II. Anatomy
- Retina
- Lines the globe inner surface and contains light sensitive Neurons that transmit signals to the Optic Nerve
- Photoreceptors (rods and cones) comprise the inner, sensory layer of the Retina
- Macula
- Retinal region responsible for central Vision
- Lies two disc diameters lateral to the optic disc
- Fovea
- Subpart of the Macula with highest Visual Acuity (and highest cone density)
- Images
Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)
Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)
Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)
III. Physiology: Neurons
- Background
- Light is refracted from Cornea and lens through pupil onto the Retina
- Photoreceptors (rods and cones) lie on the deepest portion of the Retina, adjacent to the Choroid
- Light passes through both Retinal plexiform layers to reach the photoreceptors
- Photoreceptor pigments are composed of cis-Retinal (from Vitamin A or Retinol) and an opsin Protein
- Rhodopsin (rods) and Iodopsin (cones) represent the two photoreceptors pigments
- When light strikes the photoreceptor's cis-Retinal molecule, the Retinal temporarily assumes its trans form
- Trans-Retinal in turn triggers a Neuronal impulse, and then Retinal returns to its cis form
- Light stimulates photoreceptors and generates electron flow
- Signals are then passed from the deepest Retina, back to the superficial Retina and out the Optic Nerve
- Signals are passed from the inner plexiform to outer plexiform layer via bipolar cells
- Ganglion cells (individual Neurons of the Optic Nerve) are triggered in the outer plexiform layer
- Lateral inhibition prevents excessive lateral signal spread
- Facilitated by horizontal cells, amacrine cells and interpexiform cells
- Step 1: Photoreceptor Cells
- Rods (black and white Vision)
- Rods are most concentrated on the periphery
- More sensitive than cones to dim light (or night Vision)
- Decreased Visual Acuity compared with cones
- Rod to bipolar cell ratio may approach 1:1000
- Cones (color Vision)
- Concentrated at center of Retina (fovea)
- High Visual Acuity compared with rods
- Cone to bipolar cell ratio approaches 1:1
- Rods (black and white Vision)
- Step 2: Outer Plexiform Layer (input from photoreceptors, adjacent to pigment epithelium and Choroid)
- Horizontal Cells
- Transmit signals horizontally (within the outer plexiform layer) between rods, cones and bipolar cells
- Bipolar Cells
- Transmits signals from photoreceptors and horizontal cells to inner plexiform layer
- Horizontal Cells
- Step 3: Inner Plexiform Layer (output to Optic Nerve, closer to vitreous)
- Amacrine Cells
- Transmits signals horizontally (within the inner plexiform layer) between bipolar and Ganglion cells
- Ganglion Cells
- Form individual Neurons of the Optic Nerve
- Amacrine Cells
- Inhibitory Cells
- Interplexiform Cells
- Transmit feedback inhibitory signals back from inner plexiform to outer plexiform layer
- Inhibit lateral signal spread
- Interplexiform Cells
IV. Physiology: Color Vision
- Retina have three types of color detecting cone photoreceptors that roughly correlate with red, green and blue Perception
- Photoreceptors are most sensitive to light at peak wavelengths along the light spectrum
- Long (Red): 564–580 nm
- Medium (Green): 534–545 nm
- Short (Blue): 420–440 nm
- Each of the cone types are stimulated by light of a wider range that overlaps with other cone types
- However, cone firing will be maximal when stimulated in their peak range
- Combination of the individual firing of these three cone types will be interpreted as specific color variations
- Disorders
- See Color Blindness
V. References
- Guyton and Hall (2006) Medical Physiology, Elsevier, Philadelphia, p. 626-39