Special Senses

Sight- Eyes

The Sense of Sight Accessory organs: Eyelids- 4 layers: skin, muscle, connective tissue, conjunctiva (mucous membrane) moved by orbicularis oculi muscles (close) and levatator palpebrae superioris (open)

Lacrimal apparatus- lacrimal gland- secretes tears continuously– tears contain lysozyme which protects against bacteria

6 extrinsic muscles- movement of eye

Structure of the Eye Cornea Helps focus entering light rays Connective tissue with thin layer of epithelium No blood vessels Unusually regular fiber pattern

Sclera (white of eye) Opaque due to large, disorganized collagenous and elastic fibers For protection and attachment

Choroid coat Contains blood vessels Produces melanin which absorbs excess light

Ciliary body Forms internal ring around front of eye (muscle fibers and ligaments)

Iris Colored portion of eye (pigmented versus non-pigmented) Thin diaphragm composed of connective tissue and muscle fibers Circular and radial muscle fibers control size of pupil (stimulated by photons of light)

Aqueous humor Fills space between cornea and lens Nourishes, helps hold shape

Glaucoma- aqueous humor made quicker than can be removed; pressure builds, damage results

Lens Lies directly behind iris and pupil Epithelial cells (cytoplasm is transparent part) Change shape to focus

Cataract- fluid in lens becomes cloudy and opaque

Vitreous humor- holds the shape of the back of the eye

Light entering the eye is refracted

Retina- contains photoreceptors Rods- more sensitive in low light, gives general outline, seen as black and white Cones- less sensitive in low light, sharp picture, color

Fovea- cones only, sharpest vision

Optic disk- where nerve fibers from retina enter optic nerve Known as the “blind spot”

Night blindness- fewer working rods (lack of vitamin A) Day blindness- lack of working cones

Visual pigments In rods- rhodopsin breaks down into opsin and retinal when struck by photons. This initiates a chemical rxn (action potential) which is sent to the visual cortex. (occipital lobe) In bright light nearly all rhodopsin is broken down, reducing rod sensitivity

In cones- 3 different light sensitive proteins connected to retinal Depending on which one is stimulated, brain interprets that color Erythrolabe- red Chlorolabe- green Cyanolabe- blue

If all are stimulated at once- white

1. Normal Color Vision: A: 29, B: 45, C: --, D: 26 4 Sex-Linked Traits:  1. Normal Color Vision:  A: 29,  B: 45,  C: --,  D: 26  2. Red-Green Color-Blind:  A: 70,  B: --,  C: 5,  D: --  3. Red Color-blind:  A: 70,  B: --,  C: 5,  D: 6  4. Green Color-Blind:  A: 70,  B: --,  C: 5,  D: 2

A few enter the thalamus to stimulate visual reflexes Axons of the retina form optic nerves which join at optic chiasma in which certain fibers cross (medial) and other fibers do not (lateral) A few enter the thalamus to stimulate visual reflexes

Hearing- Ears Also static and dynamic equilibrium

Outer Ear Composed of auricle (“ear”) and meatus (auditory canal) collects and directs sound waves ends at tympanic membrane (ear drum) glands secrete cerumen

Middle Ear Small air-filled, mucosa-lined cavity within temporal bone transfers vibrations via the malleus (hammer), incus (anvil) and stapes (stirrup) pressure needs to be equalized

Inner (internal) Ear Maze of bony labyrinth (cavity): cochlea (spiral) vestibule semicircular canals Filled with fluid Lined with mechanoreceptors- hair cells Cochlear nerve sends info to temporal lobe (auditory cortex) Maculae receptors report changes of position (static equilibrium)- with tiny stones Bending of cupula indicates rotation (dynamic equilibrium)

Smell- Olfactory bulbs Chemical sense- chemoreceptors called olfactory receptors Thousands of smells Very sensitive- adapt quickly Pathways closely tied to limbic system

Taste- Taste buds Replaced every 7-10 days!! Five basic tastes: sweet, sour, bitter, salty, and umami Taste tied to smell