Disc Shedding

Disc Shedding

The retina contains two types of photoreceptor – rod cells and cone cells. There are about 6-7 million cones that provide color vision to the eye, and they are very concentrated in a central spot in the retina, called the macula. However, the rods are much more numerous – they weigh in at about 120 million – and are also more sensitive than the cones. These rods are responsible for scotopic (night) vision, our most sensitive motion detection, and our peripheral vision. Vertebrate photoreceptors are composed of a photosensitive outer segment, an inner segment that contains the cell’s metabolic machinery (endoplasmic reticulum, Golgi complex, ribosomes, mitochondria), and a synaptic terminal at which contacts with second-order neurons of the retina are made. The photosensitive outer segment is connected to the inner segment by a modified, nonmotile cilium, and consists of a series of discrete membranous discs that are apparently derived from the plasma membrane in the region of the connecting cilium.