Intro | Amacrine Cells | Back of the Eyeball | Bipolar Cells | Blind Spot & Optic Nerve | Cone Receptors | Retinal Ganglion Cells | Horizontal Cells | Rod Receptors
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Rod Receptors are named for their rod-like shape. These receptor cells are specialized for night and peripheral vision, and outnumber the cones by about 125 million to 6.4 million. Rods are particularly sensitive to dim light, hence their role in night vision. No rods are found in the fovea. Although rods are found in abundance throughout the retina peripheral to the fovea, the density of rods is greatest surrounding the fovea (20 degrees from the fovea) and gradually decreases toward the periphery of the retina where they reach their lowest density at approximately 80 degrees from the fovea. When lighting is dim, it is best to avert your gaze slightly away from the object of interest. This will bring the object into focus at the portion of the retina where rods are plentiful.
Opposite to most neurons, when a rod is activated by light it is in a hyperpolarized state (as opposed to depolarized state). While at rest, rod cells transmit a steady inhibitory input to the bipolar cells. When transduction occurs, the rate of this inhibition is slowed. The resulting depolarization of bipolar cells causes them to release more neurotransmitter molecules and the ganglion cells are excited.
The transduction process in rods involves the conversion of photopigments. Rods contain a photopigment, 11-cis-retinal, which is a form of vitamin A bound to a protein called rhodopsin. One photon of light has the energy to convert this photopigment into a molecule of all-trans-retinal. This conversion alters the configuration of the opsin portion of the molecule, resulting in a closing of Na+ and Ca+ channels on the cell membrane and hyperpolarization. When a photon strikes a photopigment, a G protein called transducin is activated. The activated G protein in turn activates phosphodiesterase molecules, which destroy cyclic GMP and close the Na+ and Ca+ channels in the photoreceptor. When excited, rods release the excitatory neurotransmitter glutamate.