Sight

• Conjunctiva


  • Thin layer of epithelial cells that lines the inside of your eyelids from the eye
  • Protects the cornea from friction
  • Helps moisturize the cornea, protects it from dust and debris

  
• Cornea


  • First part of the eye that the light hits
  • Thick, transparent sheet of fibrous tissue
  • Lines the anterior 1/6th of the eye (continuous with the sclera, which lines the posterior 5/6th of the eye)
  • Protects the eye
  • Starts to bend incoming light

  
• Anterior chamber


  • A space filled with aqueous humour (essentially just water and salt, secreted by the ciliary body)
  • Provides pressure to maintain shape of eyeball
  • Allows nutrients and minerals to supply cells of cornea/iris
• Pupil


  • The opening in the middle of the iris
  • The pupil can get bigger or smaller, based on the degree of iris muscular relaxation/contraction that is occuring
  • Physical opening to allow eye to enter the eye

  
• Iris


  • Consists of two different muscles that constricts/relaxes to change the size of the pupil
  • Change the amount of light that can enter the eye by changing the size of the pupil
  • Also confers eye colour

  
• Lens


  • Bi-convex (i.e. curved on both ends)
  • Attached to suspensory ligaments, which are themselves attached to a ciliary muscle
  • Bends the light so it goes to the back of eyeball
  • Focuses light specifically on the fovea of the retina
  • Change in shape of lens adjust how much it bends the light

  
• Posterior chamber


  • Part of the anterior segment
  • Represents a narrow space behind the iris, surrounding the equator of the lens and separating it from the ciliary body
  • Filled with aqueous humor
• Vitreous chamber


  • Part of the posterior segment
  • Represents the distance between the posterior of the lens and the inner retinal layer
  • Filled with vitreous humour, a transparent jelly-like substance (water, salt, albumin, other proteins)
  • Vitreous humour provides pressure (structure) to eyeball
  • Vitreous humour provides nutrients to inside of eyeball

  
• Retina


  • Lines the entire back of the eyeball
  • Tinted red
  • Filled with photoreceptors (rods and cones)
  • Rods allow us to see light, dark, and shades of grey
  • Cones allow us to see colour, and discern high levels of detail
  • Most of the retina is covered primarily in rods
  • Macula is a special part of retina rich in cones, with some rods
  • Fovea is a special part of macula, completely covered in cones, no rods
  • Photoreceptors (rods and cones) convert the ray of light from a physical waveform to an electrochemical impulse, which can then travel along the optic nerve and to the brain for interpretation

  
• Choroid


  • Network of blood vessels
  • Pigmented black in humans
  • Nourish the retina
• Sclera


  • The white outer layer of the eye
  • Composed of thick fibrous tissue that covers posterior 5/6th of eyeball (and is continuous with the cornea, which covers the anterior 1/6th)
  • Acts as an attachment point for muscles
  • Provides an extra layer of protection and structure of eyeball

  
• Anterior segment


  • Front 1/3rd of the eye
  • Includes structures that are in front of the vitreous humour (cornea, iris, ciliary body, and lens)
  • Contains two fluid-filled spaces: Anterior chamber (between the cornea and the iris), Posterior chamber (between the iris and the vitreous)
• Posterior segment


  • Back 2/3rd of the eye
  • Includes structures include the vitreous humour and back (retina choroid, and optic nerve)

  
• Blind spot - location where nerve fibers come together to form the optic nerve
• Sensation


  Requires a physical stimulus (e.g. light) to be converted into a neural impulse by some specialized cell (e.g. photoreceptor)
• Light


  • An electromagnetic wave (the sun is one of the most common sources)
  • Electromagnetic (EM) spectrum contains everything from gamma rays and x-rays (low wavelength) to AM/FM radio waves (long wavelength)
  • Visible light is in the middle of the EM spectrum, from violet (400nm) to red (700nm), with the lowest to highest wavelength being VBGYOR
• Rods


  • Count: 120 million
  • Function: Night vision
  • By default, the rod is 'turned on'
  • When light hits, the rod 'turns off' (see phototransduction cascade)
• Cones


  • Count: 6-7 million (fewer than the number of rods)
  • Function: Colour vision
  • Trichromatic theory of vision suggests that we have 3 types of cones that are each sensitive to a specific colour: red (60%), green (30%), and blue (10%) cones
  • Almost all cones are centered in fovea, found at the center of the macula

  
• Transduction


  Transformation of energy from one form to another (e.g. light energy → electrical energy by rods and cones)
• Transmission


  The electrical activation of one neuron by another neuron
• Processing


  Neural transformation of multiple neural signals into a perception
• Perception


  Conscious sensory experience of neural processing
• Bipolar cells


  • Found in the retina
  • Send visual signals from the rods and cones to the ganglion cells
• Ganglion cells


  • Found in the retina
  • Receive signals from the bipolar cells
• Optic nerve


  • Visual signals, after having been picked up by the rods and cones and transferred to bipolar cells to the ganglion cells, finally leave the eye through the optic nerve, which is really just the axons of the ganglion cells

  
• Phototransduction cascade


  Rod (ON by default) → Light hits rod (rod turns OFF) (converts light to a neural impulse) → bipolar cell (turns ON) → retinal ganglion cell (turns ON) → optic nerve → brain (for interpretation)

  
• Inside rod are thousands of optic discs stacked on top
• Optic nerve


  The axons from the ganglion cells band together into a bundle (optic nerve) and go through the hole at the back of the eye (your 'blind spot')

  
• Phototransduction cascade (PTC)


  A set of events occurring at the molecular level of a rod that causes a rod (which is ON by default) to turn OFF after being hit by light
• Phototransduction cascade process


  1. Inside rod are thousands of optic discs stacked on top of one another

  2. Optic disks are rich in different types of proteins

  3. Important protein: rhodopsin

  4. When light enters the eye, it hits the rods of the retina, and some of the light will hit the retinal molecule directly, providing energy that induces a conformation change in the shape of the retinal molecule from a bent (11-cis retinal) to a straight conformation (11-trans retinal)

  5. When the retinal molecule changes shape, this causes rhodopsin to change shape too, since the two are closely linked molecules

  6. The shape change of rhodopsin causes transducin to dissociate from rhodopsin

  7. The α subunit of transducin is now liberated to bind to another disk protein – phosphodiesterase (PDE)

  8. cGMP –––PDE–––> GMP

  9. Thus, when light hits, ↓[cGMP]

  10. cGMP is usually bound to Na+ channels to keep them open (and thus keeping the cell ON)

  11. When ↓[cGMP], Na+ channels close, causing the cell to hyperpolarize (and thus turn OFF)

  12. When rods are turned OFF, the on-center bipolar cells are being turned ON

  13. When bipolar cell turns ON, retinal ganglion cell also turns ON, which sends the signal to the optic nerve and then to brain

  
• Photoreceptors (rods and cones)


  • Contain thousands of optic discs – large, membrane bound structures that contain proteins (rhodopsin in rods, photopsin in cones) that are implicated in the phototransduction cascade (which, upon light stimulation, results in the firing of APs to the brain)

  
• Rods


  • Located in the periphery
  • 20x more numerous than cones (120 million vs. 6 million)
  • 1000x more sensitive to light than cones
  • Better at detecting light and 'telling us' when light is present
  • Slow recovery time

  
• Cones


  • Concentrated in the fovea (few dispersed through rest of eye)
  • 3 different cell types that primarily detect color (but also detect some light)
  • Fast recovery time

  
• Blind spot is where the optic nerve connects to the retina, whereby there are no cones or rods

  
• In the periphery, light must go through a bundle of axons, causing some light energy to be lost, which causes less light to hit the rods
• The fovea is the 'dimple' in the retina, which is the primary location for cones, and since no axons exist at the fovea, there are no axons to get in the way of incoming light, so light hits cones in the fovea directly and the cones get more light, allowing for high resolution

  
• The left and right visual fields are contralaterally processed, meaning that the left visual field is processed by the right hemisphere of the brain, and vice versa
• Visual field processing


  1. All our right visual field goes to left side of brain

  2. All our left visual field goes to right side of brain

  3. All axons from the nasal side of each eye cross to the other side

  4. All axons leading from the temporal side do not cross the optic chiasm

  
• Feature Detection


  When looking at an object, you need to break it down into its component features (colour, form, and motion) to make sense of what you are looking at
• Parallel Processing


  Allows us to detect all this information (colour, form, and motion) at the same time