1. Eye Flashcards
Eyelids
Function
Function:
- Protect the cornea from mechanical damage
- Prevent excess light from entering the eye and damaging the retina by squinting
- Helps spread tears over the cornea and conjunctiva
- Wipes dust particles off the cornea by blinking
Eyelashes
Function
Function:
- Help to shield eye from dust particles
Tear Glands
Function
Function:
- Secrete tears to wash away dust particles
- Keep cornea moist for atmospheric oxygen to dissolve so that oxygen can diffuse into the cornea
- Lubricate the conjunctiva, helping to reduce friction when the eyelids move
Conjunctiva
Structure & Function
Structure:
- Thin, transparent membrane covering the sclera
Function:
- Secretes mucus to keep the front of the eyeball moist
Sclera
Structure & Function
Structure:
- Tough, white outer covering of the eyeball
- Continuous with cornea
Function:
- Protects eyeball from mechanical damage
Choroid
Structure & Function
Structure:
- Middle layer of the eyeball between the sclera and retina
Function:
- Contains black pigments to prevent internal reflection of light
- Contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products
Retina
Structure & Function
Structure:
- The intermost layer of the eyeball
- Contains photoreceptors (rods and cones)
Function:
- Light-sensitive layer where images are formed
Cornea
Structure & Function
Structure:
- Transparent layer in front of eye (beneath the conjunctiva)
- Continuous with sclera
Function:
- Refracts light towards lens
Iris
Structure & Function
Structure:
- Consists of 2 sets of involuntary muscles, the circular muscles and radial muscles
- Surrounds the pupil
- In front of the lens
Function:
- Adjusts the amount of light entering the eye by changing the size of the pupil
Pupil
Structure & Function
Structure:
- Hole in the centre of iris
- Appears black because the choroid is visible through it
Function:
- Allows light to enter the eye
Lens
Structure & Function
Structure:
- Transparent, biconvex, elastic crystalline body
Function:
- Changes curvature or thickness to refract light onto the retina
Suspensory Ligaments
Structure & Function
Structure:
- Connective tissue
- Attached to the edge of the lens and the ciliary body
Function:
- Transfer the effect of the ciliary muscles to the lens
Ciliary Body
Structure & Function
Structure:
- Consists of ciliary muscles
Function:
- Controls the curvature or thickness of the lens
Aqueous Humour
Structure & Function
Structure:
- Transparent, watery fluid
- In front of the lens
Function:
- Refracts light onto the lens
- Serves as a medium for the diffusion of oxygen and nutrients
- Keeps eyeball firm
Vitreous Humour
Structure & Function
Structure:
- Transparent, jelly-like fluid
Function:
- Refracts light onto retina
- Keeps the eyeball firm
Fovea (Yellow Spot)
Structure & Function
Structure:
- Greatest concentration of cones but no rods
Function:
- Part of the retina that best resolves images, where images are focused onto it
- Gives detailed colour vision in bright light
Blind spot
Structure & Function
Structure:
- Found over the optic nerve
- No photoreceptors
Function:
- Area on the retina where neurones leave the eye
Optic Nerve
Structure & Function
Structure:
- Composed of sensory neurones
Function:
- Trasmists nerve impulses from photoreceptors to brain
Cones
Properties
- Cones enable us to see colours in bright light
- There are 3 types of cones: red, green and blue cones
- Each cone contains different pigments which absorbs light of different wavelengths
- Most concentrated at the yellow spot/fovea
- Cones do not work well in dim light
Rods
Properties
- Rods are more sensitive to light than cones
- Rods enable us to see in dim light, but only in black and white
- Contain the light sensitive pigment, visual purple (rhodopsin)
- Rods lie outside the fovea in the more peripheral parts of the retina
Rhodopsin (Visual Purple)
Properties
- Found in rods
- Visual purple is bleached when the eye is exposed to bright light, It takes a while for visual purple to reform when you walk from a bright place to a dimly lit room
- Requires Vitamin A for the formation of visual purple
Accommodation
Definition
Accomodation is the adjustment of the lens to change curvature or thickness so that clear images at different distances are formed on the retina.
Accomodation is achieved by the following structures:
- Ciliary muscles to alter the shape of the lens
- Suspensory ligaments which transfer the effect of the ciliary muscles to the lens
- Elasticity of the lens
Describe the changes in the eye when focusing on a near object
Process
- Ciliary muscles contract, reducing the pull on the suspensory ligaments
- Suspensory ligaments slacken
- Lens becomes thicker and more convex
- Focal length decreases
- Light rays from the near object are brought to focus on the retina
- Photoreceptors are stimulated. Nerve impulses are produced and transmitted to the brain via the optic nerve.
Describe the changes in the eye when focusing on a distant object
Process
- Ciliary muscles relax, pulling on the suspensory ligaments
- Suspensory ligaments become taut
- Lens becomes thinner and longer
- Focal length increases
- Light rays from the distant object are brought to focus on the retina
- Photoreceptors are stimulated. Nerve impulses are produced and transmitted to the brain via the optic nerve.
Simple reflexes in the Eye
Function + Types
Simple reflex actions have protective functions.
- Pupil reflex - strong light causes pupils to become smaller to protect retina
- Blink reflex - any object seen coming towards the eyes cause the eyelids to close
- Tearing reflex - dust sensed by the conjunctiva causes an increase in tear flow to wash it away
These are cranial reflex actions, where the reflex centre is in the brain.
Describe pupil reflex in bright light
Process
- Photoreceptors in the retina detect an increase in the light intensity.
- Nerve impulses are sent along the sensory neurone in the optic nerve to the brain.
- The brain returns nerve impulses along the motor neurone to the radial and circular muscles of the iris.
- The radial iris muscles relax while the circular iris muscles contract.
- The diameter of the pupil decreases, allowing less light to enter the eye.
Describe pupil reflex in dim light
Process
- Photoreceptors in the retina detect a decrease in the light intensity.
- Nerve impulses are sent along the sensory neurone in the optic nerve to the brain.
- The brain returns nerve impulses along the motor neurone to the radial and circular muscles of the iris.
- The radial iris muscles contract while the circular iris muscles relax.
- The diameter of the pupil increases, allowing more light to enter the eye.
