Eyes and Gustation

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Eyes and Gustation

• By Kevin Tran, Spencer Ayres, Brandon Shaw, and
  Morgan Ciehanski

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Vision

• We rely on our vision more than any other special
  sense
• Visual receptors are located in the eye

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Functions of accessory structures

• Protection
• Lubrication
• Secretion of tears

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Accessory structures of the eye

• Superficial Epithelium of the Eye- thin layers of
  skin around the eye and covering the eye itself
• Eyelashes- robust hairs that help prevent foreign
  materials from reaching the eye
• Eyelids continuation of the skin that protect
  and lubricate the eye

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eyelashes

• Located along the inner margin of the eye lid
• Tarsal Glands- also known as Meibomian, are
  modified sebaceous glands
• Tarsal glands secrete lipid-rich products that
  keep the eye lids from sticking together

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Eyelids

• Eyelids open and close eye using muscles fibers
• Orbicularis Oculi and Levator Palpebrae
  Superioris muscles are responsible for closing
  the eye and raising the upper lid

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Epithelium of the eye

• Conjunctiva- outer surface of the eye that a
  mucous membrane covered in stratified squamous
  epithelium
• Palpebral Conjunctiva- inner surface of the
  eyelid
• Ocular Conjunctiva- the anterior surface of the
  eye
• Cornea- a transparent part of the outer fibrous
  layer

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Lacrimal apparatus

• Lacrimal Apparatus- produces, distributes, and
  removes tears
• Consists of
• Lacrimal Gland and associated ducts
• Lacrimal Canaliculi
• Lacrimal Sac
• Nasolacrimal Duct

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Lacrimal apparatus

• Lacrimal Gland- tear gland
• Lacrimal Canaliculi- small canals that lead to
  the lacrimal sac
• Lacrimal Sac- holds the tears that the lacrimal
  gland produces
• Nasolacrimal Duct- delivers tears to the nasal
  cavity on that side

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The eye

• Sophisticated visual instruments
• Contains three distinct layers or tunics
• Outer Fibrous Tunic
• Middle Vascular Tunic
• Inner Neural Tunic (retina)

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Fibrous tunic

• Outermost layer
• Consists of sclera and cornea
• Sclera- white of the eye made of collagen and
  elastic fibers
• Provides mechanical support and some physical
  protection
• Serves as an attachment site for the eye muscles
• Contains structures that assist in the focusing
  process

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Vascular tunic

• Also known as the Uvea
• Contains blood vessels, lymphatic vessels, and
  the intrinsic muscles of the eye
• Provides a route for blood vessels and lymphatics
  that supply tissues of the eye
• Regulating the amount of light the eye receives

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Vascular tunic

• Secreting and reabsorbing the aqueous humor that
  circulates the eye
• Controls the shape of the lens
• Contains the iris
• Visual receptors, or Photoreceptors, located in
  neural tunic

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Iris

• Iris- visible through the corneal surface,
  contains the blood vessels, pigment cells, and
  smooth muscle fibers
• Pupillary muscles- muscles that contract and
  changes the diameter of the pupil
• Pupil- central opening of the iris

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Pupillary muscles

• Pupillary Constrictor Muscles- when it contracts,
  the pupil decreases (more light)
• Pupillary Dilator Muscles- contraction enlarges
  the pupil (less light)

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Neural Tunic

• Also known as the Retina
• Retina helps process visual information
• Contains two parts pigmented part and neural
  part
• Pigmented part absorbs light
• Neural part is in control of processing
• Also contains photoreceptors
• Photoreceptors- cells that detect light

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Organization of retina

• Rods and cones
• Rods- highly sensitive to light, dont see
  colors
• Cones- sees colors, provide sharper clearer
  images
• Optic Nerve- transmits the visual images picked
  up from the rods and cones and delivers them to
  the brain

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Rods and cones

• Macula Lutea- has no rods
• Fovea- contains highest cone concentration
• Fovea is the site of the sharpest vision

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Structure of the eye

• The eye is hollow
• Two cavities
• Posterior cavity
• Anterior cavity is filled with aqueous humor

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Posterior cavity

• Or Vitreous Chamber, contains the vitreous body
• Vitreous Body- or Vitreous Humor, gelatinous
  substance that makes up most of the volume of the
  posterior cavity
• Helps stabilize the shape of the eye

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Anterior Cavity

• Divided into two chambers
• Anterior chamber
• Posterior chambers
• Chambers are filled with Aqueous Humor
• Aqueous Humor- fluid that circulates within the
  anterior cavity, passing through the chambers of
  the pupil

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Anterior chamber

• Extends from the cornea to the iris

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Posterior chamber

• Extends between the iris and the lens

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lens

• Lies posterior to the cornea
• Primary function is to focus the visual image on
  the photoreceptors
• Focus happens by the change in shape of the lens
• Lens fibers are in the interior of the lens

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Lens fibers

• Lost their nucleus and organelles
• Slender and elongated
• Filled with transparent proteins called
  crystallins
• Crystallins- responsible for clarity and focusing
  power of the lens

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transparency

• Depends on precise combination of structural and
  biochemical characteristics
• Lose of balance produces cataracts

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refraction

• The light that is collected by the photoreceptors
  in refracted, or bent when passing from one
  medium to another
• Pencil in water
• Refraction occurs when passing light through the
  cornea and then into the lens

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refraction

• Greatest amount of refraction occurs when light
  passes through the air into the corneal tissues
• Tissues have a density similar to water
• When you opne your eyes underwater you cant see
  as easily because the air-water refraction has
  been eliminated and replaced with water to water,
  thus light remains unbent and

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Additional refraction

• Light passes through the aqueous humor into the
  dense lens
• This lens provides extra refraction thats needed
  to focus the light rays from an object to a focal
  point
• Focal Point- a specific point of intersection of
  the retina

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Focal distance

• Focal Distance- distance between the center of
  the lens and its focal point
• Determined by two factors
• Distance from object to the lens
• Shape of the lens

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Distance from the object to the lens

• The closer an object is to the lens, the greater
  the focal distance

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The shape of the lens

• The rounder the lens the more refraction occurs,
  so a very round lens has a shorter focal distance
  than a flatter one

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accommodation

• Accommodation- focusing images on the retina by
  changing the shape of the lens to keep the focal
  length constant
• To view nearby objects the lens becomes rounder
• The lens flattens when we view a distant object
• Lens are held in place by suspensory ligaments

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accommodation

• Greatest amount of refraction is needed for
  viewing objects up close
• Inner limit of clear vision is called the near
  point of vision
• Children can see things up close but as time goes
  on the lens becomes stiffer and less responsive
• Aging effects the near point of vision

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Astigmatism

• If light doesnt pass properly the image is
  distorted
• Astigmatism- the degree of curvature in the
  cornea or lens varies from one axis to another
• Image distortion may be so minimal people dont
  even notice the condition

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Image reversal

• Light originates at a single point either near or
  far
• However and object in view is a complex light
  source that is treated as a large number of
  individual points
• These individual points creates a miniature image
  of the original but is upside down and backwards
• The brains compensates for this image reversal
  and we dont notice it

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Visual activity

• Visual activity- clarity of vision
• Rated against a normal standard (20/20, 20/15,
  etc.)
• Considered legally blind when vision falls below
  20/200, even with glasses or contact lenses

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blindness

• Terms implies a total absence of vision due to
  damage of the optic pathways
• Common causes are
• Diabetes mellitus
• Cataracts
• Glaucoma
• Corneal scarring
• Detachment of the retina
• Hereditary factors

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Scotomas

• Abnormal blind spots that may appear in the field
  of vision
• Permanent in a fixed position
• Result from a compression of the optic nerve,
  damage to the photoreceptors, of damage to the
  visual pathway
• Also Floaters, which a small spots that drift
  across the field of vision, generally temporary
  phenomena

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Color vision

• Objects appear to have color if they reflect or
  transmit photons from one portion of the visible
  spectrum and absorbs the rest
• Photons stimulate rods and cones
• Photons of all colors bounce off an object or
  rods themselves are stimulated, the object will
  appear white
• If photons are absorbed by the object (none reach
  the retina), the object appears black

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Cone types

• Blue cones, green cones, and red cones
• Each have a sensitivity to a different range of
  wavelengths
• Stimulation to different combos of wavelength
  creates color vision
• Color discrimination results from the integration
  of info from all three types of cones
• EXAMPLE Yellow is formed from a combo of highly
  stimulate green cones, less strongly stimulated
  red cones, and relatively unaffected blue cones

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Color blindness

• People who are unable to distinguish certain
  colors have a form of color blindness
• Happens when one or more classes of cones aren't
  functional
• Either lack of cones or unable to function
  properly
• Most common type is red-green color blindness
  red cones are missing so a person cant tell the
  difference between red and green light

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Effects of Aging on the eye

• Senile cataracts- lens loses transparency,
  blurred vision
• Accommodation problems- the near point of vision
  gradually increases with age

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Eye diseases

• Conjunctivitis- or pinkeye, due to damage and/or
  irritation of the conjunctival surface
• Cataract- balance in the lens becomes disturbed
  and the lens loses transparency they can result
  from injury, radiation, or reaction to drugs, as
  well as aging
• Glaucoma- eye disease in which the optic nerve is
  damaged in a characteristic pattern

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Professions dealing with the eye

• Optometrist- concerned with the health of the
  eyes and related structures as well as vision,
  visual systems, etc. they are trained to fit
  lens to improve vision and diagnose and treat
  diseases of the eye
• Ophthalmologist- a specialist in medical and
  surgical eye problems
• Optician-use prescriptions written by an
  optometrist or an ophthalmologist to fit and sell
  eyeglasses, contact lenses and other eyewear

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Taste

• Special sense given to us by the tongue
• Taste sensation(s) is due to the presence of
  taste receptors on the tongue

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Taste buds

• Made of specialized epithelial cells and taste
  receptors
• Contain around 40 cells of different types/stages
• Basal cells -gt Stem Cells in the tongue
• Gustatory cells -gt Mature daughter cells, grow in
  stages
• Around 3000 in the adult tongue

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Lingual papillae

• Epithelial projections on the tongue
• Three types Filiform Papillae, Fungiform
  Papillae, Circumvallate Papillae
• Taste Buds located on the papillae

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Filiform papillae

• Do not contain taste buds
• Provide friction to move things around the mouth

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Fungiform papillae

• A contain around five taste buds
• A little bigger than filiform papillae

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Circumvallate papillae

• Can contain up to 100 taste buds
• Largest of the three types of papillae
• Forms a V at the back of the tongue

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Gustatory discrimination

• Four Primary sensations Sweet, sour, salty,
  bitter
• Two less well known Umami Umami, Water
• Different regions of the tongue are more prone to
  certain tastes than others
• All sensations have same structure in the taste
  bud, just slightly different receptor mechanisms
• Respond much more readily to unpleasant tastes
  than to pleasant

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Taste receptor underpinnings

• Dissolved chemicals bind to the receptor proteins
  in gustatory cell
• Cell releases neurotransmitter, which generates
  action potential in nervous system

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Aging on taste

• With age, the number of functioning taste buds
  decreases, meaning youre less sensitive to
  various tastes
• Number decreases dramatically after 50

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Taste video link

• http//bigthink.com/videos/from-tongue-to-brain-th
  e-neurology-of-taste