week 10

• Visual System

  Convert light energy into electrical signal into nerve impulses that can be interpreted by brain

  
• What factors do you need to consider for the visual system

  • Optically clear media required-Transparent (c.f. translucent)• Detection mechanism-Light has to reach this in focus• Conversion to electrical signal-Travels to, then is processed by, the brain
• Visual System = Human Camera• A camera requires:• A system of lenses (cornea and crystalline lens) to focus the image on...• A light-sensitive film / CCD chip (retina)• A power supply (central retinal artery, digestive, respiratory, cardio vascular system etc.)• A light-proof, protective case (sclera, orbit and skull) but there is no processing, no understanding (no occipital cortex, no brain!)

  
• Protecting the Eye• The eye is positioned in the eye socket within the skull and surrounded by fatty tissue• Eyelids close over the eye when we are asleep, and react quickly to approaching danger (“blink reflex”)• The tear film covers the front of the eye and helps to wash away debris• The sclera is a tough, opaque layer which keeps the eye rigid and light-proof• The conjunctiva ‘seals’ the front of the eye and prevents entry of foreign bodies.

  
• Protecting the Eye: the cornea

  
• Focusing the light: cornea and crystalline lens

  
• Cornea is the front surface of the eye and is curved like a lens• Makes up 2/3rds of the eye’s focussing power(~43-44 D out of 60)
• Crystalline lens is inside the eye• Makes up 1/3rd of the eye’s focussing power (~20 D out of 60)• Can change shape to focus far and near
• Refractive error is any error in the focussing of theeye• Mismatch between eye refractive power (~ 60 D) and eye length (~23mm)
• Emmetropia:(focus on the retina: optics match axial length)
• Myopia(focus "before" retina: optics too strong OR eye too long)
• Hypermetropia(focus“behind” retina: optics too weak OR eye too short)
• Tear film• O2 from atmosphere dissolves in the tear film• O2 and nutrients to anterior cornea

  
• Aqueous humour• Fluid in the anterior chamber• Relationship to IOP• O2 and nutrients to posterior cornea

  
• Blood vessels• O2 and nutrients to uveal tract• O2 and nutrients to retina

  
• Physiology of SeeingLight detected by photoreceptors.Chemical reaction converts to an electrical signal.Signal passed to different types of nerve cells for further processing.Leaves the eye via the optic nerve to head to the brain.Retina is upside down!

  
• Seeing: Transmitting Visual Information to the Brain

  • The information collected by the retina is transmitted to the brain via the optic nerve• Sensory nerve (vs motor)• This is an extension of the brain, and is the only directly visible part of the CNS (and vascular system)• Brain injuries or disease along the visual pathway will affect vision• Different brain areas deal with different types of visual function

  
• Prefix pseudo-=false e.g. pseudomyopia
• Prefix circum-=around
• prefix extra-=outside of
• prefix intra-=inside
• Prefix retro-=backwards
• prefix eryth-=red
• Suffix -opia=condition of vision, particularly a visual defect
• Suffix -itis=inflammation in a certain area
• Direction of eye movements

  -version used for movement of both eyes

  -duction used for movement of only one eye