Physiology Neurophysiolgy of vision

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mct
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Physiology Neurophysiolgy of vision
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2013-10-17 14:30:49
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Lecture Nine
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Neurophysiology of vision
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  1. Visible light
    The (small) portion of the electromagnetic spectrum that we can see.
  2. Illuminance
    Amount of light received by an object (measured in lux)
  3. Luminance
    Amount of light emitted by object, candelas per square metre (cd/mm2)
  4. Albedo
    Ratio of illuminance
  5. Visual performance
    • Scotopic: function of eye in very low light
    • Mesopic: At threshold of cone vision, low light
    • Photopic: eye function under well lit conditions, colour vision
  6. Overview of the eye
    • Suspensory ligament
    • Ciliary body
    • Conjunctive
    • Iris
    • Pupil
    • Lens
    • Cornea
    • Aqueous humor
    • Vitreous humor
    • Extrinsic eye muscle
    • Choroid
    • Retina
    • Sclera
    • Fovea
    • Optic nerve
    • Optic disc
    • Blood vessels
  7. Muscles of the eye
    • Dorsal rectus (CN III)
    • Lateral rectus (CN VI -abducens)
    • Ventral rectus (CN III)
    • Medial rectus (CN III)
    • Dorsal oblique (CN IV - trochlear)
    • Ventral oblique (CN III)
  8. General Principles
    • Objects are represented inverted on the retina
    • Our brain 'turns' this image right side up
    • Eye structures (being liquid) being the light rays that previously had been in air
  9. Accommodation
    • Accomodation is process of bending (refracting) light rays to focus them on the retina
    • For viewing distant objects no accomodation is required
    • Accomodation is required for viewing close objects
    • Horse may not accomodate. Possibly has unique retinal structure.
  10. Accommodation nerves
    • Parasympathetic constricts pupils and promotes accommodation (near vision)
    • Sympathetic nervous system dilates pupils and flattens lens (far vision)
  11. Ten layers of retina
    • 1. Pigment epithelium
    • 2. Photosensitive layer (rods and cones)
    • 3. External limiting membrane
    • 4. External nuclear layer (cell bodies and cones)
    • 5. External plexiform layer
    • 6. Internal nuclear layer
    • 7. Internal plexiform layer
    • 8. Ganglion layer
    • 9. Nerve fiber layer
    • 10. Internal limiting membrane
  12. Pigment epithelium
    Nurture rods and cones. Ensures that photoreceptor discs are recycled
  13. Photosensitive layer
    Contains area of photoreceptive cells that responds to light
  14. External limiting membrane
    Delimits the radial astrocytes that support main classes of retinal neurons
  15. External nuclear layer
    Nuclei of photoreceptors
  16. External plexiform layer
    Contains synapses between photoreceptor cells & bipolar neurons & horizontal cells
  17. Internal nuclear layer
    Nuclei of bipolar cells
  18. Internal plexiform layer
    Synapses between bipolar neurons and ganglion neurons. Also amacrine cells
  19. Ganglion layer
    Cell bodies and nuclei of ganglion neurons
  20. Nerve fiber layer
    Axons from ganglion neurons
  21. Visual processing by retina is accomplished by 5 cell types
    • Amacrine cell
    • Horizontal cell
    • Ganglion cell
    • Bipolar cell
    • Photorecptor cells (rods and cones)
  22. Activation of rods is via rhodopsin
    • Light (photons) causes a conformational chagne in retinal (vitamin A derivative)
    • The conformational change leads to hyperpolarization of the rod cell by closure of
  23. Conversion
    • Much processing occurs at the retina itself
    • If it were one to one, we would need a much larger optic nerve (>11 times larger)
    • Total of 1 million ganglion cells
    •    Greater convergence at rod levels.
  24. Tapetum lucidium
    • Reflective layer of cells behind the retina
    • Reflecting light back into the photorecptors boosts ability to detect low light levels
    • Does reduce acuity
    • Other species such as ourselves have a dark absorbing band behind our retina
  25. Binocular versus Monocular vision
    • Binocular Vision:
    • Stereopsis (depth perception)
    • Binocular summation
    • Predator animals tend to have greater binocular vision since the eyes are placed forward
    • Prey animals on the other hand tend to have eyes placed on the side of their heads to provide maximum field of view and detect predators
  26. Vision is a percept
    Occipital and association cortices responsible for our perception of objects
  27. Central Processing
    • Action Potentials travel to the lateral geniculate along the optic nerve. Axons from the lateral geniculate from the optic radiations which synapse in the occipital cortex in area V1. More sophisticated processing occurs in downstream areas (V2, 3 ect)
    • Images from the left visual field are processed in the right occipital cortex, images from the right visual field are processed in the left occipital cortex
  28. Central Processing (Visual Cortex)
    • Simple cells in the visual cortex respond to lines placed in certain areas of the visual field at certain orientations
    • Complex cells respond to edges or lines in a particular orientation no matter where they are in the visual field
  29. Two visual streams from the visual cortex
    • One to the temporal lobe ("what" area)
    • One to the priotel cortex (where area)
  30. Menace Response
    • Component of neurological exam
    • Standard part of neurological exams. Not used is isolation. Use of other blink reflexes also required.
    • Tests integrity of cranial nerve pathways
    • Reflex blinking that occurs following a fast movement of an object
    • Learned behavior
    • Requires cortical circuits to be present. Also cereballar circuits
    • Cranial Nerves (II, Optic; VII, Facial)
  31. Dazzle Reflex
    • Strong light causes an immediate closing of the eyelids
    • Controlled from subcortical areas of the brain.
  32. Sympathetic Innervation of eye
    • Axon courses to the eye associated with the ophthalmic nerve of CNV, which enters the periorbita through the orbital fissure
    • Trigeminal ganglion
    • Axon courses through tympano-occipital fissure
    • Typmanic bulla
    • Cranial cervical ganglion
    • Vagosympathetic trunk
    • Ramus communicans
    • T1-T3 spinal nerve roots
    • Tectotegmental spinal pathway in lateral funiculus
    • First order sympathetic neurons begin in the hypothalamus
  33. Parasympathetic innvervation (constriction)
    Pupillary reflex
    • When light is shone into eye, the pupil constricts
    • This is called the direct pupillary light reflex. Light activates the photoreceptors and ganglion cells which project to the pretectal nucleus. They then synapse on the parasympathetic nucleus of the third cranial nerve. The axons then travel to the ciliary ganglion, which then innervates the constrictor smooth muscle of the iris. Axons also cross the midline at the pretectal nucleus. Thus when light is shone into one eye, the other eye's pupil also constricts.
    • The pupillary reflex tests the integrity of cranial nerve II and the occulomotor nerve (III), the retina and the iris.

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