Chapter 16 The Special Senses

1. Chapter 16The Special Senses • Smell, taste, vision, hearing and equilibrium • Housed in complex sensory organs • Ophthalmology is science of the eye • Otolaryngology is science of the ear

2. Chemical Senses • Interaction of molecules with receptor cells • Olfaction (smell) and gustation (taste) • Both project to cerebral cortex & limbic system • evokes strong emotional reactions

3. Olfactory Epithelium • 1 square inch of membrane holding 10-100 million receptors • Covers superior nasal cavity and cribriform plate • 3 types of receptor cells

4. Olfaction: Sense of Smell • Odorants bind to receptors • Na+ channels open • Depolarization occurs • Nerve impulse is triggered

5. Adaptation & Odor Thresholds • Adaptation = decreasing sensitivity • Olfactory adaptation is rapid • 50% in 1 second • complete in 1 minute • Low threshold • only a few molecules need to be present • methyl mercaptan added to natural gas as warning

6. Gustatory Sensation: Taste • Taste requires dissolving of substances • Four classes of stimuli--sour, bitter, sweet, and salty • 10,000 taste buds found on tongue, soft palate & larynx

7. Anatomy of Taste Buds • An oval body consisting of 50 receptor cells surrounded by supporting cells • A single gustatory hair projects upward through the taste pore • Basal cells develop into new receptor cells every 10 days.

8. Physiology of Taste • Complete adaptation in 1 to 5 minutes • Thresholds for tastes vary among the 4 primary tastes • most sensitive to bitter (poisons) • least sensitive to salty and sweet

9. Accessory Structures of Eye • Eyelids or palpebrae • protect & lubricate • Tarsal glands • oily secretions keep lids from sticking together • Conjunctiva • stops at corneal edge • dilated BV--bloodshot

10. Eyelashes & Eyebrows Eyeball = 1 inch diameter 5/6 of Eyeball inside orbit & protected • Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight • Sebaceous glands are found at base of eyelashes (sty) • Palpebral fissure is gap between the eyelids

11. Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme.

12. Extraocular Muscles • Six muscles that insert on the exterior surface of the eyeball • . • 4 rectus muscles -- superior, inferior, lateral and medial • 2 oblique muscles -- inferior and superior

13. Tunics (Layers) of Eyeball • Fibrous Tunic(outer layer) • Vascular Tunic (middle layer) • Nervous Tunic(inner layer)

14. Fibrous Tunic -- Description of Cornea • Transparent • Helps focus light(refraction) • astigmatism • Transplants • common & successful • no blood vessels so no antibodies to cause rejection

15. Fibrous Tunic -- Description of Sclera • “White” of the eye • Dense irregular connective tissue layer -- collagen & fibroblasts • Provides shape & support

16. Vascular Tunic -- Choroid & Ciliary Body • Choroid • pigmented epithilial cells (melanocytes) & blood vessels • provides nutrients to retina • black pigment in melanocytes absorb scattered light • Ciliary body • ciliary processes • folds on ciliary body • secrete aqueous humor • ciliary muscle • smooth muscle that alters shape of lens

17. Vascular Tunic -- Iris & Pupil • Colored portion of eye • Shape of flat donut suspended between cornea & lens • Hole in center is pupil • Function is to regulate amount of light entering eye

18. Vascular Tunic -- Description of lens • Avascular • Crystallin proteins arranged like layers in onion • Clear capsule & perfectly transparent

19. Nervous Tunic -- Retina • Posterior 3/4 of eyeball • Optic disc • optic nerve exiting back of eyeball • Central retina BV • fan out to supply nourishment to retina • visible for inspection • hypertension & diabetes • Detached retina • trauma (boxing) • fluid between layers • distortion or blindness View with Ophthalmoscope

20. Rods & Cones--Photoreceptors • Rods----rod shaped • shades of gray in dim light • 120 million rod cells • discriminates shapes & movements • distributed along periphery • Cones----cone shaped • sharp, color vision • 6 million • fovea of macula lutea • densely packed region • at exact visual axis of eye • 2nd cells do not cover cones • sharpest resolution or acuity

21. Pathway of Nerve Signal in Retina • Light penetrates retina • Rods & cones transduce light into action potentials • Rods & cones excite bipolar cells • Bipolars excite ganglion cells • Axons of ganglion cells form optic nerve leaving the eyeball (blind spot) • To thalamus & then the primary visual cortex

22. Aqueous Humor • Continuously produced by ciliary body • Flows from posterior chamberinto anterior through the pupil • Glaucoma • increased intraocular pressure that could produce blindness • problem with drainage of aqueous humor

23. Major Processes of Image Formation • Refraction of light • by cornea & lens • light rays must fall upon the retina • Accommodation of the lens • changing shape of lens so that light is focused • Constriction of the pupil • less light enters the eye

24. Definition of Refraction • Bending of light as it passes from one substance (air) into a 2nd substance with a different density(cornea) • In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens

25. Refraction by the Cornea & Lens • Image focused on retina is inverted & reversed from left to right • Brain learns to work with that information • 75% of Refraction is done by cornea -- rest is done by the lens

26. Near Point of Vision and Presbyopia • Near point is the closest distance from the eye an object can be & still be in clear focus • 4 inches in a young adult • 8 inches in a 40 year old • lens has become less elastic • 31 inches in a 60 to 80 year old • Reading glasses may be needed by age 40 • presbyopia • glasses replace refraction previously provided by increased curvature of the relaxed, youthful lens

27. Correction for Refraction Problems • Emmetropic eye (normal) • can refract light from 20 ft away • Myopia (nearsighted) • eyeball is too long from front to back • glasses concave • Hypermetropic (farsighted) • eyeball is too short • glasses convex (coke-bottle) • Astigmatism • corneal surface wavy • parts of image out of focus

28. Constriction of the Pupil • Constrictor pupillae muscle contracts • Prevents light rays from entering the eye through the edge of the lens • Sharpens vision by preventing blurry edges • Protects retina very excessively bright light

29. Convergence of the Eyes • Binocular vision in humans has both eyes looking at the same object • As you look at an object close to your face, both eyeballs must turn inward. • convergence

30. Photoreceptors • Photopigment is integral membrane protein of outer segment membrane • photopigment membrane folded into “discs” & replaced at a very rapid rate • Photopigments = opsin (protein) + retinal (derivative of vitamin A) • rods contain rhodopsin • cone photopigments contain 3 different opsin proteins permitting the absorption of 3 different wavelengths (colors) of light

31. Color Blindness & Night Blindness • Color blindness • inability to distinguish between certain colors • absence of certain cone photopigments • red-green color blind person can not tell red from green • Night blindness (nyctalopia) • difficulty seeing in low light • inability to make normal amount of rhodopsin • possibly due to deficiency of vitamin A

32. Light and Dark Adaptation • Light adaptation • adjustments when emerge from the dark into the light • Dark adaptation • adjustments when enter the dark from a bright situation • light sensitivity increases as photopigments regenerate • during first 8 minutes of dark adaptation, only cone pigments are regenerated, so threshold burst of light is seen as color • after sufficient time, sensitivity will increase so that a flash of a single photon of light will be seen as gray-white

33. synapse in thalamus & visual cortex Brain Pathways of Vision

34. Processing of Image Data in the Brain • Visual information in optic nerve travels to • occipital lobe for vision • midbrain for controlling pupil size & coordination of head and eye movements • hypothalamus to establish sleep patterns based upon circadian rhythms of light and darkness

35. Visual fields • Left occipital lobe receives visual images from right side of an object through impulses from nasal 1/2 of the right eye and temporal 1/2 of the left eye • Left occipital lobe sees right 1/2 of the world • Fibers from nasal 1/2 of each retina cross in optic chiasm

36. Anatomy of the Ear Region

37. External Ear • Function = collect sounds • Structures • auricle or pinna • elastic cartilage covered with skin • external auditory canal • curved 1” tube of cartilage & bone leading into temporal bone • ceruminous glands produce cerumen = ear wax • tympanic membrane or eardrum • epidermis, collagen & elastic fibers, simple cuboidal epith. • Perforated eardrum (hole is present) • at time of injury (pain, ringing, hearing loss, dizziness) • caused by explosion, scuba diving, or ear infection

38. Middle Ear Cavity

39. Middle Ear Cavity • Air filled cavity in the temporal bone • Separated from external ear by eardrum and from internal ear by oval & round window • 3 ear ossicles connected by synovial joints • malleus attached to eardrum, incus & stapes attached by foot plate to membrane of oval window • stapedius and tensor tympani muscles attach to ossicles • Auditory tube leads to nasopharynx • helps to equalize pressure on both sides of eardrum • Connection to mastoid bone =mastoiditis

40. Inner Ear---Bony Labyrinth Vestibule canals • Bony labyrinth = set of tubelike cavities in temporal bone • semicircular canals, vestibule & cochlea lined with periosteum & filled with perilymph • surrounds & protects Membranous Labyrinth ampulla

41. Inner Ear---Membranous Labyrinth • Membranous labyrinth = set of membranous tubes containing sensory receptors for hearing & balance and filled with endolymph • utricle, saccule, ampulla, 3 semicircular ducts & cochlea

42. Anatomy of the Organ of Corti • 16,000 hair cells have 30-100 stereocilia(microvilli ) • Microvilli make contact with tectorial membrane (gelatinous membrane that overlaps the spiral organ of Corti) • Basal sides of inner hair cells synapse with 1st order sensory neurons whose cell body is in spiral ganglion

43. Vibrating object causes compression of air around it = sound waves audible range is 20 to 20,000 Hz hear best within 500 to 5000 cycles/sec or Hz speech is 100 to 3000 Hz Frequency of a sound vibration is pitch higher frequency is higher pitch Greater intensity (size) of vibration, the louder the sound measured in decibels (dB) Conversation is 60 dB; pain above 140dB OSA requires ear protection above 90dB Sound Waves

44. Deafness • Nerve deafness • damage to hair cells from antibiotics, high pitched sounds, anticancer drugs • the louder the sound the quicker the hearing loss • fail to notice until difficulty with speech • Conduction deafness • perforated eardrum • otosclerosis

45. Physiology of Hearing • Auricle collects sound waves • Eardrum vibrates • slow vibration in response to low-pitched sounds • rapid vibration in response to high-pitched sounds • Ossicles vibrate since malleus attached to eardrum • Stapes pushes on oval window producing fluid pressure waves in scala vestibuli & tympani • oval window vibration 20X more vigorous than eardrum • Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane • Microvilli are bent producing receptor potentials

46. Overview of Physiology of Hearing

47. Cochlear Implants • If deafness is due to destruction of hair cells • Microphone, microprocessor & electrodes translate sounds into electric stimulation of the vestibulocochlear nerve • artificially induced nerve signals follow normal pathways to brain • Provides only a crude representation of sounds

48. Physiology of Equilibrium (Balance) • Static equilibrium • maintain the position of the body (head) relative to the force of gravity • Dynamic equilibrium • maintain body position (head) during sudden movement of any type--rotation, deceleration or acceleration

49. Detection of Position of Head • Movement of stereocilia or kinocilium results in the release of neurotransmitter onto the vestibular branches of the vestibulocochler nerve

50. Detection of Rotational Movement • When head moves, the attached semicircular ducts and hair cells move with it • Nerve signals to the brain are generated indicating which direction the head has been rotated