SENSES Three types of senses: 1. SOMATIC SENSES: Light touch (being touched by a feather), heat, cold, vibration, pressure, pain. These are routinely tested by doctors in a physical exam, especially for people with diabetes and lupus. SENSES 2. PROPRIOCEPTORS are found in the muscles, joints, and tendons. They measure the amount of movement, force, and position of the body. Proprioception is often tested by having the patient close their eyes and saying if their fingers are up or down. Proprioceptors send information to the cerebellum. That’s how you know your legs are crossed before you stand up. Somatic senses (including pain) and proprioception are NOT considered special senses. SENSES 3. SPECIAL SENSES: Smell, taste, vision, hearing, equilibrium (balance). OLFACTORY SENSE (smell) Olfactory receptors are CHEMORECEPTORS; a special type of neuron which senses particular chemicals and triggers an action potential. Chemoreceptors are at the roof of the nasal cavity. There are hundreds of thousands of types, and they can smell a wide variety of substances. They are extremely sensitive, and can detect parts per billion, as in the scent of natural gas…just a few molecules! The olfactory nerve goes through the cribiform plate to the OLFACTORY BULB (one of the shortest nerves in the body) and into the limbic system. OLFACTORY SENSE (smell) Scientists who are trying to find a way to make neurons divide to heal nerve injuries often study the body’s only mitotic neurons. These neurons are the olfactory receptors. People who experience imaginary odors have what are called “unicate fits”. Olfactory Receptors Figure 16.3a, b Some towns are old factory towns. Some are ol’ factory towns! GUSTATORY SENSE (taste) Sensed on taste buds, which are located mostly on the tongue surface, but are also on the palate, pharynx, and a few on the lips. Taste buds have specialized cells, which increase surface area and have chemoreceptors. They are surrounded by support cells (like glia). They synapse on sensory neurons, which go to the facial nerve. Someone with a damaged facial nerve can not easily taste sweet, sour, or salty substances. Taste buds are the only parts of the nervous system that can regenerate completely. The taste information is sent to the primary gustatory (taste) cortex, located in the parietal lobe of the brain. Taste Buds Figure 16.1a, b Taste Bud GUSTATORY SENSE (taste) How many different tastes are there? Dozens. Salt, sweet, bitter, and sour are only a few. Where are they located on the tongue? All tastes are located all over the tongue. The picture in the book was drawn 120 years ago by an anatomist that knew his drawing was not right; he just wanted to use it as a starting point for further experimentation. GUSTATORY SENSE (taste) Taste appreciation is also involved in texture (a mealy apple is not as good), temperature (cold pizza tastes different than warm), and smell (perfume or cigarette smoke clog the senses and decrease taste). There are dozens of taste receptors, hundreds of thousands of smell receptors, so the subtly of taste is from smell. Foods people like are in opposite proportion to the numbers of taste receptors for that. People that love sweets have FEWER taste receptors for sweets, so they crave more taste of sweet things. If you dislike something, it’s because you have lots of receptors for it. Also, as you get older, you become less tolerant of sweets and more tolerant of bitter tastes (like beer and coffee). Fun Facts The catfish has over 27,000 taste buds. (What could be so tasty on the bottom of a pond?) Flies taste with their feet. Tongue with bacteria • What did the right eye say to the left eye? • Between you and me, something smells! THE EYE Structures Surrounding the Eye The eye is in the orbit of the skull for protection. Within the orbit are 6 extrinsic eye muscles, which move the eye. There are 4 cranial nerves: Optic (II), Occulomotor (III), Trochlear (IV), and Abducens (VI). Eyelids are PALPEBRA, and eyelashes are CILIA. Fun fact: there is a new medicine to make you grow longer eyelashes! People of Asian descent have an EPICANTHIC FOLD in the upper eyelid; no functional difference. Around the eyeball are glands. Surface Anatomy of Eye Extrinsic Eye Muscles Figure 16.6a, b FUN FACTS Do eyelashes grow back? Eyelashes take about four to eight weeks to grow back. What purpose do eyebrows serve? To keep the sweat and rain out of our eyes. The arch shape of the eyebrow diverts rain or sweat down our cheeks, keeping our eyes dry. Eyebrows also help us to communicate. Sometimes a raised eyebrow is more effective than words. Eyelashes GLANDS OF THE EYE 1. LACRIMAL GLANDS are the largest set. They are on the superior lateral eyelid and they produce tears, which drain into the nasal cavity via the LACRIMAL DUCT. The function is to moisten and lubricate the eye surface, and it has enzymes to kill bacteria (which thrive in warm, moist conditions). Figure 16.5b GLANDS OF THE EYE LACRIMAL CARUNCLE (“little meat”) is the spot on the medial corner of the eye. It makes an oily secretion like a sebaceous gland. The function is to lubricate the eye a little bit for the eyelids. When the secretion dries, it is called “sand” in the eyes. 2. GLANDS OF THE EYE 3. TARSAL GLANDS are sebaceous glands on the inside of the eyelid, and produce sebum, which is an oil to lubricate the eyeball. Therefore, the oil component found in tears is produced by the tarsal glands. The tarsal glands and the lacrimal caruncle make a waterproof surface so the eye won’t dry out. When tarsal glands are clogged = CHALAZION Chalazion GLANDS OF THE EYE 4. CILLIARY SEBACEOUS GLANDS go to only the cilia. When clogged = STY. Sty Dry Eye Syndrome FUN FACTS An ostrich's eye is bigger than it s brain! Is the human eye fully grown at birth? A typical newborn's eye is around 18 millimeters in diameter. A fully grown adult's eye measures 24-25 millimeters. A fully developed eyeball is about two-thirds the size of a ping-pong ball. This means a human eye grows only about 28% over the course of its life. Fish supposedly have the ability to increase their eye size "steadily over the course of their entire lives," a talent lost on us. Ora serrata Rods and cones The Eyeball 1. CONJUNCTIVA is like a Saran Wrap covering around the eye and under the eyelids. It’s made of stratified columnar epithelium (the first time in the body we’ve seen this tissue). It also has lots of goblet cells to secrete moisture for those areas. Deep to the epithelium is loose connective tissue with lost of small blood vessels, which are not seen unless the conjunctiva becomes inflamed: Blood-shot eyes: just from being tired PINK EYE (layman’s term), known as CONJUNCTIVITIS (from bacteria, very contagious). The Eyeball 2. FIBROUS TUNIC is the next layer, and has 2 parts: A. SCLERA is the white of the eye, made of dense irregular connective tissue. It is continuous with the dura mater of the brain. The eye is part of the brain. The sclera protects the eye. B. CORNEA is clear, and avascular (no blood supply) except around the periphery. Therefore, there is no tissue rejection when it is transplanted into another person; there is also no need to find a donor match. It has lots of pain receptors, so a scratched cornea is very painful. Its function is to be the main focuser of light for the eye. If damaged, need a corneal transplant. Scleritis Conjunctivitis The Eyeball 3. VASCULAR TUNIC is deep to the fibrous tunic. It has several structures. A. CHOROID has lots of blood vessels and pigment. The function of the pigment is to make sure light does not enter from the sides. The blood vessels provide blood supply to the other layers. B. CILIARY MUSCLES surround the lens. C. SUSPENSORY LIGAMENTS (also known as the zonule) hold the lens in place. D. LENS functions to continue to focus the light after it passes through the cornea. It changes shape to allow you to distinguish close from far. The lens changes shape by the ciliary muscles pulling on the suspensory ligaments. The Vascular Tunic PLAY Vascular Tunic (Uvea) Figure 16.8 Rods and cones Figure 16.7a Figure 16.9a Ciliary Muscles When you are looking far away, the ciliary muscles are relaxed, the lens is stretched into a wide circle, and the suspensory ligaments are tight. When you look up close, the ciliary muscle contracts and gets smaller, to the ligaments relax. Constantly looking close puts strain on the ciliary muscles = EYE STRAIN. Pupils Fun Fact: -When you are looking at someone you love, your pupils dilate, and they do the same when you are looking at someone you hate. PROBLEMS WITH THE LENS With age, the lens loses flexibility, and is less likely to round up. It stays in the position for seeing far, so there is trouble focusing on things that are near = PRESBYOPIA (“old eyes”). Occurs around age 45-50. The lens cannot accommodate. PROBLEMS WITH THE LENS Clouding of the lens leads to a clinical condition known as CATARACTS. Treatment is to remove the lens and replace it with a plastic one (which is not flexible either). If the lens yellows, you can’t see the color blue. After surgery, can see blue again. Cataract Cataract Vision IRIS (the colored part of the eye) The function is to constrict or dilate the pupil (opening) to allow light in. Therefore, it regulates the amount of light passing to the visual receptors of the eye. If there is a lot of pigment, eye is brown; a medium amount = green, small amount = blue, no pigment = pink (albino). FUN FACTS Why are all babies born with blue eyes? Melanin is a brownish pigment that adds color to your hair, eyes, and skin. At the time babies are born, melanin hasn't yet been "deposited" in the eyes' iris. Hence, they appear blue. After about six months, eyes change color depending on the amount of melanin. If you have a lot of it, your eyes will turn brown or black. If you have little, they'll stay blue. And if you have no melanin, your eyes may appear pink. Interestingly, as the site notes, human beings aren't the only creatures with freaky color-morphing eyeballs. Kittens experience the same phenomenon. How can someone have two different colored eyes? Eye color is a polygenic trait. Many babies are born with blue eyes. Their eyes change color later as they begin to produce more melanin. RETINA The retina is on top of the choroid layer. The retina is made up of PHOTORECEPTORS, which are sensors for light. Rods and cones Figure 16.7a Optic Nerve Fovea centralis Rods and Cones Two types of photoreceptors: Rods and Cones. 1. CONES (red, green, and blue) they have less light sensitivity (poor at night) but see colors well. There is a region on the retina that has the highest concentration of cones; it is called the FOVEA CENTRALIS. The fovea centralis is the very center point of a small circular region called the MACULA. When you want details, focus the light on the macula, because there are a lot of cones there. The other layers contain a mixture of both. Figure 16.7a Test for Colorblindness Rods and Cones 2. RODS (chartreuse = yellowish green) have more light sensitivity (can see well at night) but does not see colors well. Above the photoreceptors are layers of neurons whose axons become the optic nerve. Optic Nerve Fovea centralis Retina Figure 16.10a Photoreceptors Figure 16.11 Blind Spot at the Optic Nerve The region where the optic nerve and blood vessels goes in and out of the eye has no photoreceptors = BLIND SPOT. Hold your hands out at 45° and that’s the location of the blind spot. You can still see your hands because the other eye sees it. Close your right eye and look for your right hand and you’ll find the blind spot. Figure 16.7a Find your blind spot! Stare at the center of X and move head closer until one red spot disappears Blind Spot The light takes a path through the lens to the blood vessels, so this is the only place in the body where you can see blood vessels directly. The doctor can diagnose hypertension. On a clear, bright day, look at the blue sky and you can see the shadow of your own blood vessels on the photoreceptors as criss-cross lines in field of vision. The little moving dots are your blood cells. Normal Retina The visual information travels from the retina deep into the brain through the optic chiasma (not visible with an opthalmascope). From here, it goes into the occipital lobe of the brain, where it is processed. Cranial Nerves II, III, IV, VI OPTHALMASCOPE An opthalmascope is the instrument used to look inside the eye. The doctor can see the optic disc, fovea centralis, macula, the lens, retina, blood vessels, but of course, not the optic chiasma, since that is on the brain surface, external to the eye. PROBLEMS WITH VISION FLOATERS are when a capillary breaks and cells break off. Floaters don’t actually move, the eye just tries to track them. Floaters Floaters Floaters can best be seen when the person looks at a clear blue sky or white wall. RETINAL DETACHMENT The retina separates from the underlying choroid. Retinal detachment can be caused from a blow to the eye, or may occur spontaneously. Usually caused by an injury like a blow to the eye with a baseball, punch, or airbag to the eye. It may not cause blindness immediately. Although the detached portion contains capillaries, it is separated from the main blood supply, so if it is not lasered back into place immediately, permanent blindness can result. Cells in the retina die from lack of oxygen. Manifests as a shimmering light. This is considered a medical emergency and needs immediate treatment. Those who are most vulnerable to spontaneous detachment are those who are nearsighted. HYPEROPIA (far-sighted) eyes are too short MYOPIA (nearsighted) eyes are too long The Eye as an Optical Device Figure 16.14a–c Hyperopia and Myopia Normal eyes are perfect spheres. When the eyeball is not a perfect sphere, the lens has to accommodate as much as possible, and corrective glasses are usually needed. Myopic eyes are elongated (overhead projector is in focus, but move it backward, gets fuzzy). Even badly nearsighted eyes are only 1mm from normal. Treatments are glasses or Lasix, which is laser surgery on cornea, when it’s shaved so it focuses light farther back to reach the retina. Lasik Surgery Hyperopia and Presbyopia Hyperopia and presbyopia have some features in common, but a key difference between these two conditions is that in hyperopia the lens can accommodate, but in presbyopia it cannot. ASTIGMATISM • ASTIGMATISM is when the cornea has an irregular shape. Part of the field of view is out of focus. • The eyeball changes shape until age 24. Astigmatism Test Astigmatism Vision MACULAR DEGENERATION The size of the macula is the size of the printed letter “O” in 14 pt font. When the macula degenerates, you lose a lot of sight. This is the most common cause of blindness in the US. It’s due to bleeding in the eye, causing scar tissue. The retina does not get enough oxygen, and the cells die. Macular degeneration allows vision in the periphery, but they can’t read or drive. Macular Degeneration DIABETIC RETINOPATHY This is when the high sugar levels destroy the photoreceptors in the retina. The blood vessels also swell and rupture and the clots block vision. Some of this damage can be repaired by using a laser to evaporate the blood clots, but any damage to the photoreceptors is permanent. It can lead to blindness. Diabetic Retinopathy Diabetic Retinopathy Diabetic Retinopathy vision INTERNAL STRUCTURES OF THE EYE There are two cavities Anterior Cavity Posterior Cavity Anterior Cavity 1. ANTERIOR CAVITY is anterior to the lens, and is filled with AQUEOUS HUMOR, similar to plasma, supplies nutrients to the cornea and lens. Figure 16.7a GLAUCOMA GLAUCOMA is increased pressure within the anterior chamber of the eye. It leads to blindness. This form of blindness is more common in thirdworld countries because we have tests to detect it and treat it. The test measures how much pressure there is here by seeing how easily the cornea is deformed, either with air or direct pressure. How many of you have had this test? Glaucoma 2. POSTERIOR CAVITY is filled with VITREOUS HUMOR, which is jelly-like, and helps give shape to the eyeball. It leaks out from a cut, you’ll go blind because the body can’t replace it. The four main things that cause blindness are macular degeneration, cataracts, glaucoma, and diabetic retinopathy. AMBLYOPIA AMBLYOPIA = Lazy Eye. In a child, one eye will track and focus, the other won’t. If untreated in children, eventual blindness in weak eye because the brain will shut down in the occipital lobe. Treatment is to patch the good eye to force the bad eye to make the connections, or a surgery to weaken the muscle to make the strong side just as weak. Other Eye Problems Cancer of the Eye: Choroidal Melanoma Live worm in the eye NOTE If a child is blind until age 4-5, and then you restore the sight, he will still be blind because the brain doesn’t form properly. With kids who have astigmatism or weak eye muscles, one eye stops seeing (or sees double). The thalamus in the brain will shut off all the signals from the bad eye. NOTE To protect your eyesight throughout your life, use the 20-20-20 rule: Every 20 minutes look up for 20 seconds at something 20 feet away. The rest of this lecture is not on the exam! The red lines are straight! Look at the dot and move your head forward and back Look at the dot and move your head forward and back Stare at the + for 30 seconds and the pink dots will disappear! FUN FACTS ABOUT VISION Why do you cry when you cut onions? It releases an enzyme that reacts with the amino acids in the onion that create an acid that diffuses into the air and irritates your eyes. Scientists have tried to make a non=crying onion, but it turns out that enzyme is needed for the onion flavor. You can heat the onion before chopping or chop under running water. Or order take-out. Do cucumbers relieve puffy eyes? No, they are just cool and filled with water, so the cooling effect reduces the swelling. FUN FACTS ABOUT VISION Can carrots help improve your vision? This myth dates back to WWII when the British Royal Air Force was attempting to hide the fact that they developed a new radar system to shoot down German bombers. They bragged that the great accuracy of thie British fighter pilots at night was a result was a result of them being fed an enormous amount of carrots. It is true that carrots contain beta carotene which converts to vitamin A which is needed to prevent night blindness, but we only need a small amount, and having more than that will not improve vision. In fact, too many carrots can cause your skin to turn orange. FUN FACTS ABOUT VISION Can you lose a contact lens in the back of your head? No; the underside of the eyelid is connected to the sclera (white part of the eye). Why do you get bags under your eyes when you are tired? The skin under the eyes is the thinnest in the body, and it allows the dark, venous blood to show through. Dark rings tend to be genetic and get worse as you age. Good rest and nutrition minimizes the rings. FUN FACTS ABOUT VISION Should you put a steak on a black eye? It doesn’t do any more good than an ice pack Does hysterical blindness really exist? Yes. There is a device that can check for someone faking blindness; they put a black and white spinning pinwheel in front of the eyes, and if you can see, your eyes will automatically move a little back and forth. FUN FACTS ABOUT VISION Why do you see stars when you are hit in the head? It happens to Wile E. Coyote every time he gets hit on the head with an anvil by the Road Runner. Yes, it can happen, and it indicates there is a concussion. The visual area of the brain has hit against the inside of the skull. Will staring at an eclipse make you go blind? The intense light burns some of the cells in the retina, but it doesn’t cause complete blindness. FUN FACTS ABOUT VISION Why is it impossible to sneeze with your eyes open? Sneeze impulse affects a variety of body parts, including the abdomen, chest, neck, and face. During a sneeze, the impulses that travels through your face causes your eyelids to blink. This response is entirely automatic. There's nothing you can do about it. Sneezing puts a lot of pressure on your head and respiratory system, so blinking is probably a protective mechanism. The point is that all of these responses (the abdominal contraction, the sharp burst of air out of your lungs, the general lunging movement) are intertwined. FUN FACTS ABOUT VISION Can a person who is blind from birth "see images" in their dreams? People who are visually impaired from birth appear to lack visual imagery in their dreams but have a “very high percentage of gustatory, olfactory, and tactual sensory references," something very unusual for sighted dreamers to experience. FUN FACT OR FICTION? Extensive computer usage can cause dry eyes. Fact. Extensive computer use can lead to dry eyes. A person experiencing minor amount of dryness will feel much worse after prolonged computer use. Studies have shown that computer users tend to stare at the screen without blinking for a long time, which may cause dry eyes. Dry eyes are one of the factors leading to Computer Vision Syndrome. Thus frequent blinking is essential to lubricate the eyes and prevent them from drying. Eating carrots will improve your vision Fallacy. It is true that carrots are rich in Vitamin A, which is an essential vitamin for our eyes. However, we require only a small amount of this vitamin for good vision. A well- balanced diet, with or without carrots, provides all the Vitamin A necessary for good vision. FUN FACT OR FICTION? Sitting close to a television set, movie or computer screen can harm your eyes. Fallacy. Our eyes are not harmed by viewing these at a short distance. There is, however, a greater likelihood of experiencing eye fatigue or a headache. FUN FACT OR FICTION? Reading in dim light is harmful to your eyes. Fallacy. For centuries, all night time reading and sewing was done by candlelight or with gas or kerosene lamps. Reading in dim light does not damage the eyes. However, good lighting does make reading easier and prevents eye fatigue, especially for people who wear bifocals. Computers and eyestrain Computers can put stress on the eyes causing headaches, blurred vision and eye fatigue. The following steps can be taken to reduce eyestrain: Computer screen is at least 18 to 26 inches away from your eyes. Arrange the monitor so that the top line of on-screen text is at eye level. Placing a monitor too high exposes more of the eye, causing it to dry out. Arrange lighting to minimize glare and reflections. Keep the computer screen clean and dust-free to minimize glare. Take frequent vision breaks to stretch your body and rest your eyes. Remember to blink often to keep your eyes moist. Place reference material alongside and as close to the computer screen as possible to avoid frequent head and eye movements and focusing changes. Television and your eyes Never watch TV in a completely darkened room. The best lightning conditions consist of a back light and dim general light in the room. Place the set to avoid glare and reflections from lamps, windows and other bright sources. View from a distance at least five times the width of the television screen. Have the set at approximately eye level. Wear lenses prescribed for vision correction. Avoid staring at the screen. Briefly look away from the picture, around the room. Avoid eye strain while reading Keep the reading material at the best distance: place knuckles under your chin and book under the elbow. Try to avoid reading while lying on your back, stomach, or side. Make sure that there is good light on close work tasks and good room light as well. A light positioned behind and over one's shoulder works well for reading. Also, looking up from time to time from your reading can help keep your eyes relaxed.