptors: • mechanoreceptors • thermoreceptors • nociceptors

24. Mechanoreceptors • The tactile senses of touch and pressure are served by a variety of mechanoreceptors. • can sense light touch & pressure to deep touch & pressure, vibration, stretching, hair movement

25. Thermoreceptors • free nerve endings that detect hot or cold in the skin • other sensors deep in the body regulate temp of the blood, helping the hypothalamus regulate body temp

26. Nociceptors (noci- as in noxious): • two kinds for temperature (hot or cold) • Two for pain (rapid pricking pain or slow burning pain) • important for a number of reasons: • often indicates danger • can make us aware of injury or disease

27. Pathway of communication to 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

28. our sense of touch can be tested using calipers to test our two-point threshold • acuityis greatest in the most densely nerve-packed areas of the body

29. EFFECTS OF AGING: • people lose sensory receptors as they age • age 10: ~50 touch receptors/square mm • age 50: ~10/square mm • nevertheless, there is little midlife loss of sensitivity (there are still more receptors than nerve fibers) • preliminary results show a 50% loss in touch acuity by around age 70

30. THE EAR: HEARING & BALANCE • The ear is divided into 3 major areas: • outer ear (hearing) • middle ear (hearing) • inner ear (hearing & balance)

31. Parts of the outer ear: • pinna or auricle = shell-shaped flap that collects sound waves & channels them in • Helix (the rim) + lobe • 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

32. 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

33. 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 • in children they are short & nearly horizontal, which permits bacteria to spread from the pharynx to the middle ear = otitis media

34. The boundary between the middle & inner ear is the oval window • The inner ear is a cavity called thebony labyrinth

35. 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

36. cochlea = “snail” • houses the organ of Corti, the actual organ for hearing • organ of Corti consists of hair cells (actually specialized microvilli) embedded in the basilar membrane • 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 vestibulocochlear nerve • filled with perilymph, fluid similar to and continuous with cerebrospinal fluid

38. MECHANISMS OF HEARING: • A vibrating object creates pressure waves in the surrounding air • 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 • stirrup transmits the vibrations to the inner ear through the oval window

39. pressure waves pass through the cochlea and bends the hair cells • a receptor potential results, & neurotransmitters are released • neurotransmitters cross the synapse & cause an action potential in a sensory neuron that travels through the auditory nerve to the brain

40. 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

41. 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

43. BALANCE & EQUILIBRIUM: • inside the utricle & saccule there are hair cells embedded in a gelatinous membrane with tiny crystals of calcium carbonate called otoliths • gravity pulls on the otoliths & bends the hair cells as the position of the head changes • Nerves send messages that the brain interprets to determine where your body is at rest

45. the semicircular canals provide information about the body in motion • consist of fluid-filled membranous ovals oriented in 3 different planes • at the base of each is an enlarged area called the ampulla which contains hair cells called crista

46. As the body moves, the hair cells are bent & then they straighten • The bending generates impulses carried by the 8th cranial nerve to the brain where they are interpreted as starting or stopping, or changing direction

47. 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

48. THE SENSE OF VISION: • Our eyes: • can detect many colors • can form images of objects near & far • can respond to minute amounts of light energy

49. 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

50. THE ANATOMY OF VISION: • The outer surface of the eyeball is a tough whitish layer of c.t. 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