Once light passes through the cornea, is inverted by the lens, and travels through the eyeball, it hits the retina, the innermost layer of eye that contains neurons. While it would be convenient to be able to funnel light directly to the brain, the brain cannot decode pure light. Instead, the visual information has to be encoded as an electrical signal (a process called phototransduction) that the brain can understand. This is the primary function of the retina.
Within the retina, there are five basic types of neurons: photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells. They are arranged in layers; the bipolar cells are at the front, and the photoreceptors are at the back.
The photoreceptors, bipolar cells, and ganglion cells are connected to each other in a kind of hierarchical chain. The horizontal and amacrine cells connect the other neurons to each other, allowing for a kind of communication of comparison and contrast. Horizontal cells facilitate lateral communication between photoreceptors and bipolar cells, and amacrine cells exist between bipolar and ganglion cells.
The Role of Photoreceptors as the First Step of Phototransduction
The process of phototransduction begins with the photoreceptors, which are at the back of the retina. Because light has to pass the other neurons before arriving at the photoreceptors, it may seem counterintuitive that this is where the process begins. Why not put photoreceptors closer to the light source? The answer lies in the pigment epithelium, the thin wall behind the neurons. This wall keeps the receptors functioning by removing dead components of the cells, providing fresh chemicals, and supplying nourishment via blood from capillaries.
There are two types of photoreceptors: rods and cones. They differ in their size and shape but demonstrate a similar structure. These cells are primarily concerned with detecting light, but once they detect light, they have to pass that information to the other neurons.
How Rods and Cones Tell the Brain it is Dark
Most neurons in the brain are silent until another neuron comes along and “tells” them to start firing. In the retina, it is just the opposite. Photoreceptors generate electrical signals by default, which means that in the dark, they are sending a signal. The molecule cGMP keeps ion channels open, allowing sodium and calcium ions to enter the cell, producing the electric signal.
This signal leads the photoreceptor to release the neurotransmitter glutamate. In all other parts of the body, when a neuron releases glutamate, it causes the next neuron in the chain to carry an electrical signal too. In the eye, however, the release of glutamate keeps the next cell (a bipolar cell) from firing; it has an inhibitory effect. Because the bipolar cell does not fire, the message stops there, and the brain does not receive any information.
How Rods and Cones Tell the Brain There is Light
Inside every photoreceptor are a slew of chemicals and proteins, and the presence of light sets these substances into a sequence of specific events.
The first three substances important to detecting light are retinal, opsins, and transducin. Retinal is a molecule (and a form of vitamin A) that sits in the pocket of an opsin. An opsin (e.g. rhodopsin in rods) is a protein tuned to a particular region in the visual light spectrum. When a bit of light contacts the photoreceptor, the molecule retinal changes shape, which makes the opsin protein change shape. This change causes the activation of tranducin, a protein messenger.
Once transducin is activated, it activates the enzyme phosphodiesterase, a substance that in essence chews up the cGMP that was keeping those ion channels open. When cGMP starts to disappear, the ion channels close, and the cell stops releasing glutamate.
With no glutamate in the synapse, there is nothing to keep the bipolar cell from firing, so it generates an electrical signal that gets passed on to the ganglion cells, which take the message up to the brain.
Understanding Phototransduction Through Analogy
This whole process seems complicated, and it is, but the very basic concept can be likened to a car. The neurotransmitter glutamate is like the brakes on a car. When the driver holds a foot to the brakes, the car does not move, but when the foot is lifted off the brake, the car moves. In the retina, the photoreceptors are always releasing glutamate—i.e. keeping their foot on the brakes. When light strikes the cell, however, they stop releasing the neurotransmitter, or take their foot off the brakes, allowing the message to pass on to the next neuron to fire, or allowing the car to move.
For more information on what the brain does with the signals generated by the retina, please see "How Vision Works in the Brain."
References
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W., LaMantia, A., McNamara, J. O., & White, L. E. (Eds.). (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.
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