HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
HBIO4 > The Eye
Welcome, Guest!
Login with Facebook | Login

Detecting light - the eye

Table: Structure and function of the eye

Conjunctiva

Protection of cornea

Sclera

- Protection
- Attachment for eye muscles

Cornea

Refracts (→focuses) and allows passage of light

Choroid

Pigment prevents light reflection within the eyeball by absorbing light

Ciliary body

- Accommodation
- Secretion of humour

Iris

Regulates passage of light

Lens

Refracts light

Retina

Contains light receptors

Fovea

Contains only cone cells

Blind spot

Optic nerve (sensory nerve fibres) leave the eyeball

Humour

Maintains shape of the eyeball

Transmissive and refractive properties of the eye

  • Light/photons travel through transparent media in a light ray
    • Rays reflect at a predictable angle when they strike an object
    • Rays passing through mediums of different density refract (change angle)
  • Accommodation → focus of rays from near/distant objects by changing shape of lens
  • Light rays form an image in the retina [EXAM]
    • Refraction / by lens or cornea / shape of lens changes


NEAR ACCOMMODATION

DISTANT ACCOMMODATION

CILIARY MUSCLES

CONTRACT

RELAX

TENSION IN SUSPENSORY LIGAMENTS

REDUCED

INCREASED

SHAPE OF LENS

FAT, ROUNDED

THIN, FLAT

RESULT

LIGHT BENDS

LIGHT BENDS LESS

FOCUSES

DIVERGING LIGHT RAYS

PARALLEL LIGHT RAYS

Rod and cone cells

  • Retina contains 4 layers → synapse between them
    • Cone and rod cells (light-sensitive receptor)
      • Inner segment → nucleus, mitochondria, ribosomes, synaptic region
      • Outer segment → membranous disks containing pigments
    • Bipolar neurones (relay neurone)
    • Ganglion cells (sensory neurones)
    • Axon of ganglion cells → optical nerve
      • Send impulses to the brain
  • Light passes through neurones before it strikes the retina
  • There are no cone and rod cells where the optic nerves pass through the retina; this point is called the blind spot

Table: Features of rod cells and cone cells


FEATURE

ROD CELLS

CONE CELLS

Number in retina

More

Fewer

Distribution

- Evenly throughout the retina
- Absent from the fovea
- Only type of light receptor at the periphery of the retina

Present in the fovea

Shape of outer segment

Rod shaped

Cone shaped

Sensitivity to

Dim light

Bright light

Visual acuity

Poorly resolved images

Well-resolved images

Light-sensitive pigments

- Only rhodopsin
- Monochromic vision

- Iodopsin
- Sensitive to blue, green, blue light
- Trichromatic vision (combination)

Synapse with relay cells

Several rod cells synapse with same relay cell

Each cone cell synapses with just one relay cell

Table: Absorption of light by rhodopsin creates a generator potential in rod cells


In the dark (rod cell)

In light (rod cell)

Opsin + Cis-Retinal → Rhodopsin

Rhodopsin → Opsin + Trans-Retinal

Causes sodium channels to open

Causes sodium channels to close

Membrane depolarised

Membrane hyperpolarised

Neurotransmitter released into inhibitory synapse [rod → bipolar cell]

No neurotransmitter released into inhibitory synapse

Bipolar neurone hyperpolarised → no impulse

Bipolar neurone depolarised → AP

No neurotransmitter released into excitatory synapse [bipolar → ganglion cell]

Neurotransmitter released into excitatory synapse

No action potential

Action potential along ganglion neurone

Resynthesis of rhodopsin

  • TRANS-RETINAL + OPSIN → RHODOPSIN ATP → ADP + PI
  • Mitochondria in inner segment synthesis ATP
  • Slow reaction compared to rhodopsin breakdown by light
  • Bright light into dim light conditions → poor vision until rhodopsin is resynthesised
  • Retinal is a derivative of vitamin A

Connection Between Sensory Cells and The Neurone of the Optic Nerve

  • Rod cells are working in dim light conditions
    • Several rod cells synapse with one relay cell → retinal convergence
    • Impulse by summation, thus, rod cells collectively cause generator potential
    • Poor visual acuity but high sensitivity to dim light
  • Cone cells are working in bright light
    • Each cone cell synapses with each individual relay cell
    • Several impulses pass along the optic nerve to the brain
    • High visual acuity (ability of the brain to resolve images)

Perceiving - the brain

  • Visual pathway
    • Binocular vision
      • Overlap of visual fields of both eyes
      • Right visual field focuses on left retina and vice versa
    • Impulses from rod and cone cells travel within optic nerve
    • Arrive at optic chiasma
      • Half of the fibres cross over to the other side
      • Right brain processes the left visual field of both eyes and vice versa
    • Impulses continue in the optic tract and pass to the lateral geniculate nucleus (LGN)
    • From there, impulses pass to the visual cortex in the occipital lobe
  • Binocular vision allows the judgment of distances
  • Top-down process
    • Perception depends on content of the image and is processed
    • Knowledge, expectations, or thoughts influence perception
    • Evidence from visual illusions
    • Constructivism
  • Bottom-up process
    • Perception from physical characteristic of stimulus
    • Only information received from eye is used for perception
    • Realism