rod
rod In the retina of the vertebrate eye, the more common of the two types of light receptor (compare CONE), which is sensitive to very low levels of stimulation. Many rods (in humans about 150) are connected by bipolar cells to a single ganglion cell whose axon is a nerve fibre in the optic tract; stimuli to the rods are summated (i.e. added together) to produce a total stimulus that must exceed a threshold before an impulse is discharged by the ganglion cell. Because the contribution of many individual rods is summated to produce a single impulse the resulting perceived image is not clearly defined. There are estimated to be about 65 × 106 rods in a single human retina and the eyes of some animals (e.g. opossums) have retinas containing only rods and no cones.
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rod
