Student : Chen–Fung Tsen Advisor : Sheng-Lung Huang

大綱 Introduction Histology of human retina Structure of photoreceptor Details of rods & cones --Function --Response to light --Distribution

Introduction The only neurons that are directly sensitive to light are the photoreceptor cells, and is capable of phototransduction. Photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential. The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. A third class of photoreceptor cells, the photosensitive ganglion cells. These cells do not contribute to sight directly, but are thought to support circadian rhythms and pupillary reflex.

Histology of human retina

Structure of rods and cones Photoreceptor’s distinguishing feature is the presence of large amounts of tightly packed membrane that contains the photopigment rhodopsin. The tight packing is needed to achieve a high photopigment density, which allows a large proportion of the light photons that reach the photoreceptor to be absorbed.

Cones Responsible for color vision Function best in relatively bright light. Cone cells are densely packed in the fovea Require significantly brighter light (i.e., a larger numbers of photons) in order to produce a signal. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods. Because humans have three kinds of cones, which have different response curves and thus respond to variation in colour in different ways, we have trichromatic vision.

Rods A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than cones. They have only one type of light- sensitive pigment. Rods have little role in colored vision. Rod cells also respond more slowly to light than cones do, so stimuli they receive are added over about 100 milliseconds.

Response to light The photoreceptor proteins in the three types of cones differ in their sensitivity to photons of different wavelengths. Cones respond to both the wavelength and intensity of light. Cones cannot detect color by themselves; rather, color vision requires comparison of the signal across different cone types. Purkinje effect: as intensity dims at twilight, the rods take over, peak sensitivity of vision shifts towards the rods' peak sensitivity (blue- green). Wavelength responsiveness of rods compared to that of three types of cones. The dashed gray curve is for rods.

Distribution The human retina has approximately 5 million cones and 120 million rods. Fovea : both color vision and the highest visual acuity. The rods are absent from the fovea. At a few degrees away from it their density rises to a high value and spreads over a large area of the retina. These rods are responsible for night vision, our most sensitive motion detection, and our peripheral vision. No photoreceptors are found at the blind spot, the area where ganglion cell fibers are collected into the optic nerve and leave the eye

Comparison of rods & cones

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