The Special Senses: Smell, Taste, Vision, Hearing, and Equilibrium

1. Chapter 17 The Special Senses Lecture Outline

2. Chapter 17The 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 Principles of Human Anatomy and Physiology, 11e

3. Chemical Senses • Interaction of molecules with receptor cells • Olfaction (smell) and gustation (taste) • Both project to cerebral cortex & limbic system • evokes strong emotional reactions Principles of Human Anatomy and Physiology, 11e

4. Anatomy of olfactory receptors • The receptors for olfaction, which are bipolar neurons, are in the nasal epithelium in the superior portion of the nasal cavity (Figure 17.1). • They are first-order neurons of the olfactory pathway. • Supporting cells are epithelial cells of the mucous membrane lining the nose. • Basal stem cells produce new olfactory receptors. Principles of Human Anatomy and Physiology, 11e

5. Olfactory Epithelium • 1 square inch of membrane holding 10-100 million receptors • Covers superior nasal cavity and cribriform plate • 3 types of receptor cells Principles of Human Anatomy and Physiology, 11e

6. 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. Principles of Human Anatomy and Physiology, 11e

7. Physiology of Olfaction - Overview • Genetic evidence suggests there are hundreds of primary scents. • In olfactory reception, a generator potential develops and triggers one or more nerve impulses. • Adaptation to odors occurs quickly, and the threshold of smell is low: only a few molecules of certain substances need be present in air to be smelled. • Olfactory receptors convey nerve impulses to olfactory nerves, olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system. • Hyposmia, a reduced ability to smell, affects half of those over age 65 and 75% of those over 80. It can be caused by neurological changes, drugs, or the effects of smoking . Principles of Human Anatomy and Physiology, 11e

8. Olfaction: Sense of Smell • Odorants bind to receptors • Na+ channels open • Depolarization occurs • Nerve impulse is triggered Principles of Human Anatomy and Physiology, 11e

9. 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 Principles of Human Anatomy and Physiology, 11e

10. 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 Principles of Human Anatomy and Physiology, 11e

11. GUSTATORY: SENSE OF SMELL • Taste is a chemical sense. • To be detected, molecules must be dissolved. • Taste stimuli classes include sour, sweet, bitter, and salty. Principles of Human Anatomy and Physiology, 11e

12. Gustatory Sensation: Taste • Taste requires dissolving of substances • Four classes of stimuli--sour, bitter, sweet, and salty • Other “tastes” are a combination of the four taste sensations plus olfaction. • 10,000 taste buds found on tongue, soft palate & larynx • Found on sides of circumvallate & fungiform papillae • 3 cell types: supporting, receptor & basal cells Principles of Human Anatomy and Physiology, 11e

13. 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. Principles of Human Anatomy and Physiology, 11e

14. Physiology of Taste • Receptor potentials developed in gustatory hairs cause the release of neurotransmitter that gives rise to nerve impulses. • 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 Principles of Human Anatomy and Physiology, 11e

15. Gustatory Pathway • First-order gustatory fibers found in cranial nerves • V • 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 • provides conscious perception of taste Principles of Human Anatomy and Physiology, 11e

16. VISION • More than half the sensory receptors in the human body are located in the eyes. • A large part of the cerebral cortex is devoted to processing visual information. Principles of Human Anatomy and Physiology, 11e

17. Accessory Structures of Eye - Overview • Eyelids or palpebrae • protect & lubricate • epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva • Tarsal glands • oily secretions • Conjunctiva • palpebral & bulbar • stops at corneal edge Principles of Human Anatomy and Physiology, 11e

18. Eyelids • The eyelids shade the eyes during sleep, protect the eyes From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva (Figure 17.4a). • The tarsal plate gives form and support to the eyelids. • The tarsal glands secrete a fluid to keep the eye lids from adhering to each other. • The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids and is reflected onto the anterior surface of the eyeball. • Eyelashes and eyebrows help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun. Principles of Human Anatomy and Physiology, 11e

19. 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 Principles of Human Anatomy and Physiology, 11e

20. Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme. Principles of Human Anatomy and Physiology, 11e

21. 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 Principles of Human Anatomy and Physiology, 11e

22. Tunics (Layers) of Eyeball • The eye is constructed of three layers (Figure 17.5). • Fibrous Tunic(outer layer) • Vascular Tunic (middle layer) • Nervous Tunic(inner layer) Principles of Human Anatomy and Physiology, 11e

23. 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 Principles of Human Anatomy and Physiology, 11e

24. 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) Principles of Human Anatomy and Physiology, 11e

25. 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 Principles of Human Anatomy and Physiology, 11e

26. 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 Principles of Human Anatomy and Physiology, 11e

27. 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 Principles of Human Anatomy and Physiology, 11e

28. 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 Principles of Human Anatomy and Physiology, 11e

29. Vascular Tunic -- Suspensory ligament • Suspensory ligaments attach lens to ciliary process • Ciliary muscle controls tension on ligaments & lens Principles of Human Anatomy and Physiology, 11e

30. 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 Principles of Human Anatomy and Physiology, 11e

31. Photoreceptors • shapes of their outer segments differ • Rods • specialized for black-and-white vision in dim light • allow us to discriminate between different shades of dark and light • permit us to see shapes and movement. • Cones • specialized for color vision and sharpness of vision (high visual acuity) in bright light • most densely concentrated in the central fovea, a small depression in the center of the macula lutea. Principles of Human Anatomy and Physiology, 11e

32. Photoreceptors • The macula lutea is in the exact center of the posterior portion of the retina, corresponding to the visual axis of the eye. • The fovea is the area of sharpest vision because of the high concentration of cones. • Rods are absent from the fovea and macula and increase in density toward the periphery of the retina. Principles of Human Anatomy and Physiology, 11e

33. 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 Principles of Human Anatomy and Physiology, 11e

34. Rods & Cones--Photoreceptors • Rods----rod shaped • shades of gray in dim light • 120 million rod cells • 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 (acuity) Principles of Human Anatomy and Physiology, 11e

35. 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 Principles of Human Anatomy and Physiology, 11e

36. Lens • The eyeball contains the nonvascular lens, just behind the pupil and iris. • The lens fine tunes the focusing of light rays for clear vision. • With aging the lens loses elasticity and its ability to accommodate resulting in a condition known as presbyopia. Principles of Human Anatomy and Physiology, 11e

37. 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 Principles of Human Anatomy and Physiology, 11e

38. Eye Anatomy • The pressure in the eye, called intraocularpressure, is produced mainly by the aqueous humor. • The intraocular pressure, along with the vitreous body, maintains the shape of the eyeball and keeps the retina smoothly applied to the choroid so the retina will form clear images. • Glaucoma • increased intraocular pressure • problem with drainage of aqueous humor • may produce degeneration of the retina and blindness Principles of Human Anatomy and Physiology, 11e

39. 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 Principles of Human Anatomy and Physiology, 11e

40. 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 Principles of Human Anatomy and Physiology, 11e

41. 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 Principles of Human Anatomy and Physiology, 11e

42. 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 Principles of Human Anatomy and Physiology, 11e

43. Accommodation & the Lens • Accommodation is an increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects (figure 17.10c). • Convex lens refract light rays towards each other • Lens of eye is convex on both surfaces • Viewing 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 • The near point of vision is the minimum distance from the eye that an object can be clearly focused with maximum effort. Principles of Human Anatomy and Physiology, 11e

44. 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 Principles of Human Anatomy and Physiology, 11e

45. Refraction Abnormalities • Myopia is nearsightedness (Figure 17.11). • Hyperopia is farsightedness (Figure 17.11). • Astigmatism is a refraction abnormality due to an irregular curvature of either the cornea or lens. Principles of Human Anatomy and Physiology, 11e

46. 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 Principles of Human Anatomy and Physiology, 11e

47. 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 Principles of Human Anatomy and Physiology, 11e

48. 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. • In convergence, the eyeballs move medially so they are both directed toward an object being viewed. • required so that light rays from the object will strike both retinas at the same relative point • extrinsic eye muscles must coordinate this action Principles of Human Anatomy and Physiology, 11e

49. Physiology of Vision • The first step in vision transduction is the absorption of light by photopigments (visual pigments) in rods and cones (photoreceptors) (Figure 17.12). Principles of Human Anatomy and Physiology, 11e

50. Photoreceptors • Named for shape of outer segment • Receptors transduce light energy into a receptor potential in outer segment • Photopigment is integral membrane protein of outer segment membrane • photopigment membrane is folded into “discs” & replaced at a very rapid rate • Photopigments • opsin (protein) + retinal (derivative of vitamin A) Principles of Human Anatomy and Physiology, 11e