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