Chapter 15: The Special Senses

Chapter 15: The Special Senses Objectives: 1. Describe the structure and function of accessory eye structures, eye layers, the lens, and humors of the eye. 2. Trace that pathway of light through the eye to the retina, and explain how light is focused for distant and close vision. 3. Describe the events involved in the stimulation of photoreceptors by light, and compare and contrast the roles of rods and cones in vision. 4. Note the cause and consequence of astigmatism, cataract, glaucoma, hyperopia, myopia, presbyopia and color blindness. 5. Compare and contrast light and dark adaptation. 6. Trace the visual pathway to the visual cortex, and briefly describe the steps in visual processing. 7. Describe the location, structure, and afferent pathways of taste and smell receptors, and explain how these receptors are activated. 8. Describe the structure and general function of the outer, middle and internal ears. 9. Describe the sound conduction pathway to the fluids of the internal ear, and follow the auditory pathway from the spiral organ (of Corti) to the temporal cortex. 10. Explain how one is able to differentiate pitch and loudness and localize the source of sounds. 11. Explain how the balance organs of the semicircular canals and the vestibule help maintain dynamic and static equilibrium. 12. List possible causes and symptoms of otitis media, deafness, Ménière s syndrome, and motion sickness. 13. List changes that occur in the special sense organs with aging. I. The Eye and Vision A. Eye and Associated Structures 1. 70% of all sensory receptors are in the eye 2. Most of the eye is protected by a cushion of fat and the bony orbit 3. Accessory structures include a. Eyebrows b. Eyelids c. Conjunctiva 1

d. Lacrimal apparatus e. Extrinsic eye muscles B. Eyebrows 1. Coarse hairs that overlie the supraorbital margins 2. Functions include: a. Shading the eye b. Preventing perspiration from reaching the eye C. Palpebrae (Eyelids) 1. Protect the eye anteriorly 2. Lacrimal caruncle a. Contains glands that secrete a whitish, oily secretion (Sandman s eye sand) D. Eyelashes 1. Project from the free margin of each eyelid 2. Initiate reflex blinking 3. Lubricating glands associated with the eyelids E. Conjunctiva 1. Transparent membrane that: a. Lines the eyelids as the palpebral conjunctiva b. Covers the whites of the eyes as the ocular conjunctiva c. Lubricates and protects the eye F. Lacrimal Apparatus 1. Consists of the lacrimal gland and associated ducts 2. Lacrimal glands secrete tears 3. Tears a. Contain mucus, antibodies, and lysozyme b. Enter the eye via superolateral excretory ducts c. Exit the eye medially via the lacrimal punctum d. Drain into the nasolacrimal duct G. Structure of the Eyeball 1. A slightly irregular hollow sphere with anterior and posterior poles 2. The wall is composed of three tunics a. Fibrous b. Vascular c. Sensory 3. The internal cavity is filled with fluids called humors 4. The lens separates the internal cavity into anterior and posterior segments 5. Fibrous Tunic 2

a. Forms the outermost coat of the eye and is composed of: 1) Opaque sclera (posteriorly) a) The sclera protects the eye and anchors extrinsic muscles 2) Clear cornea (anteriorly) a) The cornea lets light enter the eye 6. Vascular Tunic (Uvea): Choroid Region a. Has three regions 1) Choroid a) Supplies blood to all eye tunics 2) Ciliary body a) A thickened ring of tissue surrounding the lens b) Composed of smooth muscle bundles (ciliary muscles) c) Anchors the suspensory ligament that holds the lens in place 3) Iris a) The colored part of the eye b) Pupil i. Central opening of the iris c) Regulates the amount of light entering the eye during: i. Close vision and bright light pupils constrict ii. Distant vision and dim light pupils dilate 7. Sensory Tunic: Retina a. A delicate two-layered membrane 1) Pigmented layer a) Outer layer that absorbs light and prevents its scattering 2) Neural layer, which contains: a) Photoreceptors that transduce light energy b. Ganglion cell axons: 1) Run along the inner surface of the retina 2) Leave the eye as the optic nerve c. The optic disc: 1) Is the site where the optic nerve leaves the eye 2) Lacks photoreceptors (the blind spot) d. Photoreceptors 1) Rods: a) Respond to dim light b) Are used for peripheral vision 2) Cones: 3

a) Respond to bright light b) Have high-acuity color vision c) Are found in the macula lutea d) Are concentrated in the fovea centralis H. Inner Chambers and Fluids 1. The lens separates the internal eye into anterior and posterior segments 2. The posterior segment is filled with a clear gel called vitreous humor that: a. Transmits light b. Supports the posterior surface of the lens c.holds the neural retina firmly against the pigmented layer 3. Anterior Segment a. Composed of two chambers 1) Anterior a) Between the cornea and the iris 2) Posterior a) Between the iris and the lens 4. Aqueous humor a. A plasmalike fluid that fills the anterior segment b. Supports, nourishes, and removes wastes 5. Lens a. A biconvex, transparent, flexible, avascular structure that: 1) Allows precise focusing of light onto the retina 2) With age, the lens becomes more compact and dense and loses its elasticity I. Light 1. Our eyes respond to a small portion of light called the visible spectrum 2. Different cones in the retina respond to different wavelengths of the visible spectrum 3. Refraction and Lenses a. When light passes from one transparent medium to another its speed changes and it refracts (bends) b. Light passing through a convex lens (as in the eye) is bent so that the rays converge to a focal point c. When a convex lens forms an image, the image is upside down and reversed right to left 4. Focusing for Distant Vision 4

a. Light from a distance needs little adjustment for proper focusing b. Far point of vision 1) Distance beyond which the lens does not need to change shape to focus (20 ft.) 5. Close vision requires: a. Accommodation 1) Changing the lens shape by ciliary muscles to increase refractory power b. Constriction 1) Pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye c. Convergence 1) Medial rotation of the eyeballs toward the object being viewed 6. Problems of Refraction a. Emmetropic eye 1) Normal eye with light focused properly b. Myopic eye (nearsighted) 1) Focal point is in front of the retina 2) Corrected with a concave lens c. Hyperopic eye (farsighted) 1) Focal point is behind the retina 2) Corrected with a convex lens J. Adaptation 1. Adaptation to bright light (going from dark to light) involves: a. Dramatic decreases in retinal sensitivity 1) Rod function is lost b. Switching from the rod to the cone system 1) Visual acuity is gained 2. Adaptation to dark is the reverse a. Cones stop functioning in low light K. Depth Perception 1. Achieved by both eyes viewing the same image from slightly different angles 2. Three-dimensional vision results from cortical fusion of the slightly different images 3. If only one eye is used, depth perception is lost and the observer must rely on learned clues to determine depth 5

II. Chemical Senses A. Gustation (taste) B. Olfaction (smell) C. Their chemoreceptors respond to chemicals in aqueous solution 1. Taste a. Substances dissolved in saliva 2. Smell a. Substances dissolved in fluids of the nasal membranes C. Sense of Smell 1. The organ of smell is the olfactory epithelium, which covers the superior nasal concha 2. Olfactory receptor cells are bipolar neurons with radiating olfactory cilia 3. Physiology of Smell a. Olfactory receptors respond to several different odor-causing chemicals D. Taste Buds 1. Taste buds are found in papillae of the tongue mucosa 2. Taste Sensations a. There are five basic taste sensations 1) Sweet a) Sugars, saccharin, alcohol, and some amino acids 2) Salt a) Metal ions 3) Sour a) Hydrogen ions 4) Bitter a) Alkaloids such as quinine and nicotine 5) Umami a) Elicited by the amino acid glutamate 3. Physiology of Taste a. In order to be tasted, a chemical: 1) Must be dissolved in saliva 2) Must contact gustatory hairs 4. Influence of Other Sensations on Taste a. Taste is 80% smell b. Thermoreceptors, mechanoreceptors, nociceptors also influence tastes c. Temperature and texture enhance or detract from taste 6

III. The Ear: Hearing and Balance A. The three parts of the ear are the inner, outer, and middle ear B. The outer and middle ear are involved with hearing C. The inner ear functions in both hearing and equilibrium D. Receptors for hearing and balance: 1. Respond to separate stimuli 2. Are activated independently E. Outer Ear 1. The auricle (pinna) is composed of: a. The helix (rim) b. The lobule (earlobe) 2. External auditory canal 3. Tympanic membrane (eardrum) a. Thin connective tissue membrane that vibrates in response to sound b. Transfers sound energy to the middle ear ossicles c. Boundary between outer and middle ears F. Middle Ear (Tympanic Cavity) 1. Pharyngotympanic tube connects the middle ear to the nasopharynx a. Equalizes pressure in the middle ear cavity with the external air pressure G. Ear Ossicles 1. The tympanic cavity contains three small bones a. Malleus b. Incus c. Stapes d. Transmit vibratory motion of the eardrum to the oval window H. Inner Ear 1. Bony labyrinth a. Contains 1) Vestibule 2) Cochlea 3) Semicircular canals b. Filled with perilymph 2. Membranous labyrinth a. Series of membranous sacs within the bony labyrinth b. Filled with a potassium-rich fluid 3. The Vestibule a. The central egg-shaped cavity of the bony labyrinth 7

b. Suspended in its perilymph are two sacs: 1) Saccule a) Extends into the cochlea 2) Utricle a) Extends into the semicircular canals 3) House equilibrium receptors called maculae a) Respond to gravity and changes in the position of the head 4. The Semicircular Canals a. Three canals that each define two-thirds of a circle and lie in the three planes of space b. Membranous semicircular ducts line each canal and communicate with the utricle c. The ampulla is the swollen end of each canal and it houses equilibrium receptors in a region called the crista ampullaris 1) These receptors respond to angular movements of the head 5. The Cochlea a. A spiral, conical, bony chamber that: b. Extends from the anterior vestibule c. Contains the cochlear duct, which ends at the cochlear apex d. Contains the organ of Corti (hearing receptor) I. Sound and Mechanisms of Hearing 1. Sound vibrations beat against the eardrum 2. The eardrum pushes against the ossicles, which presses fluid in the inner ear against the oval and round windows 3. This movement sets up shearing forces that pull on hair cells 4. Moving hair cells stimulates the cochlear nerve that sends impulses to the brain 4. Properties of Sound a. Sound is: 1) A pressure disturbance (alternating areas of high and low pressure) originating from a vibrating object b. Frequency 1) Number of waves that pass a given point in a given time c. Pitch 1) Perception of different frequencies (we hear from 20 20,000 Hz) 8

d. Amplitude 1) Intensity of a sound measured in decibels (db) e. Loudness 1) Subjective interpretation of sound intensity 5. Transmission of Sound to the Inner Ear a. The route of sound to the inner ear follows this pathway: 1) Pinna 2) Auditory canal 3) Eardrum 4) Malleus 5) Incus 6) Stapes 7) Oval window 8) Stimulation of the organ of Corti 9) Generation of impulses in the cochlear nerve 6. Deafness a. Conduction deafness 1) Something hampers sound conduction to the fluids of the inner ear 2) e.g., impacted earwax, perforated eardrum, osteosclerosis of the ossicles b. Sensorineural deafness 1) Results from damage to the neural structures at any point from the cochlear hair cells to the auditory cortical cells c. Tinnitus 1) Ringing or clicking sound in the ears in the absence of auditory stimuli J. Mechanisms of Equilibrium and Orientation 1. Vestibular apparatus a. Equilibrium receptors in the semicircular canals and vestibule b. Maintains our orientation and balance in space c. Vestibular receptors monitor static equilibrium d. Semicircular canal receptors monitor dynamic equilibrium 2. Anatomy of Maculae a. Maculae are the sensory receptors for static equilibrium b. Contain supporting cells and hair cells c. Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane d. Otolithic membrane 9

1) Jellylike mass studded with tiny CaCO 3 stones called otoliths 2) Utricular hairs respond to horizontal movement 3) Saccular hairs respond to vertical movement e. Crista Ampullaris and Dynamic Equilibrium 1) The crista ampullaris (or crista): a) Is the receptor for dynamic equilibrium b) Is located in the ampulla of each semicircular canal c) Responds to angular movements IV. Developmental Aspects A. All special senses are functional at birth B. Chemical senses 1. Few problems occur until the fourth decade, when these senses begin to decline C. Vision 1. Vision is not fully functional at birth 2. Babies are hyperopic, see only gray tones, and eye movements are uncoordinated 3. Depth perception and color vision is well developed by age five 4. With age the lens loses clarity, dilator muscles are less efficient, and visual acuity is drastically decreased by age 70 10