Several structures compose the human eye. Among the most important anatomical components are the cornea, conjunctiva, iris, crystalline lens, vitreous humor, retina, macula, optic nerve, and extraocular muscles. In this post, however I'll be looking at (no pun intended!) rods and cones - little things in the retina.
The orbit is surrounded by layers of soft, fatty tissue. These layers protect the eye and enable it to turn easily.
Traversing the fatty tissue are three pairs of extraocular muscles, which regulate the motion of each eye: the medial & lateral rectus muscles, the superior & inferior rectus muscles, and the superior & inferior oblique muscles.
Cones are concentrated in the fovea centralis. Rods are absent there but dense elsewhere. Measured density curves for the rods and cones on the retina show an enormous density of cones in the fovea centralis. To them is attributed both color vision and the highest visual acuity. Visual examination of small detail involves focusing light from that detail onto the fovea centralis. On the other hand, 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.
Current understanding is that the 6 to 7 million cones can be divided into "red" cones (64%), "green" cones (32%), and "blue" cones (2%) based on measured response curves. They provide the eye's color sensitivity. The green and red cones are concentrated in the fovea centralis . The "blue" cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye's blue perception.
The cones are less sensitive to light than the rods, as shown a typical day-night comparison. The daylight vision (cone vision) adapts much more rapidly to changing light levels, adjusting to a change like coming indoors out of sunlight in a few seconds. Like all neurons, the cones fire to produce an electrical impulse on the nerve fiber and then must reset to fire again. The light adaption is thought to occur by adjusting this reset time.
The cones are responsible for all high resolution vision. The eye moves continually to keep the light from the object of interest falling on the fovea centralis where the bulk of the cones reside.
The rods are more numerous of the photoreceptors, some 120 million, and are the more sensitive than the cones. However, they are not sensitive to color. They are responsible for our dark-adapted, or scotopic, vision. The rods are incredibly efficient photoreceptors. More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions. The optimum dark-adapted vision is obtained only after a considerable period of darkness, say 30 minutes or longer, because the rod adaption process is much slower than that of the cones.
The rod sensitivity is shifted toward shorter wavelengths compared to daylight vision, accounting for the growing apparent brightness of green leaves in twilight.
While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision. If you see a dim star in your peripheral vision, it may disappear when you look at it directly since you are then moving the image onto the cone-rich fovea region which is less light sensitive. You can detect motion better with your peripheral vision, since it is primarily rod vision.
The rods employ a sensitive photopigment called rhodopsin.
Photograph details: Nikon D40. Focal length 300 mm, ISO-400, exp: 1/60 sec. F-stop f/5.6
Continue reading»
