2. The Retina • Retina is a delicate tissue composed of two layers • Sensory layer contains photoreceptors (rods and cones) that sense light • Sensory layer consists of photoreceptors (rods and cones), bipolar cells, and ganglion cells • Continuous with the optic nerve at the back of the eye

3. Photoreceptors Rods • Highly sensitive (stimulated by dim light) - good for “night vision” • Are scattered throughout the retina, most on the peripheral parts of the retina (few near center of retina) • Produce fuzzy images • Light absorbing pigment is rhodopsin (made from vitamin A) - long term vitamin A deficiency can impair night vision

4. Photoreceptors Cones • Low sensitivity (bright light is needed for stimulation) - dim light does not excite cones (you can’t see color or detail in the dark) • Are clustered at the center of the retina (fovea and surrounding macula) • Allow us to see with high visual acuity (detail) • Three types of cones - red, green, blue • Colorblindness results from lack of one or more types of cones

5. Regions of the ear • Outer ear (pinna, lobule, helix and external auditory canal) • Middle ear: Tympanic membrane (eardrum), ossicles (Malleus, Incus, Stapes), oval and round windows, auditory tube • Inner ear: cochlea, semicircular canals, vestibule

6. How does hearing occur? • Sound waves enter the external ear and vibrate the eardrum • Vibrations of the tympanic membrane are transmitted by the ossicles to the oval window to the cochlea • Fluid in the cochlea is set in motion which is detected by tiny hair cells on the basilar membrane of the cochlea which become depolarized and send impulses along the auditory nerve to the brain to be interpreted animation

7. Hair cells damaged by loud noise

8. Balance and equilibrium • Monitored by the vestibular system (semi-circular canals and maculae) • Maculae sense linear forces (nodding of the head)* • Semi-circular canals sense rotational forces of the head (spinning) • Vestibular system sends signals to brain stem and cerebellum to help maintain posture and affect eye movements

9. Olfactory Olfactory neurons are depolarized by chemicals from the air We have about 12 million olfactory cells and can recognize about 10,000 smells Gustatory Gustatory neurons are depolarized by chemicals dissolved in saliva We have about 10,000 taste buds Smell and Taste

10. Taste (Gustation) • We have about 10,000 taste buds on tongue, palate, cheek, pharynx (back of throat) • Each taste bud has 50-100 taste cells that have tiny hairs projecting through the taste pore of the taste bud • The hairs bind chemicals dissolved in saliva and impulses are sent along nerves traveling to the gustatory cortex (in parietal lobe)

11. Papillae with taste buds Taste bud

12. Smell (Olfaction) • We have about 12 million olfactory receptor cells which can recognize about 10,000 smells • Hair-like cilia from the cells project into the nasal passageways, binding chemicals that are inhaled and dissolve in mucous surrounding the cilia • Impulses are then sent along olfactory nerves traveling to the olfactory cortex (in temporal lobe) and to lower brain areas