Rods and cones
Both the photosensitive rods and cones form a regularly spaced mosaic of cells across the entirety of the retina – bar the absence of rods in the fovea centralis. Because there are 100 million rod receptors and 20 million cone receptors in each eye, rods are packed more densely per unit area. The synaptic connections of both rods and cones vary in function in different locations across the retina, reflecting the specialisations of different regions. This, for example, allows the eyes to deal with daylight and darkness and with what we see at the centre and periphery of our field of view.
Rods and cones are easily distinguished by their shape, from which they derive their names, the type of photo pigment they contain and by the distinct patterns of synaptic connections with the other neurons around them.
Neurons (nerve cells) are present throughout the human central and peripheral nervous systems and fall into three main categories: sensory, motor and interneurons. Rods and cones are both sensory neurons. Rods don’t produce as sharp an image as cone cells because they share more connections with other types of neurons. But a rod cell is believed to be sensitive enough to respond to a single photon of light whilst cone cells require tens to hundreds of photons to be activated.
The principal task of rod and cone cells alike is photo-transduction. This refers to the type of sensory transduction that takes place in the visual system. It is the process of photo-transduction that enables pigmented chemicals in the rods and cones to sense light and convert it into electrical signals. Many other types of sensory transduction occur elsewhere within the body enabling touch and hearing for example.
References: Functional Specialization of the Rod and Cone Systems: https://www.ncbi.nlm.nih.gov/books/NBK10850/