Rods and Cones

Rods and cones have different functions in vision, and the relative numbers of these two photoreceptors in the retina are partly correlated with whether an animal is most active during the day or at night.

Rods are more sensitive to light but do not distinguish colors; they enable us to see at night, but only in black and white. Because it takes more light to stimulate cones, these receptors contribute very little to night vision. Cones can distinguish colors in daylight. Color vision is found in all vertebrate classes, though not in all species. Most fishes, amphibians, reptiles, and birds have strong color vision, but humans and other primates are among the minority of mammals with this ability.

Owl Night Vision

The Tawny Owl can locate prey several metres away by the light of just one candle about 1700 feet away! Its retina has about 56,000 rods per mm². More on owl vision...

Most mammals are nocturnal, and a maximum number of rods in the retina is an adaptation that gives these animals keen night vision. Cats, usually most active at night, have limited color vision and probably see a pastel world during the day. In the human eye, rods are found in greatest density at the peripheral regions of the retina and are completely absent from the fovea, the center of the visual field (see FIGURE 49.9).

Structure of the vertebrate eye. In this longitudinal section of the eye, the jellylike vitreous humor is illustrated only in the lower half of the eyeball. The mucous membrane, or conjunctiva, surrounding the sclera (the white of the eye) is not shown. Campbell Fig 49-9.

You cannot see a dim star at night by looking at it directly; if you view it at an angle, however, focusing the starlight onto the regions of the retinas most populated by rods, you will be able to see the star. You achieve your sharpest daylight vision by looking straight at the object of interest because cones are most dense at the fovea, where there are about 150,000 color receptors per mm². Some birds have more than a million cones per mm², which enables such species as hawks to spot mice and other small prey from high in the sky. In the retina of the eye, as in all biological structures, variations represent evolutionary adaptations.

The effect of light on synapses between rod cells and bipolar cells. (a) In the dark, rhodopsin is inactive, and the rod cell membrane is highly permeable to sodium and thus depolarized. In this state, the rod cell releases glutamate and regulates the "firing" of two different classes of bipolar cells, which have opposite responses to glutamate. (b) In contrast, when light activates rhodopsin, the rod cell membrane becomes less permeable to sodium, and its membrane potential changes (it develops a receptor potential, a hyperpolarization in this case). The synaptic terminals of the rod cell then slow their release of glutamate, enhancing the activity of one class of bipolar cells and suppressing the activity of the other type. Campbell Fig 49-14.  

Photoreceptors in the vertebrate retina. (a) Photoreceptors called rod cells (rods) are very sensitive to light and function in black-and-white vision at night; cone cells (cones) account for color vision during the day. Both rods and cones are modified neurons. Visual pigments are embedded in folded membranes comprising a stack of discs in the outer segment of each rod and cone. (b) Rhodopsin, the visual pigment in the disc membrane of rods, consists of the light-absorbing molecule retinal bonded to a specific type of membrane protein, an opsin. The opsin has seven regions of alpha helix that span the disc membrane. Campbell Fig 49-11.  

Rods and cones. The retina is a thin tissue layer on the inner eye responsible for sight. Light strikes from the top. At top are nerve fibers which combine to form the optic nerve to the brain. Nerve fibers have round cell bodies with branching dendrites. Rods (green) are long nerve cells which respond to dim light, enabling images to be detected. Cones (pink) are shorter cone-like cells which detect color. Rods and cones pass visual signals through the optic nerve to the brain. Pigment cells block light from passing further.