neuro vision

• Neurophysiology of vision
• Detailed Objectives


  • Explain the mechanism by which the image is formed
  • Explain the photoreceptor mechanism
  • Illustrate the responses in the visual pathways and cortex
  • List and discuss the different aspects of visual function
• Stimulus


  Light (electromagnetic wave)
• Qualities of light distinguished by the eye

  • Brightness
  • Wavelength
• Wavelengths between 400 and 750 nanometers are called visible light (human eye)
• Photoreceptors


  Rods and cones - retina
• Structures of the eye

  • Outer layer (fibrous): cornea, corneal epithelium, conjunctiva, sclera , covers the eye except cornea
  • Middle layer (vascular): iris, choroid
  • Inner layer (neural): retina - rods and cones - photoreceptors
• Lens


  • Crystalline, situated behind the pupil, biconvex, transparent and elastic structure, avascular, receives nutrition mainly from the aqueous humor, refracts light rays and helps to focus the image of the objects on retina, focal length of human lens = 44 mm, refractory power is 23 D, supported by suspensory ligaments (zonular fibers) attached to ciliary bodies
• Iris


  Pigmented and opaque, absorbs light and reduces scatter, contains circular muscle fibers that constrict and radial fibers that dilate the pupil, adjusts the amount of light which enters the retina
• Aqueous humor


  Clear liquid, produced in the ciliary body, fills the anterior chamber of the eye, maintains the shape of the eyeball, the intraocular pressure, provides nutrients, oxygen and electrolytes to avascular structures such as lens and cornea and removes the metabolic end products from lens and cornea, normal pressure in eye is between 12-20 mm mercury, increase in pressure leads to glaucoma
• Vitreous humor


  Viscous, gelatinous fluid, fills the posterior chamber of the eye, helps maintain the shape of the eyeball
• Layers of the retina= neural tissue

  • Pigmented cell layer
  • Photoreceptor layer (rods and cones)
  • Outer nuclear layer
  • Outer plexiform layer
  • Inner nuclear layer
  • Inner plexiform layer
  • Ganglion cell layer
• Retina


  • Specialized sensory epithelium, organized in different layers, contains photoreceptors (rods and cones), interneurons (bipolar cells, horizontal cells, and amacrine cells), and ganglion cells
• Light rays must pass through the ganglion cell and bipolar cell layers to reach the rods and cones
• Pigment epithelium absorbs light rays, preventing the reflection of rays back through the retina
• Optic nerve leaves the eye at the level of optic disc, retinal blood vessels enter it at a point 3 mm medial to and slightly above the posterior pole of the globe
• There are no visual receptors over the optic disc, consequently this spot is blind (blind spot)
• Fovea centralis

  • Where light will focus, only cones, no rods, no blood vessels, region where visual acuity is greatest, affected by diabetes mellitus and hypertension (microaneurysm - tiny red spots)
• Visual pathway from the foveal portion of the retina

  • Cones
  • Bipolar cells
  • Ganglion cells
• Neural connections for the peripheral retina

  • Rods and cones
  • Many rods synapse on a single bipolar cell
• Microaneurysm


  Tiny red spots
• Layers of the retina

  • Foveal area only cones
  • Visual pathway from the foveal portion of the retina, representing the new, fast cone system
  • Neural connections for the peripheral retina, where both rods and cones are present
• Visual pathway from the foveal portion of the retina

  1. Cones

  2. Bipolar cells

  3. Ganglion cells
• Horizontal cells


  • Transmit inhibitory signals laterally in the outer plexiform layer
• Amacrine cells


  • Transmit signals laterally in the inner plexiform layer
• Visual acuity


  Degree to which the details and contours of objects are perceived, usually defined in terms of the shortest distance by which two lines can be separated and still be perceived as two lines
• Intensity discrimination


  Ability to perceive differences in light intensity
• Photoreceptors


  • At fovea: Mainly cones
  • Blind spot: No rods no cones
  • Extra foveal both rods and cones mainly rods
• Achromatic interval


  Interval from when an object is first seen and the time when that object is seen with color
• Duplicity theory


  Two inputs to the brain from rods and cones
• Snellen letter charts viewed at distance of 6m
• Scotopic (rods) vision


  Dim light vision
• Photopic (cones) vision


  Daylight vision
• Rods, cones, amacrine, horizontal bipolar only produce graded potential - local- short distance
• Ganglionic cells generate all-or-none action potentials transmitted over appreciable distances
• Hyperpolarizing responses


  Responses of the rods, cones, and horizontal cells
• Depolarizing responses


  Responses of the bipolar cells
• Depolarizing potentials and spikes

  Produced by amacrine cells, may act as generator potentials for the propagated spikes produced in the ganglion cells
• Rhodopsin


  Photosensitive pigment composed of opsin (a protein belonging to the superfamily of G protein–coupled receptors) and retinal (an aldehyde of vitamin A)
• Photoreception: transduction process in rods and cones that converts light energy into electrical energy


  Photoisomerization of retinal leads to transduction process