Recap: Focusing (Accommodation) Definition: Adjustment of the thickness of the lens of the eye so that clear images of objects at different distances are formed on the retina.

Recap: Focusing (Accommodation) Parts of eye When viewing a near object When viewing a distant object Ciliary muscles (circular) Suspensory ligaments Lens Contract Slacken (become loose) Become thicker (more convex) Relax Become taut (pulled tight) Become thinner (less convex)

THE BIOLOGY VERSION

Imagine you woke up from a bad dream… All you can see is … darkness

You thought it would be a good idea to get a drink from the kitchen… So you pushed open the door… And…

The living room lights were all switched on! Not so scary after all huh?

The first thing you observed or experienced? Discomfort in your eye (Ouch!) WHY?

Recall Which part of the eye allows light to enter and reach the retina? Pupil

Recall Where is the image formed in the eye? Retina

Entry of Light into the Eye Too much light enters the eye through the pupil The high intensity of light reaches the retina of the eye The size of pupil is altered Pain is detected

Does the Brain only process images? Is there another role for the Brain in vision?

The Brain & Amount of Light Entering the Eye Impulses from the retina are sent to the Brain The Brain detects the light intensity and returns the impulse to control the size of the pupil.

Recall Which part of the eye controls the size of the Pupil? Iris

The Iris Circular muscles Consists of: - Circular Muscles - Radial Muscles

Size of pupil related to amount of light in surroundings High surrounding light intensity Large amounts of light enter the eye However, excess light could damage the eye Hence, the pupil becomes smaller (constricts) to allow lesser light to enter

Size of pupil related to amount of light in surroundings Low surrounding light intensity Little light enter the eye Affect ability to see surroundings The pupil enlarges (dilates) to allow more light to enter

Altering the diameter of the pupil Circular muscles contract Radial muscles relax Pupil becomes smaller Circular muscles relax Radial muscles contract Pupil becomes larger PUPIL BECOMES SMALLER (CONSTRICTS) PUPIL ENLARGES (DILATES) High surrounding light intensityLow surrounding light intensity

The Pupil Reflex Parts of eye In dim light In bright light Circular iris muscles Radial iris muscles Pupil dilates(bigger) contracts(smaller) contractrelax relax contract

Pupil Reflex Action Receptor/Sensory neurone Motor/Effector neurone Central Nervous System (CNS) Receptor Effector Stimulus (Dim light/Bright light) Response ( Pupil constricts/dilates) (Rods and cones)(Iris muscles) ( Brain )

The Pupil Reflex Action In dim light In bright light In bright light Light-sensitive cells in the retina detect the light intensity Impulses are sent along the optic (sensory) nerve to the brain The brain returns impulses along a motor nerve to the iris muscles The radial iris muscles contract while the circular iris muscles relax The diameter of the pupil increases, allowing more light to enter Light-sensitive cells in the retina detect the light intensity Impulses are sent along the optic (sensory) nerve to the brain The brain returns impulses along a motor nerve to the iris muscles The circular iris muscles contract while the radial iris muscles relax The diameter of the pupil decreases, allowing less light to enter

What we learn today. Pupil Reflex (Bright Light) Pupil Reflex (Dim Light) Pupil Reflex Action