Anatomy Ch.15- The Special Senses

visual pigments

- a light-absorbing molecule called retinal that combines with proteins called opsin to form four types of visual pigments -depending on the type of opsin to which it is bound, retinal absorbs different wavelengths of the visible spectrum -retinal is chemically related to vitamin A and is made from it. the cells of the pigmented layer of the retina absorb vitamin A from the blood and serve as the local vitamin A depot for rods and cones -can assume a variety of 3-D forms, each form called an isomer. when bound to opsin, retinal has a bent shape called 11-cis-retinal -when the pigment absorbs a photon of light, retinal twists and snaps into a new configuration: all-trans-retinal. this change causes opsin to change shape and assume its activated form -the capture of light by visual pigments is the only light-dependent stage, and this event initiates a whole chain of chemical and electrical reactions in rods and cones that ultimately causes action potentials to be transmitted across the optic nerve -the cone opsins differ both from the opsin of the rods and from one another -the naming of the cones reflects the colors (wavelengths) of light that each cone variety absorbs best -blue cones respond maximally to wavelengths around 420 nm, green cones to wavelengths of 530 nm, and red cones to wavelengths at or close to 560 nm -how do we see other colors besides blue, green, and red? : cones' absorption spectra overlap, and our perception of intermediate hues results from differential activation of more than one type of cone at the same time. when all cones are stimulated equally, we see white

olfactory disorders (anosmia)

-"without smells" -result from head inquires that tear the olfactory nerves, the aftereffects of nasal cavity inflammation, and neurological disorders such as Parkinson's disease -anosmia may also be an early sign of Alzheimer's disease -some people have olfactory hallucinations during which they experience a particular (usually unpleasant) odor, such as rotting meat. these are usually caused by temporal lobe epilepsy involving the olfactory cortex and can occur as olfactory auras before the seizure begins or as olfactory hallucinations during the seizure

eyelids

-AKA palpebrae -are separated by the palpebral fissure and meet at the medial and lateral angles of the eye-- the medial and lateral commissures -medial commissure sports a fleshy elevation called the lacrimal caruncle which contains sebaceous and sweat glands and produces the whitish, oily secretion that sometimes collects at the medial commissure, especially during sleep -in most asian people, a vertical fold of skin called the epicanthic fold commonly appears on both sides of the nose and sometimes covers the medial commissure -are supported internally by connective tissue sheets called tarsal plates which also anchor the orbicularis oculi and levator palpebrae superioris msucles that run within the eyelid -the orbicularis muscle encircles the eye, and the eye closes when it contracts -the upper eyelid is much more mobile, mainly because of the levator palpebrae superioris muscle, which raises that eyelid to open the eye -eyelid muscles are activated reflexively to cause blinking every 3-7 second and to protect the eye from foreign objects. every time we blink, accessory structure secretions (oil, mucus, and saline solution) spread across the eyeball surface, keeping the eye moist

the auricle

-AKA the pinna -is what most people call the ear-- the shell-shaped projection surrounding the opening of the external acoustic meatus -the auricle is composed of elastic cartilage covered with thin skin and an occasional hair -its rim, the helix, is somewhat thicker, and its fleshy, dangling lobule lacks supporting cartilage -the function of the auricle is to funnel sound waves into the external acoustic meatus

middle ear

-AKA the tympanic cavity -is a small, air-filled mucosa-lined cavity in the petrous part of the temporal bone -is flanked laterally by the eardrum and medially by a bony wall with two openings, the superior oval window and the inferior round window -superiorly the tympanic cavity arches upward as the epitympanic recess, the "roof" of the middle ear cavity -the mastoid antrum, a canal in the posterior wall of the tympanic cavity, allows it to communicate with mastoid air cells housed in the mastoid process -the anterior wall of the middle ear is next to the internal carotid artery (the main artery supplying the brain) and contains the opening of the pharyngotympanic (auditory) tube -the pharyngotympanic tube runs obliquely downward to link the middle ear cavity with the nasopharynx (the superior most part of the throat), and the mucosa of the middle ear is continuous with that lining the pharynx (throat) -normally the pharyngotympanic tube is flattened and closed, but swallowing or yawning opens it briefly to equalize pressure in the middle ear cavity with external air pressure -this is important because the eardrum vibrates freely only if the pressure on both of its surfaces is the same, otherwise sounds are distorted. the ear- popping sensation of the pressures equalizing is familiar to anyone who has flown in an airplane -is spanned by the three smallest bones in the body: the auditory ossicles ("hammer", the incus ("anvil"), and the stapes ("stirrup"). the "handle" of the malleus is secured to the eardrum, and the base of the stapes fits into the oval window -tiny ligaments suspend the ossicles, and mini synovial joints link them into a chain that spans the middle ear cavity -the incus articulates with the malleus laterally and the stapes medially -the ossicles transmit the vibratory motion of the eardrum to the oval window, which in turn sets the fluids of the internal ear into motion, eventually exciting the hearing receptors -two tiny skeletal muscles are associated with the ossicles. the tensor tympani arises from the wall of the pharyngotympanic tube and inserts on the malleus. the stapedius runs from the posterior wall of the middle ear to the stapes -when loud sounds assault the ears, these muscles contract reflexively to limit the ossicles' vibration and minimize damage to the hearing receptors

eyeball

-a slightly irregular hollow sphere -its wall is composed of three layers: the fibrous, vascular, and inner layers -its internal cavity is filled with fluids called humors that help to maintain its shape -the lens, the adjustable focusing apparatus of the eye, is supported vertically within the eyeball, and divides it into anterior and posterior segments

conjunctiva

-a transparent mucous membrane -lines the eyelids as the palpebral conjunctiva and folds back over the anterior surface of the eyeball as the bulbar conjunctiva -the bulbar conjunctiva covers only the white of the eye, not the cornea -the bulbar conjunctiva is very thin, and blood vessels are clearly visible beneath it (they are even more visible in irritated "bloodshot" eyes) -when the eye is closed, a slitlike space occurs between the conjunctiva-covered eyeball and eyelids -this so-called conjunctival sac is where a contact lens lies, and eye medications are often administered into its inferior recess -the major function of the conjunctiva is to produce a lubricating mucus that prevents the eyes from drying out -inflammation of this, called conjunctivitis, results in reddened, irritated eyes -pinkeye, a conjunctival infection caused by bacteria or viruses, is highly contagious

gustatory pathway

-afferent fibers carrying taste information from the tongue are found primarily in two cranial nerve pairs -a branch of the facial nerve (VII), the chord tympani, transmits impulses from taste receptors in the anterior two-thirds of the tongue -the lingual branch of the glossopharyngeal nerve (IX) services the posterior third and the pharynx just behind -taste impulses from the few taste buds in the epiglottis and the lower pharynx are conducted primarily by the vagus nerve (X) -these afferent fibers synapse in the solitary nucleus of the medulla, and from there impulses stream to the thalamus and ultimately to the gustatory cortex in the insular. fibers also project to the hypothalamus and limbic system structures, regions that determine our appreciation of what we are tasting -an important role of taste is to trigger reflexes involved in digestion. as taste impulses pass through the solitary nucleus, they initiate reflexes (via synapses with parasympathetic nuclei) that increase secretion of saliva into the mouth and of gastric juice into the stomach -saliva contains mucus that moistens food and digestive enzymes that begin digesting starch. when we ingest foul-tasting substances, the taste may initiate protective reactions such as gagging or vomiting

visual pigments (photopigments)

-an elaborate array of pigments that change shape as they absorb light -are embedded in areas of the plasma membrane that form discs -folding the plasma membrane into discs increases the surface area available for trapping light -in rods, the discs are discontinuous-- stacked inside a cylinder of plasma membrane -in cones, the disc membranes are continuous with the plasma membrane, so the interiors of the cone discs are continuous with the extracellular space -photoreceptor cells are highly vulnerable to damage and immediately begin to degenerate if the retina becomes detached. they are also destroyed by intense light -photoreceptors' unique system for renewing their light-trapping outer segment. every 24 hrs, new components are synthesized in the cell body and added to the base of the outer segment. as new discs are made, the discs at the tip of the outer segment fragment off and pigment cells phagocytize them

special sensory receptors

-are distinct receptor cells -are highly localized in the head, either housed within complex sensory organs (eyes and ears) or in distinct epithelial structures (taste buds and olfactory epithelium)

taste disorders

-are less common than disorders of smell, in part because the taste receptors are served by three different nerves and thus are less likely to be disabled completely -causes include upper respiratory tract infections, head injuries, chemicals or medications, or head an neck radiation for cancer treatment -zinc supplements may help some cases of radiation-induced taste disorders

photoreceptors

-are modified neurons, but structurally they resemble tall epithelial cells turned upside down with their tips immersed in the pigmented layer of the retina. these tips are the receptive regions of rods and cones and are called the outer segments -moving from the pigmented layer into the neural layer, a connecting cilium joins the outer segment of a rod or cone to the inner segment. the inner segment then connects to the cell body, which is continuous with an inner fiber bearing synaptic terminals -in rods, the outer segment is slender and rod shaped and the inner segment connects to the cell body by the outer fiber

cones

-are our vision receptors for bright light and provide high-resolution color vision -need bright light for activation (have low sensitivity), but react more rapidly -have one of three different pigments that furnish a vividly colored view of the world -each cone in the fovea has a straight-through pathway via its "own personal bipolar cell" to a ganglion cell -each foveal cone has its own direct line to the higher visual centers-- permitting detailed, high-acuity (high-resolution) views of very small areas of the visual field

eyebrows

-are short, coarse hairs that overlie the supraorbital margins of the skull -they help shade the eyes from sunlight and prevent perspiration trickling down the forehead from reaching the eyes

vallate papillae

-are the largest and least numerous papillae, but have many taste buds -eight to twelve vallate papillae form an inverted V at the back of the tongue

gustatory epithelial cells

-are the receptor cells for taste -long microvilli called gustatory hairs project from the tips of all gustatory epithelial cells and extend through a taste pore to the surface of the epithelium, where they are bathed by saliva -the gustatory hairs are the sensitive portions (receptor membranes) of the gustatory epithelial cells -coiling around the gustatory epithelial cells are sensory dendrites that represent the initial part of the gustatory pathway to the brain -each afferent fiber receives signals from several gustatory epithelial cells within the taste bud -there are at least 3 kinds of gustatory epithelial cells. one kind forms traditional synapses with the sensory dendrites and releases the neurotransmitter serotonin -the other two kinds lack synaptic vesicles, but at lest one of these kinds releases ATP that acts as a neurotransmitter

focusing light on the retina

-as light passes from air into the eye, it moves sequentially through the cornea, aqueous humor, lens, and vitreous humor , and then passes through the entire neural layer of the retina to excite the photoreceptors that are next to the pigmented layer -during its passage, light is bent three times: 1) entering the cornea 2) entering the lens 3) leaving the lens -the cornea accounts for the majority of the refractory power of the eye. the refractory power is constant -on the other hand, the lens is highly elastic, and its curvature and light-bending power can actively change to allow fine focusing

olfactory pathway

-axons of the olfactory sensory neurons form the olfactory nerves that synapse in the overlying olfactory bulbs, the distal ends of the olfactory tracts -there, the filaments of the olfactory nerves synapse with mitral cells. which are second-order sensory neurons, in complex structures called glomeruli -axons from neurons bearing the same kind of receptor converge on a given type of glomerulus. each glomerulus represents a single aspect of an odor (like one note in a chord) but each odor activates an unique set of glomeruli (the chord itself) -different odors activate different subsets of glomeruli (making different chords which may have some of the same notes) -the mitral cells refine the signal, amplify it, and then relay it -the olfactory bulbs also house amacrine granule cells that inhibit mitral cells, so that only highly excitatory olfactory impulses are transmitted -when the mitral cells are activated, impulses flow from the olfactory bulbs via the olfactory tracts (composed mainly of mitral cell axons) to the piriform lobe of the olfactory cortex. from there, two major pathways take info to various parts of the brain: + one pathway brings info to part of the frontal lobe just above the orbit, where smells are consciously interpreted and identified. only some of this info passes through the thalamus + the other pathway flows to the hypothalamus, amygdaloid body, and other regions of the limbic system. there, emotional responses to odors are elicited. smells associated with danger-- smoke, cooking gas, or skunk scent-- trigger the sympathetic fight-or-flight response. appetizing odors stimulate salivation and the digestive tract, and unpleasant odors can trigger protective reflexes such as sneezing or choking

visual processing

-begins in the retina -retinal cells simplify and condense the info from rods and cones, splitting it into a number of different channels, each with its own type of ganglion cell -these channels include info about color and brightness, but also about the angle, direction, and speed of movement of edges (sudden changes in brightness or color) -edges are detected by a kind of contrast enhancement called lateral inhibition -ganglion cells pass the processed info to the lateral geniculate nuclei of the thalamus. there, info from each eye is combined in preparation for depth perception and input from cones is emphasized -the primary visual cortex contains an accurate topographical map of the retina, with the left visual cortex receiving input from the right visual field and vice versa. the processing neurons are responding to dark and bright edges (contrast info) and object orientation. surrounding areas process form, color, and movement -complex visual processing extends forward into the temporal, parietal, and frontal lobes via two parallel streams, one that identifies objects in the visual field and another that assesses the location of objects in space

refraction

-bending of a light ray -occurs when it meets the surface of a different medium at an oblique angel rather than at a right angle -the greater this angle, the greater the amount of bending -a lens is a transparent object curved on one or both surfaces. since light hits the curve at an angle, it is refracted -if the lens surface is convex (thickest in the center like a camera lens), the light rays bend so that they converge or intersect to focus at a single point -if the light is from a distant source, the rays are nearly parallel and the point at which they focus is called the focal point -the more convex the lens, the more the light bends and the shorter the focal distance (distance between the lens and focal point) -the image formed by a convex lens, called a real image, is inverted-- upside down and reversed from left to right -concave lenses, which are thicker at the edges than at the center , diverge the light (bend it outward) so that the light rays move away from each other -concave lenses prevent light from focusing and extending the focal distance

basic taste sensations

-can be grouped into one of five basic modalities: sweet, sour, salty, bitter, and umami 1) sweet: is elicited by many organic substances including sugars, saccharin, alcohols, some amino acids, and some lead salts (such as those found in lead paint) 2) sour: produced by acids, specifically their hydrogen ions in solution 3) salty: is produced by metal ions (inorganic salts)- table salt (sodium chloride) tastes the "saltiest' 4) bitter: is elicited by alkaloids as well as number of non alkaloid substances such as aspirin 5) umami: a subtle taste discovered by the Japanese, is elicited by the amino acids glutamate and aspartate, which are responsible for the "beef taste" of steak, the characteristic tang of aging cheese, and the flavor of the food additive monosodium glutamate (MSG) -a single taste cell has receptors for only one taste modality -all areas of the tongue can detect all taste modalities -taste likes and dislikes have homeostatic. umami guides the intake of proteins and a liking for sugar and salt helps satisfy the body's need for carbs and minerals many sour, naturally acidic foods are rich sources of vitamin C

strabismus

-congenital weakness of the external eye muscles may cause this -the affected eye rotates medially or laterally -to compensate, the eyes may alternate in focusing on objects -in other cases, only the controllable eye is used, and the brain begins to disregard inputs from the deviant eye, which (unless treated early) can then become functionally blind

external ear

-consists of the auricle and the external acoustic meatus -the auricle, or pinna, is what most people call the ear-- the shell-shaped projection surrounding the opening of the external acoustic meatus -the auricle is composed of elastic cartilage covered with thin skin and an occasional hair -its rim, the helix, is somewhat thicker, and its fleshy, dangling lobule lacks supporting cartilage -the function of the

lacrimal apparatus

-consists of the lacrimal gland and the ducts that drain lacrimal secretions into the nasal cavity -the lacrimal gland lies in the orbit above the lateral end of the eye and is visible through the conjunctiva when the lid is everted -it continually releases a dilute saline solution called lacrimal solution--tears-- into the superior part of the conjunctival sac through several small excretory ducts -blinking spreads the tears downward and across the eyeball to the medial commissure, where they enter the paired lacrimal canaliculi via two tiny openings called lacrimal puncta, visible as tiny red dots on the medial margin of each eyelid -from the lacrimal canaliculi, the tears drain into the lacrimal sac and then into the nasolacrimal duct, which empties into the nasal cavity at the inferior nasal meatus -lacrimal fluid contains mucus, antibodies and lysozyme, an enzyme that destroys bacteria. as a result, it cleanses and protects the eye surface as it moistens and lubricates it -when lacrimal secretion increases substantially, tears spill over the eyelids and fill the nasal cavities, causing congestion and the "sniffles". this spillover (tearing) happens when the eyes are irritated or when we are emotionally upset. in the case of eye irritation, enhanced tearing washes away or dilutes the irritating substance -because the nasal cavity mucosa is continuous with that of the lacrimal duct system, a cold or nasal inflammation often causes the lacrimal mucosa to swell. this swelling constricts the ducts and prevents tears from draining, causing "watery" eyes

activation of olfactory sensory neurons

-dissolved odorants stimulate olfactory sensory neurons by binding to receptor proteins in the olfactory cilium membranes, opening cation channels and generating a receptor potential -ultimately, an action potential is conducted to the first relay station in the olfactory bulb

depth perception

-each eye's visual field is about 170 degrees. the two visual fields overlap considerably, but each eye sees a slightly different view -the visual cortex fuses the slightly different images delivered by the two eyes, providing us with depth perception (or 3-D vision), an accurate means of locating objects in space -in contrast, many animals have panoramic vision. their eyes are placed more laterally on the head, so that the visual fields overlap very little. each visual cortex receives input principally from a single eye and a totally different visual field -depth perception depends on the two eyes working together. if only one eye is used, depth perception is lost, and the person must learn to judge an object's position based on learned cues (such as parallel lines diverge with distance and nearer objects appear larger)

neural layer of the retina

-extends anteriorly to the posterior margin of the ciliary body. this junction is called the ora serrata, the saw-toothed margin -is composed of three main types of neurons: photoreceptors, bipolar cells, and ganglion cells -signals are produced in response to light and spread from the photoreceptors (next to the pigmented layer) to the bipolar cells and then to the innermost ganglion cells, where action potentials are generated -the ganglion cell axons make a right-angle turn at the inner face of the retina, then leave the posterior aspect of the eye as the thick optic nerve -the retina also contains other types of neurons-- horizontal cells and amacrine cells-- which play a role in visual processing -receives its blood supply from two sources: vessels in the choroid supply the outer third (containing photoreceptors) -the inner two-thirds is served by the central artery and central vein of the retina, which enter and leave the eye through the center of the optic nerve -radiating outward from the optic disc, these vessels give rise to a rich vascular network. this is the only place where small blood vessels are visible in a living person. physicians may observe these tiny vessels with an ophthalmoscope for signs of hypertension, diabetes, and other vascular diseases

hyperopia

-farsightedness -occurs when the parallel light rays from distant objects focus behind the retina -individuals can see distant objects because their ciliary muscles contract almost continuously to increase the light-bending power of the lens, which moves the focal point forward on the retina -diverging light rays from nearby objects focus so far behind the retina that the lens cannot bring the focal point onto the retina even at its full refractory power -close objects appear blurry, and convex corrective lenses are needed to converge the light more strongly for close vision -usually results from an eyeball that is too short

aqueous humor

-fills the entire anterior segment -is a clear fluid similar in composition to blood plasma -forms and drains continually -moves from the posterior chamber to the scleral venous sinus , an unusual venous channel that encircles the eye in the angle at the corneoscleral junction -normally, it forms and drains at the same rate, maintaining a constant intraocular pressure of about 16 mm Hg, which helps to support the eyeball internally -supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and it carries away metabolic wastes

the ear: hearing and balance

-fluids must be stirred to stimulate the mechanoreceptors of the internal ear -our hearing apparatus allows us to hear an extraordinary range of sound, and our equilibrium (balance) receptors continually inform the nervous system of head movements and position -although the organs serving these two senses are structurally interconnected within the ear, their receptors respond to different stimuli and are activated independently of one another -the ear has three major areas: external ear, middle ear, and internal ear -external and middle ear structures are involved with hearing only and are rather simply engineered -the internal ear functions in both equilibrium and hearing and is extremely complex

vascular layer

-forms the middle coat of the eyeball -also called the uvea -is pigmented and has three regions: choroid, ciliary body, and iris

sclera

-forms the posterior portion and the bulk of the fibrous layer, is glistening white and opaque -seen anteriorly as the "white of the eye" -protects and shapes the eyeball and provides a sturdy anchoring site for the extrinsic eye muscles -posteriorly, where it is pierced by the optic nerve (cranial nerve II), it is continuous with the dura mater of the brain

activation of taste receptors:

-gustatory epithelial cells contain neurotransmitters. when a food chemical, or tastant, binds to receptors in the gustatory epithelial cell membrane, it induces a graded depolarizing potential that causes neurotransmitter release. binding of the neurotransmitter to the associated sensory dendrites triggers generator potentials that elicit action potentials in these fibers -the different gustatory epithelial cells have different thresholds for activation. in line with their protective nature, the bitter receptors detect substances present in minute amounts. the other receptors are less sensitive -taste receptors adapt rapidly, with partial adaptation in 3-5 seconds and complete adaptation in 1-5 min

visible light

-has a wavelength range of approximately 400-700 nm -travels in the form of waves, and its wavelengths can be measured very accurately -light can also be envisioned as packets of energy, called photons, traveling in a wavelike fashion at very high speeds -we can think of light as a vibration of pure energy -when light passes through a prism, each of its component waves bends to a different degree. this disperses the beam of light into a visible spectrum, or band of colors (rainbow) -red wavelengths are the longest and have the lowest energy -violet wavelengths are the shortest and most energetic -can reflect, or bounce, off a surface. this reflection of light by objects in our environment accounts for most of the light reaching our eyes -objects have color because they absorb some wavelengths and reflect others -things that look white reflect all wavelengths of light, whereas black objects absorb them all -travels in straight lines (rays) and is blocked by any nontransparent object -when light travels in a given medium, its speed is constant -when it passes from one transparent medium into another with a different density, its speed changes -light speeds up as it passes into a less dense medium and slows as it passes into a denser medium

glaucoma

-if the drainage of aqueous humor is blocked, fluid backs up -pressure with the eye may increase to dangerous levels and compress the retina and optic nerve -the eventual results is blindness unless the condition is detected and treated early -many forms of glaucoma steal sight so slowly and painlessly that people do not realize they have. problem until the damage is done -late signs include seeing halos around lights and blurred vision -examination: the intraocular pressure is determined by directing a puff of air at the cornea and measuring the amount of corneal deformation it causes -this exam should be done yearly after the age of 40 -the most common treatment is eye drops that increase the rate of aqueous humor drainage or decrease its production -laser therapy or surgery can also help

electromagnetic radiation

-includes all energy waves, from long radio waves (with wavelengths measured in meters) to very short waves (gamma waves and X rays with wavelengths of 1 nm and less) -our eyes respond to the part of the spectrum called visible light, which has a wavelength range of approximately 400-700 nm

choroid

-is a blood-vessel rich, dark brown membrane that forms the posterior five-sixths of the vascular layer -its blood vessels nourish all eye layers -its brown pigment, produced by melanocytes, helps absorb light, preventing it from scattering and reflecting within the eye (which would cause visual confusion) -has a posterior opening where the optic nerve leaves the eye

cataract

-is a clouding of the lens that causes the world to appear distorted, as if seen through frosted glass -some are congenital, but most result from age-related hardening and thickening of the lens or are a secondary consequence of diabetes mellitus -heavy smoking, frequent exposure to intense sunlight, and steroid use increase the risk for cataracts -oxidative stress and metabolic changes in the deeper lens fibers promote clumping of the crystallin proteins -fortunately, the offending lens can be surgically removed and an artificial lens implanted to save the patient's sight

night blindness (nyctalopia)

-is a condition in which rod function is seriously hampered, impairing one's ability to drive safely at night -in countries where malnutrition is common, the most common cause of night blindnesss is prolonged vitamin A deficiency, which leads to rod degeneration. vitamin A supplements restore function if they are administered early -in countries where nutrition isn't a problem, retinitis pigmentosa-- a group of degenerative retinal diseases that destroy rods-- are the most common causes of night blindness. retinitis pigmentosa results from pigment epithelial cells that are unable to recycle the tips of the rods as they get sloughed off

external acoustic meatus

-is a short, curved tube that extends from the auricle to the eardrum -near the auricle, its framework is elastic cartilage; the remainder of the canal is carved into the temporal bone -the entire canal is lined with skin bearing hairs, sebaceous glands, and modified apocrine sweat glands called ceruminous glands -these glands secrete yellow-brown waxy cerumen, or earwax, which provides a sticky trap for foreign bodies and repels insects -in many people, the ear is naturally cleansed as the cerumen dries and then falls out of the external acoustic meatus -jaw movements as a person eats, talks, and so on, move the wax out. in other people, the cerumen builds up and becomes compacted

eye

-is a sphere with a diameter of about 2.5 cm (1 inch) -only the anterior 1/6th of the eye's surface is visible -the rest is enclosed and protected by a cushion of fat and the walls of the bony orbit -the fat pad occupies nearly all of the orbit not occupied by the eye itself

bony labyrinth

-is a system of tortuous channels worming through the bone -has three regions: the vestibule, the semicircular canals, and the cochlea

internal ear

-is also calle the labyrinth because of its complicated shape -lies deep in the temporal bone behind the eye socket and provides a secure site for all of the delicate receptor machinery -has two major divisions: + the bony labyrinth is a system of tortuous channels worming through the bone. + the membranous labyrinth is a continuous series of membranous sacs and ducts contained within the bony labyrinth and (more or less) following its contours -is filled with perilymph, a fluid similar to cerebrospinal fluid and continuous with it -the membranous labyrinth is suspended in the surrounding perilymph, and its interior contains endolymph, which is chemically similar to K+ rich intracellular fluid. these two fluids conduct the sound vibrations involved in hearing and respond to the mechanical forces occurring during changes in body position and acceleration

lens

-is convex on both sides -is a transparent, flexible structure that can change shape to precisely focus light on the retina -is enclosed in a thin, elastic capsule and held in place just posterior to the iris by the ciliary zone -is avascular, blood vessels interfere with transparency -has two regions: the lens epithelium and the lens fibers -the lens epithelium, confined to the anterior lens surface, consists of cuboidal cells that eventually differentiate into the lens fibers that form the bulk of the lens -the lens fibers, which are packed tightly together like the layers in an onion, contain no nuclei and few organelles. they do contain transparent, precisely folded proteins called crystallins that form the body of the lens -since new lens fibers are continually added, the lens enlarges throughout life, becoming denser, more convex, and less elastic, all of which gradually impair its ability to focus light properly

colorblindness

-is due to a congenital lack of one or more cone pigments -is far more common in males than in females because it is inherited as an X-linked condition -as many as 8-10% of males have some form of colorblindness -the most common type is red-green colorblindness, resulting from a deficit or absence of either red or green cone pigments -red and green are seen as the same color-- either red or green ,depending on the cone pigment present -many color-blind people are unaware of their condition because they have learned to rely on other cues-- such as different intensities of the same color-- to distinguish something green from something red, such as traffic signals

vestibule

-is the central egg-shaped cavity of the bony labyrinth -lies posterior to the cochlea, anterior to the semicircular canals, and flanks the middle ear medially -in its lateral wall is the oval window -suspended in the vestibular perilymph and untied by a small duct are two membranous labyrinth sacs, the saccule and utricle -the smaller saccule is continuous with the membranous labyrinth extending anteriorly into the cochlea, and the utricle is continuous with the semicircular ducts extending into the semicircular canals posteriorly -the saccule and utricle house equilibrium receptor regions called maculae that respond to the pull of gravity and report on changes of head position

cornea

-is transparent and bulges anteriorly from its junction with the sclera -forms a window that lets light enter the eye, and is a major part of the light-bending apparatus of the eye -is well supplied with nerve endings, most of which are pain receptors. (this is why some people can't tolerate contact lenses). when the cornea is touched, blinking and increased tearing occur reflexively. even so, the cornea is the most exposed part of the eye and is vulnerable to damage from dust, slivers, and the like. luckily, its capacity for regeneration and repair is extraordinary. -has no blood vessels and so it is beyond the reach of the immune system -as a result, it is the only tissue in the body that can be transplanted from one person to another with little risk of rejection epithelial sheets cover both faces of the cornea: -external sheet, a stratified squamous epithelium that protects the cornea from abrasion, merges with the bulbar conjunctiva at the corneoscleral junction, epithelial cells that continually renew the cornea are located here. -the deep corneal endothelium, composed of simple squamous epithelium, lines the inner face of the cornea. its cells have active sodium pumps that maintain the clarity of the cornea by keeping its water content low.

macula lutea

-lateral to the blind spot of each eye -has a minute pit in its center called the fovea centralis -in this region, the retinal structures next to the vitreous humor are displaced to the sides -this allows light to pass almost directly to the photoreceptors rather than through several retinal layers, greatly enhancing visual acuity (the ability to resolve detail) -the fovea contains only cones, the macula contains mostly cones, and from the edge of the macula toward the retina periphery, cone density declines gradually -the retina periphery contains mostly rods, which decrease in density from there to the macula -only the foveae have a sufficient cone density to provide detailed color vision, so anything we wish to view critically is focused on the foveae -because each fovea is only about the size of the head of a pin, only a thousandth of the entire visual field is in hard focus (foveal focus) at a given moment -for us to visually comprehend a scene that is rapidly changing (as we drive through traffic ), our eyes must flick rapidly back and forth to provide the foveae with images of different parts of the visual field

semicircular canals

-lie posterior and lateral to the vestibule -the cavities of the bony semicircular canals project from the posterior aspect of the vestibule, each oriented in one of the three planes of space -there is an anterior, a posterior, and a lateral semicircular canal in each internal ear -the anterior and posterior canals are oriented at right angles to each other in the vertical plane, whereas the lateral canal is lies horizontally -snaking through each semicircular canal is a corresponding membranous semicircular duct, which communicates with the utricle anteriorly -each of these ducts has an enlarged swelling at one end called an ampulla, which houses an equilibrium receptor region called a crista ampullaris. these receptors respond to rotational (angular) movements of the head

focusing for close vision

-light from close objects (less than 6 m away) diverges as it approaches the eye and comes to a focal point farther from the lens -close vision demands that the eye make active adjustments -to restore focus, three processes must occur simultaneously: accommodation of the lenses, constriction of the pupils, and convergence of the eyeballs

foliate papillae

-located laterally on the tongue -contain many taste buds during childhood, but fewer with age

ottis media

-middle ear inflammation -is a fairly common result of a sore throat, especially in children, whose pharyngotympanic tubes are shorter and run more horizontally -is the most frequent cause of hearing loss in children -in acute infectious forms, the eardrum bulges and becomes inflamed and red -is sometimes treated with antibiotics

fungiform papillae

-mushroom-shaped -are scattered over the entire tongue surface, but have only one to five taste buds each

myopia

-nearsightedness -occurs when distant objects focus in front of the retina, rather than on it -myopic peon can see close objects without problems because they can focus them on the retina, but distant objects are blurred -typically results from an eyeball that is too long -correction has traditionally involved using concave lenses that diverge the light before it enters the eye -laser procedures to flatten the cornea slightly offer other treatment options

dark adaptation

-occurs when we go from a well-lit area into a dark one -initially, we see nothing but velvety blackness because 1) our cones stop functioning in low-intensity light and 2) the bright light bleached our rod pigments, and the rods are still turned off -once we are in the dark, rhodopsin accumulates, transducin returns to the outer segment, and retinal sensitivity increases -dark adaptation is much slower than light adaptation and can go on for hours. -there is usually enough rhodopsin within 20-30 minutes to allow adequate dim-light vision -during both light and dark adaptation, reflexive changes occur in pupil size -bright light shining in one or both eyes constricts both pupils (elicits the pupillary and consensual light reflexes). -these pupillary reflexes are mediated by the pretectal nucleus of the midbrain and by parasympathetic fibers -in dim light, the pupils dilate, allowing more light to enter the eye

light adaptation

-occurs when we move from darkness not bright light such as when leaving a movie matinee -all we see is white light because the sensitivity of the retina is still "set" for dim light. -both rods and cones are strongly stimulated, and large amounts of the visual pigments break down almost instantaneously, producing a flood of signals that accounts for the glare -under such conditions, compensations occur. the rod system turns off-- all of the transductions migrate to the inner segment, uncoupling rhodopsin from the rest of the transduction cascade. without transducin in the outer segment, light hitting rhodopsin cannot produce a signal -at the same time, the less sensitive cone system and other retinal neurons rapidly adapt, and retinal sensitivity decreases dramatically -within 60 seconds, the cones, initially overexcited by the bright light, are sufficiently desensitized to take over -visual acuity and color vision continue to improve over the next 5-10 min -during light adaptation, we lose retinal sensitivity (rod function) but gain visual acuity

rods

-our dim-light and peripheral vision receptors -are more numerous and far more sensitive to light than cones, but they do not provide sharp images or color vision -this is why colors disappear and the edges of objects appear fuzzy in dim light and at the edges of our visual field -are very sensitive (respond to very dim light-- a single photon), making them best suited for night vision and peripheral vision -contain a single kind of visual pigment so their inputs are perceived only in gray tones -participate in converging pathways, and as many as 100 rods may ultimately feed into each ganglion cell -rod effects are summated and considered collectively, resulting in vision that is fuzzy and indistinct

information processing in the retina

-photoreceptors do not generate action potentials (APs), and neither do the bipolar cells that are next in line. they only generate graded potentials. photoreceptors generate receptor potentials, and bipolar cells generate excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs) -primary function of APs is to carry info rapidly over long distances -retinal cells are small cells that are very close together. graded potentials can serve quite adequately as signals that directly regulate neurotransmitter release at the synapse by opening or closing voltage-gated Ca channels -light hyper polarizes photoreceptors, which then stop releases their inhibitory neurotransmitter (glutamate) -no longer inhibited, bipolar cells depolarize and release neurotransmitter onto ganglion cells. once the signal reaches the ganglion cells, it is converted into an AP. this AP is transmitted to the brain along the ganglion cell axons that make up the optic nerve

eyelashes

-project from the free margin of each eyelid -the follicles of the eyelash hairs are richly innervated by nerve endings, and anything that touches the eyelashes trigger reflex blinking -tarsal glands are embedded in the tarsal plates, and their ducts open at the eyelid edge just posterior to the eyelashes -these modified sebaceous glands produces an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together -a number of smaller, more typical sebaceous glands are associated with the eyelash follicles -modified sweat glands called ciliary glands lie between the hair follicles

chemoreceptors

-receptors for smell (olfaction) and taste (gustation) -respond to chemicals in an aqueous solution -complement each other and respond to different classes of chemicals -smell receptors are excited by airborne chemicals that dissolve in fluids coating nasal membrane -taste receptors are excited by food chemicals dissolved in saliva

extrinsic eye muscles

-six straplike muscles that control the movement of each eyeball -these muscles originate from the walls of the orbit and insert into the outer surface of the eyeball -they allow the eyes to follow a moving object, help maintain the shape of the eyeball, and hold it in the orbit -the four rectus muscles originate from the common tendinous ring at the back of the orbit and run straight to their insertion on the eyeball -their locations and the movements that they promote are clearly indicated by their names: superior, inferior, lateral , and medial rectus muscles -the actions of the two oblique muscles move the eye in the vertical plane when the eyeball is already turned medially by the rectus muscles -the superior oblique muscle originates in common with the rectus muscles, runs along the medial wall of the orbit, and then makes a right-angle turn and passes through a fibrocartilaginous loop called the trochlea suspended from the frontal bone before inserting on the superolateral aspect of the eyeball. it rotates the eye downward and somewhat laterally -the inferior oblique muscle originates from the medial orbit surface and runs laterally and obliquely to insert on the inferolateral eye surface. it rotates the eye up and laterally -the superior and inferior recti cannot elevate or depress the eye without also turning it medially because they approach the eye from a posteromedial direction -for an eye to be directly elevated or depressed, the lateral pull of the oblique muscles is necessary to cancel the medial pull of the superior and inferior recti -except for the lateral rectus and superior oblique muscles, which are innervated respectively by the abducens and trochlear nerves, the oculomotor nerves serve all extrinsic eye muscles -the extrinsic eye muscles are among the most precisely and rapidly controlled skeletal muscles in the entire body. this precision reflects their high axon-to-muscle-fiber ratio: the motor units of these muscles contain only 8 to 12 muscle cells and in some cases as few as two or three

tympanic membrane (eardrum)

-sound waves entering the external acoustic meatus eventual hit this -is a thin, translucent, connective tissue membrane, covers by skin on its external face and by mucosa internally -shaped like a flattened cone, its apex protrudes medially into the middle ear -sound waves make the eardrum vibrate -the eardrum transfers the sound energy to the tiny bones of the middle ear and sets them vibrating

visual pathway to the brain

-the axons of the retinal ganglion cells exit the eye in the optic nerves -at the X-shaped optic chiasma, fibers from the medial aspect of each eye cross over to the opposite side and then continue on via the optic tracts. each optic tract: -contains fibers from the lateral (temporal) aspect of the eye on the same side and fibers from the medial (nasal) aspect of the opposite eye -carries all the information from the same half of the visual field -because the lens system of each eye reverses all images, the medial half of each retina receives light rays from the temporal (lateral most) part of the visual field (from the far left or far right rather than from straight ahead), and the lateral half of each retina receives an image of the nasal (central) part of the visual field. -the left optic tract carries a complete representation of the right half of the visual field, and the opposite is true for the right optic tract -the pared optic tracts sweep posteriorly around the hypothalamus and send most of their axons to synapse with neurons in the lateral geniculate nuclei of the thalamus. the lateral geniculate nuclei maintain the fiber separation established at the chiasma, but they balance and combine the retinal input for delivery to the visual cortex -axons of these thalamic neurons project through the internal capsule to form the optic radiation of fiber in the cerebral white matter. these fibers project to the primary visual cortex in the occipital lobes, where conscious perception of visual images (seeing) occurs -some nerve fibers in the optic tracts send branches to the midbrain. one set of these fibers ends in the superior colliculi, visual reflex centers controlling the extrinsic muscles of the eye -another set comes from a small subset of ganglion cells in the retina that contain the visual pigment melanopsin, the circadian pigment. these ganglion cells respond directly to light stimuli and their fibers project to the pretectal nuclei, which mediate pupillary light reflexes, and to the suprachiasmtic nucleus of the hypothalamus, which functions as the "timer" to set our daily biorhythms

intrinsic eye muscles

-the ciliary body and the iris -are controlled by the autonomic nervous system

iris

-the colored part of the eye, the most anterior portion of the vascular layer -shaped like a flattened donut, it lies between the cornea and the lens and is continuous with the ciliary body posteriorly -its round central opening, the pupil allows light to enter the eye -is made up of two smooth muscle layers with bunches of sticky elastic fibers that congeal into a random pattern before birth -its muscle fibers allow it to act as a reflexively activated diaphragm to vary pupil size -in close vision and bright light, the sphincter pupillae (circular muscles) contracts and the pupil constricts -in distant vision and dim light, the dilator pupillae (radial muscles) contracts and the pupil dilates, allowing more light to enter -sympathetic fibers control pupillary dilation, and parasympathetic fibers control constriction -changes in pupil size may also reflect our interests and emotional reactions -our pupils often dilate when we see something that appeals to us, when we feel fear, and during problems solving -boredom or viewing something unpleasant causes pupils to constrict -although irises come in different colors, they contain only brown pigment -when they have a lot of pigment, the eyes appear brown or black -if the amount of pigment is small and restricted to the posterior surface of the iris, the unpigmented parts simply scatter the shorter wavelengths of light and the eyes appear blue, green, or gray -most newborn babies' eyes are slate gray or blue because their iris pigment is not yet developed -divides the anterior segment into the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris and the lens) -the entire anterior segment is filled with aqueous humor

focusing for distant vision

-the far point of vision is that distance beyond which no change in lens shape (accommodation) is needed for focusing -for the normal or emmetropic eye, the far point is 6 m (20 feet) -because distant objects appear smaller, light from an object at or beyond the far point of vision approaches the eyes as nearly parallel rays -the cornea and the at-rest lens focus the light from these distant objects precisely on the retina -during distant vision, the sphincter like ciliary muscles are completely relaxed, and tension in the ciliary zonule stretches the lens flat -the lens is as tin as it gets and is at its lowest refractory power when the ciliary muscle is at rest -the ciliary muscles relax when sympathetic input to them increases and parasympathetic input decreases

inner layer (retina)

-the innermost layer of the eyeball which develops from an extension of the brain -contains millions of photoreceptors that transduce (convert) light energy, other neurons involved in processing responses to light, and glia -consists of two layers: pigmented layer and an inner neural layer -although the pigmented and neural layers are very close together, they are not fused -only the neural layer of the retina plays a direct role in vision

olfactory epithelium

-the organ of smell that is a yellow-tinged patch of pseudostratified epithelium located in the roof of the nasal cavity -air entering the nasal cavity must stimulate the olfactory receptors before entering the respiratory passageway before -covers the superior nasal concha on each side of the nasal septum, and contains millions of olfactory sensory neurons -these are surrounded by columnar supporting cells which make up the bulk of the penny-thin epithelial membrane. the supporting cells contain a yellow-brown pigment similar to lipofuscin, which gives the olfactory epithelium its yellow hue. at the base of the epithelium lie the short olfactory stem cells -olfactory cilia typically lie flat on the nasal epithelium and are covered by a coat of thin mucus produced by the supporting cells and by olfactory glands in the underlying connective tissue. this mucus is a solvent that "captures" and dissolves airborne odorants

pigmented layer of the retina

-the outer layer, a single-cell-thick lining -is next to the choroid, and extends anteriorly to cover the ciliary body and the posterior face of the iris -these pigment cells, like those of the choroid, absorb light and prevent it from scattering in the eye -they also act as phagocytes participating in photoreceptor cell renewal and store vitamin A needed by the photoreceptor cells

fibrous layer of eyeball

-the outermost coat of the eyeball composed of dense avascular connective tissue -has two different regions: the sclera and cornea

retinal detachment

-the pattern of vascularization of the retina makes it susceptible to _________ _______ -this condition, in which the pigmented and neural layers separate (detach) and allow the jellylike vitreous humor to seep between them, can cause permanent blindness because it deprives the photoreceptors of nutrients -may happen when the retina is torn during a traumatic blow to the head or when the head stops moving suddenly and then jerks in the opposite direction (as in bungee jumping) -the symptom most often described is " a curtain being drawn across the eye", but some people see soot-like spots or light flashes -if diagnosed early, it is often possible to reattach the retina with a laser before photoreceptors are permanently damaged

accommodation of the lenses

-the process that increases the refractory power of the lens -the ciliary muscles contract, pulling the ciliary body anteriorly toward the pupil and inward, releasing tension in the ciliary zonule -no longer stretched the elastic lens recoils and bulges, providing the shorter focal length needed to focus the image of a close object on the retina -parasympathetic fibers of the oculomotor nerves control the contraction of the ciliary muscles -the closest pint on which we can focus clearly is called the near point of vision, and it represents the maximum bulge the lens can achieve -in young adults with emmetropic vision, the near point is 10 cm (4 inches) from the eye. it is closer in children and gradually recedes with age-- why elderly people must read a newspaper at arm's length -the gradual loss of accommodation with age reflects the lens' decreasing elasticity -in many people over age 50, the lens is non accommodating, a condition known as presbyopia ("old person's vision"

taste buds

-the sensory organs for taste -a few are scattered on the soft plate, inner surface of the cheeks, pharynx, and epiglottis of the larynx, but most are found in papillae -each flask-shaped taste bud consist of 50 t 100 epithelial cells of two major types: gustatory epithelial cells and basal epithelial cells

constriction of the pupils

-the sphincter pupillae muscles of the iris enhance the effect of accommodation by reducing the size of the pupil toward 2 mm. this accommodation pupillary reflex, mediated by parasympathetic fibers of the oculomotor nerves, prevents the most divergent light rays from entering the eye -such rays would pass through the extreme edge of the lens and would not focus properly, causing blurred vision

convergence of the eyeballs

-the visual goal is always to keep the object being viewed focused on the retinal foveae -when we look at distant objects, both eyes are directed either straight ahead or to one side of the same degree, but when we fixate on a close object, our eyes converge -convergence, controlled by somatic motor fibers of the oculomotor nerves, is medial rotation of the eyeballs by the medial rectus muscles so that each is directed toward the object being viewed -the closer that object, the greater the degree of convergence required -reading or viewing a smartphone screen requires almost continuous accommodation, pupillary constriction, and convergence. this is why these activities can tire the eye muscles and result in eyestrain. when your eyes tire, it helps to look up and stare into the distance occasionally to relax your intrinsic eye muscles

rhodopsin

-the visual pigment of rods is a deep purple pigment called _____ -molecules are arranged in a single layer in the membranes of each of the thousands of discs in the rods' outer segments -the breakdown and regeneration of visual pigments in cones is essentially the same as for rhodopsin. however, cones are about a hundred times less sensitive than rods, which means that it takes higher-intensity (brighter) light to activate cones formation and breakdown of rhodopsin: 1) pigment synthesis- rhodopsin forms and accumulates in the dark. vitamin A is oxidized to the 11-cis-retinal form and then combined with opsin to form rhodopsin 2) pigment bleaching: when rhodopsin absorbs light, retinal changes shape to its all-trans isomer, allowing the surrounding protein to quickly relax like an uncoiling spring into its light-activated form. eventually the retinal-opsin combination breaks down, allowing retinal and opsin to separate. the breakdown of rhodopsin to retinal and opsin is known as the bleaching of the pigment 3) once the light-struck all-trans-retinal detaches from opsin, enzymes within the pigmented epithelium reconvert it to its 11-cis isomer. then retinal heads "homeward" again to the photoreceptor cells' outer segments. rhodopsin is regenerated when 11-cis-retinal is rejoined to opsin

olfactory receptors

-there are about 350 different odorant receptors in humans -each receptor responds to one or more odorants and each odorant binds to several different receptor types. however, each receptor cell has only one type of receptor protein -some of what we call smell is really pain. the nasal cavities contain pain and temperature receptors that respond to irritants such as the sharpness of ammonia, the hotness of chili peppers, and the "chili" of menthol -impulses from these receptors reach the central nervous system via afferent fibers of the trigeminal nerves

smell transduction

-transduction of odorants uses a receptor linked to a G protein -begins when an odorant binds to a receptor. this event activates G proteins which activate enzymes that synthesize cyclic AMP (cAMP) as a second messenger -cAMP then acts directly on plasma membrane cation channels, causing them to open, allowing Na and Ca to enter -Na influx leads to depolarization and impulse transmission -Ca influx causes the transduction process to adapt, decreasing its response to a sustained stimulus. this olfactory adaptation helps explain how a person working in a paper mill or sewage treatment plant can still enjoy lunch

vitreous humor

-transmits light -supports the posterior surface of the lens and holds the neural layer of the retina firmly against the pigmented layer -contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles -forms in the embryo and lasts for a lifetime

astigmatism

-unequal curvatures in different parts of the cornea or lens also lead to blurry images -special cylindrically ground lenses or laser procedures are used to correct this problem

light transduction reactions

-what happens when light changes opsin's shape? : an enzymatic cascade occurs that ultimately results in closing cation channels that are normally kept open in the dark -light-activated rhodopsin activates a G protein called transduction. transducin activates PDE (phosphodiesterase), the enzyme that breaks down cyclic GMP (cGMP) - in the dark, cGMP binds to cation channels in the outer segments of photoreceptor cells, holding them open. this allows Na and Ca to enter, depolarizing the cell to its dark potential of about -4- mV. -in the light, PDE breaks down cGMP, th cation channels close, Na and Ca stop entering the cell, and the cell hyperpolarizes to about -70 mV

influence of other sensations on taste

-what we think of as taste is actually 80% smell -when nasal congestion blocks access to your olfactory receptors, food tastes bland. without morning smell , our morning coffee would lack its richness and simply taste bitter -the mouth also contains thermoreceptors, mechanoreceptors, and nociceptors, and the temperature and texture of rods can enhance or detract from their taste -"hot" foods such as chili peppers actually bring about their pleasurable effects by exciting pain receptors in the mouth

diplopia (double vision)

-when movements of the external muscles of the two eyes are not perfectly coordinated, a person cannot properly focus the images of the same area of the visual field from each eye and so seems two images instead of one -can result from paralysis or weakness of certain extrinsic muscles, or neurological disorders

optic disc

-where the optic nerve exits the eye, is a weak spot in the fundus (posterior wall) of the eye because it is not reinforced by the sclera -is also called the blind spot because it lacks photoreceptors, so light focused on it cannot be seen -we do not usually notice these gaps in our vision because the brain uses a sophisticated process called filling in to deal with the absence of input photoreceptors found in the neural layover are of two types: rods and cones

sty

a painful inflammation of the sebaceous glands at the base of the eyelashes

ciliary body

a thickened ring of tissue that encircles the lens and has three parts: -ciliary muscles: make up most of the ciliary body and consist of interlacing smooth muscle bundles that control lens shape -ciliary processes: near the lens, the posterior surface of the ciliary body has radiating folds called ciliary processes, which secrete the fluid that fills the cavity of the anterior segment of the eyeball -ciliary zone: extends from the ciliary processes to the lens. this halo of fine fibers encircles the circumference of the lens and helps hold it in its upright position. it also transmits tension from the ciliary muscle to the lens

basal epithelial cells

act as stem cells, dividing and differentiating into new gustatory epithelial cells

physiology of taste

for a chemical to be tasted it must dissolve in saliva, diffuse into a taste pore, and contact the gustatory hairs gustatory epithelial cells contain neurotransmitters. when a food chemical, or tastant, binds to receptors in the gustatory epithelial cell membrane, it induces a graded depolarizing potential that causes neurotransmitter release. binding of the neurotransmitter to the associated sensory dendrites triggers generator potentials that elicit action potentials in these fibers -the different

optic chiasma

if neural destruction occurs beyond the ____ _____-- in an optic tract, the thalamus, or visual cortex-- then part or all of the opposite half of the visual field is lost ex: a stroke affecting the left visual cortex leads to blindness in the right half of the visual field

accessory structures of the eye

include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles

posterior segment behind the lens

is filled with a clear gel called vitreous humor that binds tremendous amounts of water

optic nerve

loss of an eye or destruction of one ______ _____ eliminates 3D vision, the major component of depth perception, and peripheral vision on the damaged side

chalazion

obstructed tarsal glands may cause a firm, usually painless bump called a ____

papillae

peg-like projections of the tongue mucosa that men the tongue surface slightly abrasive three kinds of papillae have taste buds: + fungiform papillae + vallate papillae + foliate papillae

equilibrium

receptors are housed in the ear along with the organ of hearing

touch

reflects the combined activity of the general senses

phototransduction

the process by which light energy is converted into a graded receptor potential. it begins when a visual pigment captures a photon of light

eyeball shape

the vast majority of refractive problems are related to ______ _______-- either too long or too short-- and not to a lens that is too strong or too weak

taste transduction

three different mechanisms underlie how we taste: -salty taste is due to Na influx thru Na channel, which directly depolarizes gustatory epithelial cells -sour is mediated by H+, which acts intracellularly to pen channels that allow other cations to enter -bitter, sweet, and umami response share a common mechanism, but each occurs in a different cell. each taste's unique set of receptors is coupled to a common G protein called gustducin. activation leads to the release of Ca from intracellular stores, which causes cation channels in the plasma membrane to open, thereby depolarizing the cell and releasing the neurotransmitter ATP

special senses

vision, taste, smell, hearing, and equilibrium