Visual pathway

• Collection of structures which form the visual pathway

  Retina

  Optic nerve

  Optic chiasma

  Optic tracts

  Lateral geniculate bodies

  Optic radiations

  Visual cortex (in brain)

  Optic nerve (CNII) is responsible for transmitting special sensory information for vision
• Visual Pathway


  Route taken by light stimulus

  Once it is perceived by the retina it travels along several structures before reaching the brain
• What happens when light enters the eye

  It is then transmitted to the visual cortex where it is processed by the brain
• Optic tract


  A part of the visual pathway in the brain

  Runs from the optic chiasma to the lateral geniculate body

  Conversely the left optic tract consists of temporal retinal fibres from the left eye and nasal fibres from the right eye.
• Lateral geniculate body


  A relay Centre in the thalamus for the visual pathway
• Ganglion cell axons


  Ganglion cell axons from the retina gather at the optic disc

  Here they myelinate and form the optic nerve

  They go on to form different segments of the nerve
• Optic chiasma


  Where half of the information from each eye crosses sides and half stays on the same side
• Regions of the optic nerve


  Intra ocular

  Intra orbital

  Intra canalicular

  Intra cranial
• Intra ocular


  1-1.5mm long

  Extends from the retinal surface of optic disc to the posterior scleral foramen
• Intra orbital


  Extends from the globe to the optic foramen

  Approx 25mm long

  Has sufficient slack to allow for eye movements
• Intra canalicular


  Part of dura mater is attached to periorbita and the proximal tendon of the rectus muscles. This is firmly attached throughout the length of the optic canal (5mm)
• Intracranial


  This section starts as the nerve passes from the optic canal

  Ends as the nerve enters the optic chiasma (10mm)
• Dura mater structure 


  0.5mm thick

  Dense irregular connective tissue

  Collagen, elastin, MPS
• Dura mater function 


  Protection

  Tough outer layer
• Arachnoid mater structure 


  10um thick

  Loose connective tissue

  Meshwork of collagen and elastin
• Arachnoid mater function

  Helps retain cerebrospinal fluid in sub Arachnoid space
• Pia mater structure 


  Highly vascular

  Loose connective tissue

  Collagen, elastin, Mps, fibroblasts
• Pia mater function 


  Provides nutrients removes waste

  Defence against disease

  Produces pial septa
• Pial septa structure 


  Connective tissue

  Fibroblasts, collagen, blood vessels
• Pial septa function 


  Mechanical strength, stability

  Support for blood vessels

  Divides nerves into bundles

  Blood supply provides nutrients/ removes waste
• Cerebrospinal fluid structure


  Similar to blood plasma or aqueous

  No blood cells
• Cerebrospinal fluid function 


  Support (shock abdorber)

  Nourishment

  Prevent infection

  Microglial cells are phagocytic
• Nerve fibres structure


  Myelinated and unmyelinated axons of ganglion cells
• Nerve fibres function


  Conduct sensory info to the brain
• Oligodendrocytes structure


  Neuroglial cells
• Oligodendrocytes function 


  Produce myelin sheath around nerve fibre
• Consensual light response 


  Synchronised response

  Change in pupil size of eye opposite to eye in which light is directed

  Due to crossing over at optic chiasma
• Crossed and uncrowded fibres

  Fibres of medial retina of each eye cross
• Optic chiasma location 


  Sits below hypothalamus

  Sits above pituitary gland
• Optic chiasma facts


  Optic chiasma is bathed in cerebrospinal fluid

  Medial fibres and half of the macular bundle cross over

  Blood is supplied from the circle of Willis

  The optic chiasma is quite small (approx 13mm wide and 8mm long)
• Ipsilateral

  Eye on the same side

  Sends info to layers 2,3 and 5
• Contralateral 


  Eye on the opposite side

  Sends info to layers 1,4 and 6