Hobbies And Interests
Structure
Rods and cones share certain structural features but are quite different in appearance. Both have outer and inner segments, nuclei and synaptic bodies; however, the outer segment of a cone resembles a funnel, whereas the outer segment of a rod looks like a bar. The outer segment of a cone contains one of three different types of photopigments to absorb different wavelengths or colors of light; conversely, the outer segment of a rod contains only one type of photopigment, rhodopsin. When light hits the outer segment of the photoreceptor, an electrical message travels down its length to the synaptic body, which transmits the message to neurons.
Mechanism of Action
When light from an image enters the eye and meets the retina, the light particles strike the photopigments on the outer segments of the photoreceptors; these then translate the light into an electrical message that travels through the photoreceptor to its synaptic body. The synaptic body transfers the message to a neuron called a bipolar cell. Usually, a single cone connects directly to one bipolar cell, whereas several rods join to a single bipolar cell. The bipolar cell transfers the message to another neuron, a ganglion cell. The ganglion cell axons, which are stalk-like structures at the ends of these cells, converge to form the optic nerve. The nerve sends the message to a part of the brain called the thalamus and then to the occipital lobe at the back of the brain. These two brain centers consolidate the images and send them to the temporal lobe, where the brain translates the message into an identifiable picture.
Light Requirements
Cones function in high levels of light; rods, however, require only minimal light to respond. Rods function at night, whereas cones produce sharper images during the day. Each type of photoreceptor responds to a specific wavelength of light. Generally, rods react to a wider range of light than cones, which are more responsive to specific types of light. Because of this difference in sensitivity, rods do not produce color vision; however, different types of cones respond to specific hues, allowing the human eye to see color. Several rod cells converge onto a single neuron, increasing the amplitude of the signal in response to minute amounts of light. Cones do not produce the amplification effect because each cone basically corresponds to a single nerve cell and responds only to relatively large quantities of light.
Photoreceptor Responsivity
Rods respond only to the presence or absence of light; they do not relay information regarding color. Rods present relatively blurry, black-and-white images that may need up to an hour to adapt to low-light conditions. Cones come in three varieties, depending on the wavelength of light to which they respond. These cells allow you to view the world in different shades. The human eye reacts to light with wavelengths ranging from 380 to 760 nm. Blue cones respond best to wavelengths of 445 nm, green cones to 535 nm and red cones to 575 nm. The trichromatic theory states that the mixture of reactions in cones produces the range of colors you see.
Fovea and Blind Spot
The sharpest images produced by the eye occur in the fovea, an indentation in the middle portion of the retina. The fovea covers 15 degrees of your field of vision, which sees a total range of 200 degrees. Only images focused on the fovea appear clear. In dim light, the fovea, which consists mainly of cones, produces unfocused images. A blind spot -- the area of the retina where the nerve fibers come together to produce the optic nerve -- exists in each eye and compromises vision in one small area. This area of the retina has neither cones nor rods and produces no images.