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. Cells of the Olfactory Membrane • Olfactory receptors • bipolar neurons with cilia or olfactory hairs • Supporting cells • columnar epithelium • Basal cells = stem cells • replace receptors monthly • Olfactory glands • produce mucus • Both epithelium & glands innervated cranial nerve VII.

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

6. 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

7. Olfactory Pathway • Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb • pass through 40 foramina in cribriform plate • Second-order neurons within the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe • conscious awareness of smell begins • Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs

8. 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 • Found on sides of circumvallate & fungiform papillae • 3 cell types: supporting, receptor & basal cells

9. 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.

10. 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 • Mechanism • dissolved substance contacts gustatory hairs • receptor potential results in neurotransmitter release • nerve impulse formed in 1st-order neuron

11. Gustatory Pathway • First-order gustatory fibers found in cranial nerves • VII (facial) serves anterior 2/3 of tongue • IX (glossopharyngeal) serves posterior 1/3 of tongue • X (vagus) serves palate & epiglottis • Signals travel to thalamus or limbic system & hypothalamus • Taste fibers extend from the thalamus to the primary gustatory area on parietal lobe of the cerebral cortex • providing conscious perception of taste

12. Accessory Structures of Eye • Eyelids or palpebrae • protect & lubricate • epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva • Tarsal glands • oily secretions keep lids from sticking together • Conjunctiva • palpebral & bulbar • stops at corneal edge • dilated BV--bloodshot

13. 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

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

15. Extraocular Muscles • Six muscles that insert on the exterior surface of the eyeball • Innervated by CN III, IV or VI. • 4 rectus muscles -- superior, inferior, lateral and medial • 2 oblique muscles -- inferior and superior

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

17. Fibrous Tunic -- Description of Cornea • Transparent • Helps focus light(refraction) • astigmatism • 3 layers • nonkeratinized stratified squamous • collagen fibers & fibroblasts • simple squamous epithelium • Transplants • common & successful • no blood vessels so no antibodies to cause rejection • Nourished by tears & aqueous humor

18. Fibrous Tunic -- Description of Sclera • “White” of the eye • Dense irregular connective tissue layer -- collagen & fibroblasts • Provides shape & support • At the junction of the sclera and cornea is an opening (scleral venous sinus) • Posteriorly pierced by Optic Nerve (CNII)

19. 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

20. 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 • Autonomic reflexes • circular muscle fibers contract in bright light to shrink pupil • radial muscle fibers contract in dim light to enlarge pupil

21. Vascular Tunic -- Muscles of the Iris • Constrictor pupillae (circular) are innervated by parasympathetic fibers while Dilator pupillae (radial) are innervated by sympathetic fibers. • Response varies with different levels of light

22. Vascular Tunic -- Description of lens • Avascular • Crystallin proteins arranged like layers in onion • Clear capsule & perfectly transparent • Lens held in place by suspensory ligaments • Focuses light on fovea

23. Vascular Tunic -- Suspensory ligament • Suspensory ligaments attach lens to ciliary process • Ciliary muscle controls tension on ligaments & lens

24. 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

25. Layers of Retina • Pigmented epithelium • nonvisual portion • absorbs stray light & helps keep image clear • 3 layers of neurons (outgrowth of brain) • photoreceptor layer • bipolar neuron layer • ganglion neuron layer • 2 other cell types (modify the signal) • horizontal cells • amacrine cells

26. 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

27. 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

28. Cavities of the Interior of Eyeball • Anterior cavity (anterior to lens) • filled with aqueous humor • produced by ciliary body • continually drained • replaced every 90 minutes • 2 chambers • anterior chamber between cornea and iris • posterior chamber between iris and lens • Posterior cavity (posterior to lens) • filled with vitreous body (jellylike) • formed once during embryonic life • floaters are debris in vitreous of older individuals

29. Aqueous Humor • Continuously produced by ciliary body • Flows from posterior chamberinto anterior through the pupil • Scleral venous sinus • canal of Schlemm • opening in white of eyeat junction of cornea & sclera • drainage of aqueous humor from eye to bloodstream • Glaucoma • increased intraocular pressure that could produce blindness • problem with drainage of aqueous humor

30. 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

31. 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

32. 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 • Light rays from > 20’ are nearly parallel and only need to be bent enough to focus on retina • Light rays from < 6’ are more divergent & need more refraction • extra process needed to get additional bending of light is called accommodation

33. Accommodation & the Lens • Convex lens refract light rays towards each other • Lens of eye is convex on both surfaces • View a distant object • lens is nearly flat by pulling of suspensory ligaments • View a close object • ciliary muscle is contracted & decreases the pull of the suspensory ligaments on the lens • elastic lens thickens as the tension is removed from it • increase in curvature of lens is called accommodation

34. 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

35. 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

36. Constriction of the Pupil • Constrictor pupillae muscle contracts • Narrows beam of light that enters the eye • 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

37. 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 • required so that light rays from the object will strike both retinas at the same relative point • extrinsic eye muscles must coordinate this action

38. Photoreceptors • Named for shape of outer segment • Transduction of light energy into a receptor potential in outer segment • 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

39. 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

40. Photopigments • Isomerization • light cause cis-retinal to straighten & become trans-retinal shape • Bleaching • enzymes separate the trans-retinal from the opsin • colorless final products • Regeneration • in darkness, an enzyme converts trans-retinal back to cis-retinal (resynthesis of a photopigment)

41. Regeneration of Photopigments • Pigment epithelium near the photoreceptors contains large amounts of vitamin A and helps the regeneration process • After complete bleaching, it takes 5 minutes to regenerate 1/2 of the rhodopsin but only 90 seconds to regenerate the cone photopigments • Full regeneration of bleached rhodopsin takes 30 to 40 minutes • Rods contribute little to daylight vision, since they are bleached as fast as they regenerate.

42. 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

43. Formation of Receptor Potentials • In darkness • Na+ channels are held open and photoreceptor is always partially depolarized (-30mV) • continuous release of inhibitory neurotransmitter onto bipolar cells • In light • enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70mV) • release of inhibitory neurotransmitter is stopped • bipolar cells become excited and a nerve impulse will travel towards the brain

44. Release of Neurotransmitters

45. Retinal Processing of Visual Information • Convergence • one cone cell synapses onto one bipolar cell produces best visual acuity • 600 rod cells synapse on single bipolar cell increasing light sensitivity although slightly blurry image results • 126 million photoreceptors converge on 1 million ganglion cells • Horizontal and amacrine cells • horizontal cells enhance contrasts in visual scene because laterally inhibit bipolar cells in the area • amacrine cells excited bipolar cells if levels of illumination change

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

47. 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

48. 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

49. Anatomy of the Ear Region

50. 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