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 eyeParts of eye When viewing a When viewing a near objectnear object

When viewing a When viewing a distant objectdistant object

Ciliary musclesCiliary muscles

(circular)(circular)

Suspensory Suspensory ligamentsligaments

Lens 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 EyeToo 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 musclesConsists 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 intensity

Low surrounding light intensity

The Pupil ReflexParts of eyeParts of eye In dim In dim

lightlightIn bright In bright lightlight

Circular iris Circular iris musclesmuscles

Radial iris Radial iris musclesmuscles

Pupil Pupil dilatesdilates(bigger)(bigger)

contracts contracts (smaller)(smaller)

contractcontract relaxrelax

relaxrelax contractcontract

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 ActionIn dim lightIn dim light In bright lightIn bright lightLight-sensitive cells in the retina Light-sensitive cells in the retina detect the light intensitydetect the light intensity

Impulses are sent along the optic Impulses are sent along the optic (sensory) nerve to the brain(sensory) nerve to the brain

The brain returns impulses along The brain returns impulses along a motor nerve to the iris musclesa motor nerve to the iris muscles

The radial iris muscles contract The radial iris muscles contract while the circular iris muscles relaxwhile the circular iris muscles relax

The diameter of the pupil increases, The diameter of the pupil increases, allowing more light to enterallowing more light to enter

Light-sensitive cells in the retina Light-sensitive cells in the retina detect the light intensitydetect the light intensity

Impulses are sent along the optic Impulses are sent along the optic (sensory) nerve to the brain(sensory) nerve to the brain

The brain returns impulses along The brain returns impulses along a motor nerve to the iris musclesa motor nerve to the iris muscles

The circular iris muscles contract The circular iris muscles contract while the radial iris muscles relaxwhile the radial iris muscles relax

The diameter of the pupil decreases, The diameter of the pupil decreases, allowing less light to enterallowing less light to enter

What we learn today.

• Pupil Reflex (Bright Light)

• Pupil Reflex (Dim Light)

• Pupil Reflex Action