Eyes and Gustation By Kevin Tran, Spencer Ayres, Brandon Shaw, and Morgan Ciehanski
Feb 08, 2016
Eyes and Gustation
By Kevin Tran, Spencer Ayres, Brandon Shaw, and Morgan Ciehanski
Vision
We rely on our vision more than any other special sense
Visual receptors are located in the eye
Functions of accessory structures
Protection
Lubrication
Secretion of tears
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
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
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
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
Lacrimal apparatus
Lacrimal Apparatus- produces, distributes, and removes tears
Consists of
• Lacrimal Gland and associated ducts• Lacrimal Canaliculi• Lacrimal Sac• Nasolacrimal Duct
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
The eye
Sophisticated visual instruments
Contains three distinct layers or tunics
• Outer Fibrous Tunic• Middle Vascular Tunic• Inner Neural Tunic (retina)
Fibrous tunicO
utermost 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
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
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
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
Pupillary muscles
Pupillary Constrictor Muscles- when it contracts, the pupil decreases (more light)
Pupillary Dilator Muscles- contraction enlarges the pupil (less light)
Neural TunicA
lso known as the RetinaR
etina helps process visual informationC
ontains two parts: pigmented part and neural partP
igmented part absorbs lightN
eural part is in control of processingA
lso contains photoreceptorsP
hotoreceptors- cells that detect light
Organization of retina
Rods and cones
Rods- highly sensitive to light, don’t ‘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
Rods and cones
Macula Lutea- has no rods
Fovea- contains highest cone concentration
Fovea is the site of the sharpest vision
Structure of the eye
The eye is hollow
Two cavities
• Posterior cavity• Anterior cavity is filled with aqueous humor
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
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
Anterior chamber
Extends from the cornea to the iris
Posterior chamber
Extends between the iris and the lens
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
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
transparency
Depends on precise combination of structural and biochemical characteristics
Lose of balance produces cataracts
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
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
Additional refraction
Light passes through the aqueous humor into the dense lens
This lens provides extra refraction that’s needed to focus the light rays from an object to a focal point
Focal Point- a specific point of intersection of the retina
Focal distance
Focal Distance- distance between the center of the lens and its focal point
Determined by two factors
1. Distance from object to the lens2. Shape of the lens
Distance from the object to the lens
The closer an object is to the lens, the greater the focal distance
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
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
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
Astigmatism
If light doesn’t 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 don’t even notice the condition
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 don’t notice it
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
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
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
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
Cone typesB
lue cones, green cones, and red conesE
ach have a sensitivity to a different range of wavelengthsS
timulation to different combos of wavelength creates color visionC
olor 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
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
Effects of Aging on the eye
Senile cataracts- lens loses transparency, blurred vision
Accommodation problems- the near point of vision gradually increases with age
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
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
Taste
Special sense given to us by the tongue
Taste sensation(s) is due to the presence of taste receptors on the tongue
Taste buds
Made of specialized epithelial cells and taste receptors
Contain around 40 cells of different types/stages
Basal cells -> Stem Cells in the tongue
Gustatory cells -> Mature daughter cells, grow in stages
Around 3000 in the adult tongue
Lingual papillae
Epithelial projections on the tongue
Three types: Filiform Papillae, Fungiform Papillae, Circumvallate Papillae
Taste Buds located on the papillae
Filiform papillae
Do not contain taste buds
Provide friction to move things around the mouth
Fungiform papillae
A contain around five taste buds
A little bigger than filiform papillae
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
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
Taste receptor underpinnings
Dissolved chemicals bind to the receptor proteins in gustatory cell
Cell releases neurotransmitter, which generates action potential in nervous system
Aging on taste
With age, the number of functioning taste buds decreases, meaning you’re less sensitive to various tastes
Number decreases dramatically after 50
Taste video link
http://bigthink.com/videos/from-tongue-to-brain-the-neurology-of-taste