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

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

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

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

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

Hair cells damaged by loud noise

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

Smell and Taste 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

Taste (Gustation) We have about 10,000 taste buds on tongue, palate, cheek, pharynx (back of throat) Each taste bud has 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)

Papillae with taste buds Taste bud

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