Senses

Accessory Structures of the Eye Eyebrow: shade the eye, protect from sweat Eyelids: protects, prevents drying of the eye. Conjunctiva: membrane that lines the eyelid and covers the white of the eye. Prevents the eye from drying. Pink eye is an infection of the conjunctiva

Structure of the Eye

Structure of the Eye Cornea: transparent layer, helps focus light rays The cornea is the only tissue that can be transplanted with almost no possibility of rejection. Cornea: transparent layer, helps focus light rays Sclera: white part of the eye, protects and shapes the eye Choroid: black-brown inner layer, absorbs excess light Lens: transparent, flexible structure, focuses images Ciliary body: ring of smooth muscle, controls the shape of the lens. Cataract = clouding of lens caused by age, smoking, sun damage. Vitamin C decreases risk.

Structure of the Eye (cont.) Iris: colored part of the eye, has muscles that change size of the pupil Pupil: opening in center of iris, allows light to enter the eye Retina: contains photoreceptors Rods: fuzzy vision in low light (gray scale) Cones: sharp, color vision (red, blue, green) Optic disc:“blind spot”, lacks photoreceptors Color blindness is due to a lack of one or more cones. Red-green is most common. Humans have about 100 million rods and 3 million cones.

Structure of the Eye (cont.) The fovea is about the size of the head of a pin Fovea: depression in retina, produces the sharpest vision Vitreous humor: clear gel that transmits light and supports the lens and retina. Aqueous humor: fluid that nourishes the lens, cornea, and retina. Glaucoma=aqueous humor blocked, pressure builds on optic nerve and retina. May cause blindness.

Basics of Sight We see reflected light. Diabetes and arteriosclerosis can both cause blindness. We see reflected light. Light passes through the cornea and lens, is focused on the photoreceptors of the retina. Image on retina is upside down and reversed from left to right. The visual cortex of the cerebrum corrects this so objects are seen in the correct position. Newborns have very poor eyesight and cannot see colors. How we see color http://www.youtube.com/watch?v=nAbxfKPKorc

Smell and Taste Senses of smell and taste are very closely linked. Both are chemoreceptors (respond to changes in chemical concentrations). 80% of flavor actually comes from smell. Primary taste sensations include sweet, salty, sour, bitter, and umami. Carbs stimulate sweet, acids stimulate sour Metal ions stimulate salty flavor Alkaloids stimulate bitter taste Umami is stimulated by specific amino acids. http://www.pbs.org/wgbh/nova/body/neuroscience-taste.html (13min)

Purpose of Taste Umami = intake of protein Sweet = intake of carbohydrates Salty = intake of some minerals Sour = Vitamin C Bitter = protective, many poisonous and spoiled substances are bitter.

Did you know… Humans have about 12 million olfactory receptors, 500 receptor types, and can discern 10,000 scents. Umami is involved in sensing the taste of steak, some cheeses, and MSG. Humans have about 10,000 taste buds. Don’t believe those taste bud maps, all tastes can be elicited from all areas that contain taste buds. Spicy foods actually stimulate pain receptors.

The Ear

Structures of the Ear External ear: Auricle (pinna): collects sound waves External auditory meatus Middle ear: Tympanic membrane (eardrum) Malleus, incus, stapes Inner ear: Cochlea Semicircular canals Auditory (eustachian) tube: connects middle ear to throat. Helps maintain air pressure on each side of the tympanic membrane. Jaw movements (talking, chewing) help push old, dry earwax out of the ear. The auditory tube is usually closed. It opens when we yawn or swallow to equalized pressure in the middle ear.

How we Hear Sound waves (vibrations) enter the ear. Vibrations hit the tympanic membrane which passes the vibration to the malleus, incus, and stapes. Vibration of the stapes at the oval window causes the fluid in the cochlea to move. Movement of the fluid stimulates receptors (cochlear hair cells). Hair cells are sensitive to specific frequencies.

Semicircular canal Stapes Oval window Auditory nerve Incus Malleus Round window Cochlea Ear drum (tympanic membrane) Eustachian tube Sound Ear canal

Decibels Measure sound intensity. 50 dB = normal conversation 70 dB = noisy restaurant 90 db = lawnmower, frequent or prolonged exposure causes hearing loss. 120 dB = rock concert, causes discomfort 140 dB = gunshot, causes pain and immediate damage

Equilibrium Senses the position of body parts, helps maintain stability and posture, detects motion, and helps maintain balance. When the head moves or rotates, fluid in the semicircular canals moves causing hair cells to bend (stimulates the nerves). Also influenced by vision and stretch receptors in muscles and tendons. Motion sickness http://www.youtube.com/watch?v=evBdytnlXnM Nystagmus is a type of eye movement that occurs during and after rotation.