Sensitivity and visual acuity

The mammalian eye is a complex organ that tranduces light into patterns of nerve impulses. The transduction takes place in the retina, a layer of photosensitive cells at the back of the eye. It contains two types of receptors called rods and cones
Rods and cones are classified as secondary recpetors - they are more complex than primary receptors like pacinian corpuscle. A primary receptor consists of a single neurone. In secondary receptors, a modified epithetial cells detect changes in the stimulus and passes this information onto a seperate neurone which transmits the information as nerve impulses
Rods and cones are part of a sense organ, the eye. Sense organs are complex stimulus-gathering structures consisting of group sensory receptors
Up to 150 rods may connect via synapses to one neurone, a characteristic known as convergence. This enables a group of rods to function as a photosensitive unit, gathering light from a relatively wide area and combining its stimulatory effects. Convergence enables rods to provide photosensory information at low light intensities, but this increased sensitivity is at the expence of visual acuity (the ability to discriminate fine detail)
Consequently, in dim light when cones are not functioning, the rods provide only enough information for an ill-defined informationthat lacks colour. Also because rods are the only type of photoreceptor at the outmost edge of the retina, something 'out the corner of your eye' lacks detail
A rod cell has in its outer segement up to 1000 vesicles, each containing a photosenitive pigment called rhodopsin. Rhodopsin is made up of the protein opsin and retinal, a derivative of vitamin A. Light causes retinal to change shape, which results in retinal and opsin breaking apart.; a process called bleaching. This triggers a series of events which alters the permeability of the rods plasma membrane and contributes to the fromation of a generator potential in the sensory neurone. The generator potential results from the combined effects of all rods serving a particuar sensory neurone. When it exceeds a critical threshold level, a nerve impulse is transmitted.
Cones enable us to see colour and fine details. There are three types of cone, each containing a different form of thr pigment iodopsin which breaks down only bright light. The three pigments together are sensitive over the visible spectrum, but on is most sensitive to blue light, one to green light, and the other to red light. According to the trichromatic theory, different colours are percieved by mixing the information from the different types of cone.
Whereas rods function in groups connected to one sensory neurone, a single cone may have its own sesonry neurone. if light of suffieciently high intensity from two seperate sources falls on two such cone cells, two seperate impulses are transmitted along two sensory neurones and the brain recieves two seperate images. Thus cones provide much more detaild sensory information, but only at high light intensities
The fovea and blind spot. Over most of the retina, rods and cones are buried under a layer of blood vessels and nerve fibres which lead into the optic nerve. However, the fovea, a small depression in the retine opposite the lens, consists only of cones. The layer of nerve fibres here is thin and there are no cappilaries. This means that light falling on the fovea produces a clear, well defined visual image in colour. When a person wants to examine the fine details of an object, they eyes move automatically so that the light from the object falls on the fovea.
The eyes adapt to different levels of brightness by varying theor sesnitivity. A prelonged period in a darkened room results in photosensitive pigments being formed much faster than they are being broken down, thus increasing the sesitivity to light. This process is called dark adaptation. A prelonged period in bight light results in photosenstive pigments in rods and cones being broken down, reducing sensitivity to light. This is called light adapatiation

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Sensitivity and visual acuity

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