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Chapter 16 The Special Senses

Chapter 16 The 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. Chemical Senses. Interaction of molecules with receptor cells Olfaction (smell) and gustation (taste)

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Chapter 16 The Special Senses

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  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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. Tortora & Grabowski 9/e 2000 JWS

  5. Olfaction: Sense of Smell • Odorants bind to receptors • Na+ channels open • Depolarization occurs • Nerve impulse is triggered Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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. Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  14. Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme. Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  16. Tunics (Layers) of Eyeball • Fibrous Tunic(outer layer) • Vascular Tunic (middle layer) • Nervous Tunic(inner layer) Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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) Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  23. Vascular Tunic -- Suspensory ligament • Suspensory ligaments attach lens to ciliary process • Ciliary muscle controls tension on ligaments & lens Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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) Tortora & Grabowski 9/e 2000 JWS

  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. Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  44. Release of Neurotransmitters Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  46. synapse in thalamus & visual cortex Brain Pathways of Vision Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

  49. Anatomy of the Ear Region Tortora & Grabowski 9/e 2000 JWS

  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 Tortora & Grabowski 9/e 2000 JWS

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