Special Senses

1. Special Senses Chapter 8

2. Special Senses • Smell, taste, sight, and hearing • Equilibrium • Special sense receptors: • large, complex sensory organs (eyes, ears) • localized clusters of receptors (taste buds and olfactory epithelium) • Senses blend to give us our sensations

3. The Eye & Vision

4. External Anatomy of the Eye • Only anterior 1/6 of eye’s surface can normally be seen • Accessory structures: • Extrinsic eye muscles • Eyelids • Conjunctiva • Lacrimal apparatus

5. Accessory Structures • Extrinsic eye muscles • Six muscles are attached to outer surface of each eye • Produce gross eye movements

6. Accessory Structures • Eyelids • 4 layers: skin, muscle, connective tissue, conjunctiva (mucous membrane) • Moved by orbicularisoculimuscles (close) and levatatorpalpebraesuperioris (open)

7. Accessory Structures • Lacrimal apparatus • Consists of lacrimal gland & ducts that drain lacrimal secretions into nasal cavity • Lacrimal glands: continuously secrete dilute salt solution (tears) onto anterior surface of eyeball • Secretion contains mucus, antibodies, and lysozyme which is an enzyme that destroys bacteria • Cleans and protects eye surface as it moistens & lubricates it

8. Internal Structures • Eyeball is a hollow sphere • Wall composed of three layers: • Fibrous layer • Sclera • Cornea • Vascular layer • Choroid • Ciliary body • Iris • Sensory layer • Retina • Pigmented layer (prevent light from scattering) • Neural layer (photoreceptors)

9. Fibrous Layer • Sclera: thick, white connective tissue (“white of the eye”) • Opaque due to large, disorganized collagenous and elastic fibers • For protection & attachment • Cornea: clear “window” through which light enters eye • anterior portion of eye • many nerve endings (pain fibers), easily repairs itself • avascular, so only tissue in the body that can be transplanted without fear of rejection • helps focus entering light rays • Connective tissue with thin layer of epithelium; unusually regular fiber pattern

10. Fibrous Layer: Sclera & Cornea

11. Vascular Layer • Choroid: blood-rich nutritive tunic that contains a dark pigment • Prevents light from scattering inside the eye • Modified anteriorly to form ciliary body & iris • Produces melanin to absorb light • Contains blood vessels • Ciliary body: two smooth muscle structures to which the lens is attached with the ciliaryzonule (ligament) • Forms internal ring around front of eye (muscle fibers & ligaments) • Iris: pigmented, has an opening called the pupil through which light passes • Thin diaphragm composed of muscle tissue and connective tissue • Circular and radial muscle fibers control size of pupil (stimulated by photons of light) • Regulates the amount of light which enters the eye

12. Choroid

13. Ciliary Body

14. Sensory Layer: Retina • Retina – two layers which extends anteriorly only to the ciliary body • Pigmented layer • composed of pigment cells that absorb light and prevent light from scattering inside the light • act as phagocytes to remove dead or damaged receptor cells • store vitamin A needed for vision

15. Sensory Layer: Retina • Neural layer – contains millions of receptor cells • Photoreceptors (rods & cones): respond to light, bipolar neurons • 70% of sensory receptors are in the eyes • Rods: more sensitive in low light, gives general outline, seen as black and white • Cones: less sensitive in low light, sharp picture, color • Electrical impulse leaves the retina via the optic nerve & nerve impulses are transmitted to the optic cortex which results in vision • Fovea: all cones, sharpest vision (visual acuity) • Optic disc (“blind spot”) – where optic nerve leaves eyeball

16. Retina

17. Sensory Layer: Retina • Night blindness: fewer working rods (lack of vitamin A) • Day blindness: lack of working cones LINK

18. Sensory Layer: Retina • Rods: rhodopsin breaks down into opsin & retinal when struck by photons; initiates chemical reaction (action potential) which is sent to visual cortex (occipital lobe) • In bright light, nearly all rhodopsin I broken down, reducing rod sensitivity • Cones: three different light sensitive proteins connected to retinal – each most sensitive to a particular wavelength of visible light • Depending on which is stimulated, brain interprets that color • Erythrolabe: red • Chlorolabe: green • Cyanolabe: blue • Mixing & interpretation of color occurs in the brain, not the retina! (i.e. red light in one light & green light in another eye will be seen as yellow) Readpg. 286

19. Visual Pigments If all are stimulated at once, see white!

20. Color Blindness • Total color blindness: lack of all three cones • Partial color blindness: lack of one cone type (lack of red or green receptor is most common) • Sex-linked condition (carried on X chromosome)

21. Color Blindness

22. Internal Structures: Lens • Lies directly behind iris & pupil • Focuses light entering the eye on the retina (changes shape to focus) • Held upright in the eye by a suspensory ligament (ciliaryzonule) attached to ciliary body • Epithelial cells (cytoplasm is transparent part)

23. Cataracts • With age, lens becomes increasingly hard and opaque • Cataracts result from this process and cause vision to become hazy and distorted • Can eventually cause blindness in affected eye • Treatment: surgical removal of lens and replacement with lens implant or special cataract glasses

24. Internal Structures: Humors • Aqueous humor • Anterior to the lens, clear watery fluid • Fills space between cornea and lens • Nourishes, helps hold shape • Vitreous humor • Reinforces eyeball internally • Posterior to lens

25. Glaucoma • aqueous humor made more quickly than can be removed or drainage is blocked • pressure builds, damage results from compression of retina and optic nerve

26. Physiology of Vision • Light rays are bent (refracted) as light encounters the cornea, aqueous humor, lens, and vitreous humor • Refractory power of the lens can be changed by changing its shape (ciliary body controls shape of lens) • Accommodation: ability of eye to focus for close objects (less than 20 ft. away) • Image formed on retina is a real image (inverted)

27. Vision

29. Visual Pathways • Axons carrying impulses from retina are bundled together at posterior aspect of eyeball and issue from back of eye as optic nerve • Optic chiasma: fibers from medial side of eye cross over to opposite side of brain • Fiber tracts that result are optic tracts • Each optic tract contains fibers from the lateral side of the eye on the same side and the medial side of the opposite eye • Optic tract fibers synapse with neurons in the thalamus, whose axons form the optic radiation which runs to the occipital lobe of brain (visual interpretation occurs) • Each side of brain receives visual input from both eyes • Visual fields overlap to give humans binocular vision

31. The Ear: Hearing & Balance

32. Anatomy of the Ear • Divided into three major areas • External (outer) ear • Middle ear • Internal (inner) ear

33. External (Outer) Ear • Auricle (pinna) – “ear” • Collects and directs sound waves into the auditory canal (function largely lost in humans) • External acoustic meatus (auditory canal) • Short, narrow chamber carved into temporal bone of skull • Ceruminous glands – secrete cerumen (earwax) • Tympanic membrane (eardrum) – vibrate when sound waves strike it; separates external & middle ear

34. Middle Ear • Small, air-filled, mucosa-lined cavity within temporal bone • Transfers vibrations via the ossicles (smallest bones in body) • malleus (hammer), incus (anvil), and stapes (stirrup) • Stapes passes vibration to the oval window of the inner ear • Pharyngotympanic tube (auditory tube): pressure needs to be equalized to enable eardrum to vibrate

35. Middle Ear

36. Otitis Media • Inflammation of the middle ear • Pharyngotympanic tubes run more horizontally in children • Ear tubes – implanted in ear drum to allow pus to drain into external ear canal • Infants with bottles “propped” or fed lying flat can get fluid in their ears through the pharyngotympanic tube

37. Internal (Inner) Ear • Bony (osseous) cavity located behind the eye socket • Filled with perilymph (fluid) • Membranous labyrinth suspended in perilymph, contains endolymph • Three subdivisions • Cochlea • Vestibule • Semicircular canals • Lined with hair cells (mechanoreceptors)

38. Equilibrium • Vestibular apparatus: equilibrium receptors of ear • Static equilibrium: maculae receptors • Report on changes in position of head when body not moving (keep head erect) • Otoliths: tiny stones that roll in response to changes in pull of gravity • Dynamic equilibrium: bending of cupula indicates rotation (gelatinous cap) • Report on changes when body moving (i.e. spinning) • Receptors stimulate hair cells, which send impulses via the vestibular nerve to the cerebellum • Work together with proprioceptors for control & balance

39. Hearing • Spiral organ of corti – contains hair cells (hearing receptors) • Vibrations set cochlear fluids in motion, pressure waves cause vibrations • impulses are sent via cochlear nerve to temporal lobe (auditory cortex)

41. Chemical Senses: Smell & Taste

42. Olfaction • Olfactory receptors (chemoreceptors): receptors for sense of smell, occupy a postage stamp-size area in roof of each nasal cavity • 10-100,000,000 receptors in nose • Olfactory filaments (axons) make up olfactory nerve (cranial nerve I) which conducts impulses to olfactory cortex of brain • Olfactory lobes of brain (gray matter) – situated over nose (bottom of frontal lobe) • Olfactory impulses closely tied to limbic system • Sensitive receptors, just a few molecule can activate them (thousands of smells) • Adapt rather quickly when exposed to unchanging stimulus

44. Gustation: Taste • Taste buds: specific receptors for taste widely scattered in the oral cavity • Live 7-10 days! • protection (low threshold – bitter) • 10,000 taste buds (mostly on tongue) • Five tastes: sweet, salty, sour, bitter, and umami (savory) • Flavors: combination of 5 tastes and olfactory and touch sensations • 1st order neuron  medulla  hypothalamus or thalamus (limbic)  parietal lobe (conscious perception of taste)