The Eye

• Transduction


  process by which a receptor converts a specific stimulus (light, chemical, mechanical) into an electrical impulse
• Cornea


  • transparent layer covering the front part of the eyeball
  • refracts incoming light to pass through lens and reach photoreceptors in the retina
• Retina


  • multi-layered sheet of photoreceptor cells together with neurons which supply the optic nerve
  • Function: photoreceptors transduce light into electrical impulses which are interpreted as images in the occipital lobe of the brain
• Conjunctiva


  • thin transparent layer covering the cornea except where the iris is present
  • Function: produces mucus which moistens and lubricates the eyes & is primary layer which protects eye against infection
• Sclera


  • outer layer of tough connective tissue visible as white of eye
  • continuous with the cornea
  • Function: protects inner structure of the eye
• Aqueous Humour


  • layer of watery fluid present between the lens and cornea
  • Functions:
  • refracts incoming light
  • maintains shape of eyeball by exerting hydrostatic pressure
  • cleans and nourishes the lens and cornea
• Iris


  • thin, opaque pigmented muscular tissue responsible for eye colour
  • Function: control amount of light entering the eye by opening/closing the pupil through dilation/constriction
• Pupil


  • hole in the iris through which light enters the eye
  • Function: allow entry of light into the eye using 2 antagonistic muscles
• Antagonistic Muscles which Control Pupil Size

  • Pupillary sphincter:
  • under parasympathetic control
  • constricts the pupil, allowing less light into eye e.g. in bright light
  • Pupillary dilator
  • under sympathetic control
  • dilates the pupil, allowing more light into the eye e.g. in dim light
• Lens


  • transparent, elastic biconvex structure made up of protein fibres
  • Functions:
  • focus light onto the retina
  • separate the aqueous and vitreous humours
  • Shape can be adjusted by ligaments and muscles which suspend it - accommodation
• Ciliary Body


  • circular tissue found where sclera meets cornea
  • contains tissue, blood vessels and ciliary muscle
  • Functions:
  • accomodation
  • produces the aq. humour
  • anchors the lens in place
• Ciliary Muscle


  • circular sheet of smooth muscle around the lens
  • Function: plays a role in adjusting the shape of the lens during accommodation
• Suspensory Ligaments


  • connective tissue which attach the lens to the ciliary body
  • Function: play a role in adjusting shape of lens during accommodation
• Vitreous Humour


  • layer of jelly-like fluid between the lens and the retina
  • Functions:
  • maintains the shape of the eye
  • role in refracting light
• Choroid


  • layer of blood vessels between sclera and retina, continuous with ciliary body and iris
  • covered in black pigment layer which absorbs stray light which would interfere with clear vision
  • Function: provide blood and nutrients to retina
• 3 cellular layers of the retina

  1. Photoreceptor cell layer - composed of rods and cones
  2. Intermediate layer - consists of horizontal, bipolar and amacrine cells
  3. Internal surface layer - contains ganglion cells which fire impulses to optic nerve
• Intermediate Layer cells

  • Horizontal cells - connect neighbouring pairs of photoreceptor and bipolars cells; play a role in sharpening contrast between light and dark patterns
  • Bipolar cells - synapse with photoreceptor cells and relay impulses to ganglion cells
  • Amacrine cells - connect neighbouring pairs of bipolar and ganglion cells; role in sensitivity to light
• Blind Spot


  where the axons of the ganglion cells converge and leave the eye as the optic nerve, therefore no sensitivity to light in this region due to absence of photoreceptor cells
• Basic Structure of Rod/Cone Cells

  • Outer segment consisting of stacked membranous lamellae/discs with light-sensitive pigment embedded in surface
  • Inner segment containing cell organelles which produces proteins, ATP...
  • Synaptic terminal which makes contact with other neurons e.g. bipolar cells
• Rods vs Cones - Pigments

  Rods - rhodopsin

  Cones - 1 of 3 types of iodopsin
• Rods vs Cones - Functions

  Rods - monochromatic vision & night vision

  Cones - colour vision (trichromatic theory) & day vision
• Rods vs Cones - Sensitivity

  Rods - high, so sensitive even to low light intensities

  Cones - low, require high light intensities to be stimulated
• Rods vs Cones - Acuity

  Rods - low acuity, hence image not precise/sharp

  Cones - high acuity, creating sharp, precise image
• Rods vs Cones - Abundance

  Rods are more abundant than cones
• Rods vs Cones - Distribution

  Rods - relatively distributed around retina

  Cones - concentrated at fovea centralis
• Rods vs Cones - Outer Segment

  Rods' outer segment has many more stacks/discs than that of cones, therefore accounting for higher sensitivity
• Rods vs Cones - Synapse Ratio with Bipolar Cell


  1 bipolar cells synapses with many rod cells - leading to convergence due to spatial summation

  1 bipolar cells synapses with a single cone cell - no convergence, hence accounting for higher acuity and no blurriness
• Sensitivity of Photoreceptors


  ability/degree to which they can detect/be stimulated by light
• Acuity


  refers to sharpness/accuracy of image created, reflected in the ability to distinguish between nearby objects
• Fovea Centralis


  centre of the retina and region of highest visual acuity due to the presence of only cone cells
• Convergence


  ability of multiple sensor cells [e.g. rod cells] to synapse/connects with a single sensory neuron [e.g. bipolar cell]
• Convergence in the Retina

  when rods cells stimulated simultaneously, there is spatial summation of EPSPs in bipolar cells which are enough to generate an AP even in dim light - producing an image with low acuity due to many sources but allowing for higher sensitivity
• Lack of Convergence in Cone Cells

  in cone cells, there is no convergence, i.e. a single cone cells synapses with a single bipolar cell - this 1:1 relationship accounts for higher acuity as there is no integration of information from multiple sources but each part of image is detected by a different cell ; however leads to lower sensitivity as there is no spatial summation
• Accommodation


  reflex mechanism by which the shape of the lens is adjusted (to change its focal length) to focus light reflecting off of objects in view onto the retina depending on the distance between the object and the eye
• Accommodation - when object is close to eye:


  ciliary muscles contract, there is no tension exerted on lens by suspensory ligaments, therefore lens bulges/has more spherical shape, such that it has a short focal length [most refractive] and bends light to a sharp angle to focus it on retina
• Accommodation - when the object is close to the eye:


  ciliary muscles relax, so suspensory ligaments pull the lens to acquire a flatter, thinner shape, such that its focal length increases and bends light less/to smaller angle to focus light onto retina
• Rhodopsin


  • light sensitive pigment found embedded in the membranous discs of outer segment of rod cells
  • consists of:
  • Opsin - lipoprotein
  • Retinal - carotenoid pigment derived from Vit A ; cis-retinal fits into opsin protein acting as a prosthetic group
• Light Absorption in Rod Cells

  1. Light falls on rhodopsin

  2. Cis-retinal begins converting into isomer trans-retinal

  3. Rhodopsin complexes break down (as trans-retinal does not fit exactly with opsin) - BLEACHING

  4. Na+ channels close

  5. Hyperpolarisation of the rod cell due to the Na+K+ pump (i.e. no dark current)

  6. Hyperpolarisation prevents the release of neurotransmitter glutamate from synaptic terminal of rod cell

  7. No release of glutamate means the bipolar cells is no longer inhibited and can therefore generate APs
• Bleaching


  Rhodopsin complexes break down (as trans-retinal does not fit exactly with opsin)
• Bleaching


  Causes Na+ channels to close