How the Retina Works
Much of the construction of an image takes place in the retina itself through the use of specialized neural circuits
Anatomy and Physiology
Understanding the anatomy of the primate retina is essential to understanding its function. Again, the photoreceptors lie in a layer against the back of the eyeball. In the second of three cell layers, called the inner nuclear layer, lie one to four types of horizontal cells, 11 types of bipolar cells and 22 to 30 types of amacrine cells. The numbers vary depending on species. The surface layer of the retina contains about 20 types of ganglion cells. Impulses from the ganglion cells travel to the brain via more than a million optic nerve fibers. The spaces separating these three layers are also anatomically distinct. The region containing synapses linking the photoreceptors with bipolar and horizontal cell dendrites is known as the outer plexiform layer; the area where the bipolar and amacrine cells connect to the ganglion cells is the inner plexiform layer.
Decades of anatomical studies have shed light on how the retina works. Imaging techniques ranging from old-style Golgi silver staining, first used over a century ago by Ramón y Cajal, to electron microscopy and modern-day antibody staining have revealed the shapes and sizes of the retina's cell types and how the different cells connect to form synapses. Staining techniques have revealed electrical junctions between cells and the identity and location of neurotransmitter receptors and transporters. We now know that the neurotransmitter (chemical signal) passed through the vertical pathways of the retina—from photoreceptors to bipolar cells to ganglion cells—is glutamate. The horizontal and amacrine cells send signals using various excitatory and inhibitory amino acids, catecholamines, peptides and nitric oxide.
Electrophysiological investigations of the retina started 60 years ago. Studies of the optic nerve fibers showed that they could be stimulated to give traditional depolarizing action potentials, like those observed in other neurons. However, the first recordings of impulses within the retina by Gunnar Svaetichin in the 1950s showed very odd responses to light. Neurons in the outer retina—it was not immediately clear which cells he was recording from—responded to stimulation not with depolarizing spikes but with slow hyperpolarization. These "S potentials" are now known to originate with the photoreceptors and to be transmitted to horizontal cells and bipolar cells. The membrane hyperpolarization starts on exposure to light, follows the time course of a light flash and then returns to the baseline value when the light is off. This reflects the counterintuitive fact that both rods and cones release neurotransmitters during the dark, when the membrane is depolarized and sodium ions flow freely across the photoreceptors' cell membranes. When exposed to light, ion channels in the cell membranes close. The cells go into a hyperpolarized state for as long as the light continues to shine on them and do not release a neurotransmitter.
Although both rods and cones respond to light with a slow hyperpolarizing response, they report quite different image properties. Rods, detecting dim light, usually respond to relatively slow changes. Cones, dealing with bright signals, can detect rapid light fluctuations. In both cases, photoreceptors begin the process of decomposing images into separate parts. Both rods and cones respond to light directly over them. Thus, their receptive fields are very narrow.
An image continues to be broken into component elements at the first synapses of the visual pathway, those between photoreceptors and bipolar cells. Different bipolar cells have different types of receptors for the neurotransmitter glutamate, allowing the cells to respond to photoreceptor input differently (Figure 7). Some bipolar cells are tuned to faster and some to slower fluctuations in the visual signal; some glutamate receptors resensitize rapidly and others more gradually. The cells thus fire either quickly in succession or relatively slowly in response to the same amount of stimulation. These receptors respond to glutamate by activating what's known as an OFF pathway in the visual process, detecting dark images against a lighter background. (Recall that photoreceptors constantly release glutamate unless exposed to light.) Other bipolar cells have inhibitory glutamate receptors; in other words, they prevent the bipolar cell from firing when the cell is exposed to the neurotransmitter. These receptors activate the ON pathway, detecting light images against a darker background.