6.1.10 The Human Retina

• The eye is a sense organ containing receptors sensitive to light intensity and colour
• Receptors are groups of specialised cells that can generate an electrical impulse in a sensory neurone
• The eye contains two types of receptor cell:
  • Rod cells which are sensitive to light intensity
  • Cone cells which are sensitive to different wavelengths of visible light (colour)

Structure

• The purpose of the eye is to receive light and focus it onto the retina at the back of the eye
• The retina is where the rod and cone cells are located
  • There are roughly 125 million rods and 7 million cones distributed on the retina

The eye is an organ made from several different types of tissue. All of the structures function together to allow light to hit the retina, which sends signals to the brain.

Eye structure and function table

Sensitivity to light and colour

• The rods and cones contain optical pigments that absorb light and then break down
  • Rod cells contain rhodopsin
  • Cone cells contain iodopsin

• The breakdown of optical pigments results in a generator potential being produced
• The pigments within the receptors are broken down by different conditions
  • Rhodopsin within rods breaks down in dim light
  • Iodopsin within cones breaks down in bright light only

• The receptors have different sensitivities to light
  • Sensitivity refers to the amount of light required to stimulate the receptor

• Rods are very sensitive to even low light intensities
  • They allow humans to distinguish between light and dark objects when light is very dim
  • They do not allow humans to see in colour

• Cones are less sensitive to light
  • They are sensitive to different wavelengths of light
  • There are three cone types found in human eyes, each contains a different optical pigment
  • Each pigment is sensitive to light of a different wavelength
    • Red-sensitive cones are sensitive to wavelengths of light that correspond to the colour red
    • Blue-sensitive cones are sensitive to wavelengths of light that correspond to the colour blue
    • Green-sensitive cones are sensitive to wavelengths of light that correspond to the colour green

  • The combined effect of all three pigments allows humans to observe all the other colours that are on the visible spectrum

Visual acuity

• Acuity is the ability to distinguish two separate points, otherwise known as 'visual clarity'
  • Those who can read the smallest letters on an opticians board have high visual acuity

• There are over 130 million receptors (rods and cones) on the retina
• Receptors that are hit by light rays become stimulated and those that are not hit by light rays remain unstimulated
• Once a receptor is stimulated it can send impulses to the brain
• The brain is able to interpret the pattern of impulses to form an image
• There is no direct connection between rods and cones and the central nervous system
• There are synapses connecting the rods and cones to bipolar neurones
• The bipolar neurones connect to ganglion cells via synapses
• The ganglion cells have axons that extend to the optic nerve which is directly connected to the brain
• Due to the high number of receptors on the retina, it is not possible for there to be individual connections between each receptor and the brain
• The way that rods and cones are connected to the optic nerve affects visual acuity
  • Visual acuity is essentially the resolution or amount of detail that is perceived in an image
  • It is measured by how far apart two spots of light need to be in order to be seen separately

Rod cells provide lower visual acuity

• Multiple rod cells synapse with a single bipolar cell
• Multiple bipolar cells synapse with a single ganglion cell
• Approximately 100 rod cells are synaptically connected to a single ganglion cell
• The brain is not able to interpret which impulses are sent by specific rods
• If multiple rod cells connected to the same bipolar cell detect light, only one impulse from the bipolar cell is sent
• Therefore, the brain receives a general, not specific, understanding of the fields of vision that are light or dark

• A single cone cell synapses with a single bipolar cell
• A single bipolar cell synapses with a single ganglion cell
• If two cones are stimulated to send an impulse the brain is able to interpret these as two different spots of light

Rods and cones are connected to the optic nerve in different ways. The type of connection affects visual acuity.

• As cone cells detect only one of three colours (red, green or blue) the brain will receive information about the colour of light detected by the stimulated cone cell and where this light is
• This is because the brain knows which bipolar cell connects to which cone cell

Each cone produces its own impulse when stimulated

Summation

• There is a benefit to how the rods are connected to the optical nerve
• Each rod is very sensitive to light however a single stimulated rod is unlikely to produce a large enough generator potential to stimulate the bipolar cell for the conduction of nerve impulses
• When a group of rods are stimulated at the same time the combined generator potentials are sufficient to reach the threshold and stimulate the bipolar cell for the conduction of nerve impulses onwards towards the optic nerve
• This additive effect of rods is known as summation
• Summation produces a less sharp image but enables organisms to see in much dimmer light than cones allow
• Nocturnal animals tend to have mostly or solely rods present in their eyes