The Eye

Main features

Light control

  • The iris controls the size of the pupil to regulate the amount of light entering the eye
  • The circular and radial muscles in the iris work antagonistically to adjust the size of the pupil

Focusing

  • The lens changes shape to focus light rays and allow clear vision at different distances
  • The ciliary muscles attached to the lens control its shape and adjust the degree of refraction

Light receptors - rods and cones

Rods are sensitive to low light conditions and provide black and white vision

Cones are sensitive to colour and function in bright conditions, providing detailed vision

The fovea, located in the center of the retina, contains only cones and provides the sharpest vision

Structure and Function

Aqueous and Vitreous Humour

  • Aqueous humour is a watery fluid that maintains the shape of the front part of the eye
  • Vitreous humour is a jelly-like fluid that maintains the shape of the back part of the eye and supports the lens

Blind spot

  • A small area on the retina where the optic nerve leaves the eye
  • No light-sensitive cells are present, so any image falling here is not seen

Ciliary Body

  • Contains the ciliary muscle, which is attached to the lens
  • The ciliary muscle changes the shape of the lens to allow for focusing at different distances

Cornea

  • A transparent tissue that covers the front of the eye
  • It refracts (bends) light more than any other part of the eye

Iris

  • A muscular layer that contains circular and radial muscles
  • It controls the size of the pupil and the amount of light entering the eye
  • Also contains a pigment that absorbs light

Lens

  • A transparent biconcave structure that refracts light passing through the eye
  • It can change shape to adjust the degree of refraction and focus light

Optic Nerve

  • A bundle of sensory nerve fibres that transmit visual signals from the eye to the brain

Choroid

A layer of pigmented cells between the sclera and retina

It absorbs light and prevents internal reflection within the eyeball

Contains blood vessels that supply the retina and other layers with blood

Retina

The inner layer of the eyeball that contains rods and cones

Also contains bipolar neurons and ganglion cells that transmit visual signals to the brain

Sclera

  • The tough, opaque outer covering of the eyeball
  • It protects the eyeball against mechanical damage and provides attachment points for eye muscles

Conjunctiva

  • A thin, transparent membrane that protects the cornea against damage
  • Tears lubricate the surface of the conjunctiva

Adjustment of Pupil Size

Iris Muscles

The iris consists of circular and radial muscles that control the size of the pupil

The circular muscle constricts the pupil, while the radial muscles dilate the pupil

These muscles work antagonistically to adjust the size of the pupil

Reflex response

The adjustment of pupil size is a reflex response controlled by the autonomic nervous system

The iris responds rapidly to changes in light intensity to protect the sensitive rod and cone cells

Very bright light can overstimulate the cells and cause damage

Accommodation

Focusing Light

The eye adjusts the refraction of light to focus on objects at different distances

Light is refracted/bent as it passes through the cornea and lens

The cornea provides most of the refraction, but the lens can adjust the degree of refraction

Lens Adjustment

The lens changes shape to adjust the degree of refraction

For distant objects, the ciliary body relaxes, causing the lens to become thinner and less converging

For near objects, the ciliary body contracts, causing the lens to become thicker and more converging

Reflex Actions

  • Accommodation and the control of light entering the eye are reflex actions
  • They are automatic responses and not under voluntary control

Rods and Cones

Structure

Both rods and cones have an outer segment containing a photosensitive pigment

They also have an inner segment with a pair of cilia leading to the cell's nucleus

Rods and cones synapse with bipolar neurons, which then synapse with ganglion cells

Differences

Rods are more numerous and evenly distributed throughout the retina, while cones are concentrated in the fovea

Rods are very sensitive and operate in dim light, while cones operate only in bright light

Rods are insensitive to colour and provide monochromatic vision, while cones are sensitive to red, green, or blue light and provide colour vision

Rods have poor resolution and visual acuity, while cones have good resolution and high acuity

Pigments

Rods contain rhodopsin

Cones contain iodopsin

Rhodopsin is more sensitive than iodopsin and is readily broken down by light

Synapses and visual acuity

Several rods synapse on one bipolar neuron, allowing for retinal convergence

This increases the sensitivity of rods to low light intensity but decreases visual acuity

Cones synapse individually with bipolar neurons, providing high visual acuity and precise colour vision

Trichromatic Theory of Colour Vision

Types of cone cell

There are three types, each sensitive to a different wavelength of light

Blue cones respond to blue light, red cones respond to red light, and green cones respond to green light

The perception of color depends on the relative stimulation of these three types of cone cells

Colour perception

Pure blue, red, or green light will only stimulate the corresponding cone cells

Most light is a combination of different wavelengths, and the degree of stimulation of each type of cone determines colon perception

For example, yellow light stimulates both red and green cones, resulting in the perception of yellow

Structure of the Retina

Layers

Contains millions of light-sensitive cells (rods and cones) and the neurons with which they synapse

Rods and cones form a layer inside the choroid, followed by a layer of bipolar neurons

Beyond the bipolar neurons, there is a layer of ganglion cells

The axons of ganglion cells form the optic nerve, which carries visual signals to the brain

Inverted Arrangement

The retina is said to be inverted because neurons lie over the photoreceptor cells

Light has to pass through the neurons before reaching the rods and cones, which can compromise light detection

The blind spot is where the optic nerve leaves the eye, and no light-sensitive cells are present

Retinal Convergence

Occurs when several rods synapse on one bipolar neuron

This allows for summation of sub-threshold stimuli and increases visual sensitivity

Cones synapse individually with bipolar neurons, providing higher visual acuity

Binocular Vision

Advantages

Binocular vision is the ability to see with both eyes and create a single 3D image

The visual fields of both eyes overlap, allowing for accurate judgment of distance and depth perception

Binocular vision provides larger visual field, better visual acuity, and stereoscopic vision

Predatory Species

Predatory species, like humans and foxes, have eyes positioned at the front of their head for binocular vision

Binocular vision is important for accurately judging distance and position for hunting

Humans have a binocular visual field of 180°

Prey Species

Prey species, like rabbits, have laterally placed eyes on the side of their head

This gives them a wider field of view to detect potential predators

Prey species do not prioritize 3D vision and do not have stereoscopic vision