Eye

1. describe the structure of the eye as seen in front view and in horizontal section 2. state the principal functions of component parts of the eye in producing a focused image of near and distant objects on the retina 3. describe the pupil reflex in response to bright and dim light

Sclera

Sclera

Structure: Tough, white outer covering of the eyeball; continuous with cornea

Function: Protects eyeball from mechanical damage

Choroid

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

Suspensory ligaments

Structure: connective tissue

Function:

• Attach to the edge of the lens to the ciliary body

• Transfer the effect of the ciliary muscle to the lens

Ciliary body

Structure: Consist of ciliary muscles

Function: Controls the curvature or thickness of the lens

Retina

Structure: The innermost layer of the eyeball, containing photoreceptors (rods and cones)

Function: Light-sensitive layer where images are formed

Cornea

Structure: transparent layer in front of eye (beneath the conjunctiva), continuous with sclera

Function: Refracts light towards lens

Iris

Structure: consist of two sets of involuntary muscles, the circular muscles and radial muscles. Surrounds pupil, in front of the lens.

Function: Adjusts amount of light entering the eye by changing the size of pupil

Pupil

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: transparent, biconvex, elastic crystalline body

Function: Changes curvature or thickness to refract light (at varying distances) onto the retina

Aqueous humour

Structure: transparent, watery fluid

Function:

• Refracts light onto lens and keeps eyeball firm

• Serves as a medium for the diffusion of oxygen and nutrients

Vitreous humour

Structure: transparent jelly-like fluid

Function: Refracts light onto retina and keeps eyeball firm

Fovea (yellow spot)

Structure: greatest concentration of cones but no rods

Function:

• Part of retina that is best resolves images; where images are focused onto it

• Gives detailed colour vision in bright light

Blind spot

Structure: found over the optic nerve; no photoreceptors

Function: An area on the retina where neurones leave the eye

Optic nerve

Structure: composed of sensory neurones

Function: Transmits nerve impulses from photoreceptors to brain

Photoreceptors in the retina

Photoreceptors consists of rods and cones, which are stimulated by the light reflected from objects. Nerve impulses are sent to visual centre of the brain via the optic nerve.

Cones

• Cones enable us to see colours in bright light

• There are 3 types of cones: red, blue and green 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

• Rods are more sensitive to light than cones.

• They enable us to see in dim light, but only in black or white.

• They contain the light sensitive pigment, visual purple (rhodopsin).

• Visual purple is bleached when the eye is exposed to bright light. It takes a while for the 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

• Rods lie outside the fovea in the more peripheral parts of the retina

Accommodation

The adjustment of the lens to change curvature or thickness so that clear images at different distances are formed on the retina.

Focusing on near objects

• Ciliary muscles contract, reducing the pull on the suspensory ligaments

• Suspensory ligaments slacken

• Lens becomes thicker and more convex

• Decreasing its focal length

• 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

Focusing on distant objects

• Ciliary muscles relax, pulling on the suspensory ligaments

• Suspensory ligament become taut

• Lens becomes thinner and longer

• Increasing its focal length

• 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

Pupil reflex in bright light

• Photoreceptors in the retina detect an increase in light intensity (stimulus)

• Nerve impulses are sent along the sensory neuron in the optic nerve to the brain (reflex center)

• The brain returns impulses along the motor neuron to the circular and radial muscles of the iris

• Circular iris muscles contract while the radial iris muscles relax

• Diameter of the pupil decreases, allowing less light to enter

Pupil reflex in dim light

• Photoreceptors in the retina detect a decrease in light intensity (stimulus)

• Nerve impulses are sent along the sensory neuron in the optic nerve to the brain (reflex center)

• The brain returns impulses along the motor neuron to the radial and circular muscles of the iris

• Radial iris muscles contract while the circular iris muscles relax

• Diameter of the pupil increases, allowing more light to enter

Blink reflex

Any object seen coming towards the eyes causes the eyelids to close

Tearing reflex

Dust sensed by the conjunctiva causes an increase in tear flow to wash it away