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Retina: The Photoreceptor Layer

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The Retina is the photoprocessing, taking light energy and transforming it into electrical signals, layer of the eye. The receptor containing portion of the Retina is an extension of the central nervous system.
The Retina is made up of three layers:

  1. The outermost layer (farthest from incoming light), which contains the Rods and Cones
  2. The middle layer containing the Bipolar Neurons
  3. The inner layer (closest to incoming light), which contains the ganglion cells. The axons of the Ganglion Cells make up the optic nerve.

Light must pass through the first two layers before reaching the Rods and Cones. This is true except in the Fovea The Fovea is a Retinal depression located in the exact center of the Retina. Here the bipolar and the ganglion cell layers do not exists and light strikes the receptors directly. This area contains no Rods and contains the highest density of Cones, which allows for extreme acuity.

The Rods and Cones consist of three parts:

  1. The outer segment - contains the discs which detect the incoming light
  2. The inner segment - contains the cells nucleus, mitochondria, and the other organelles
  3. The synaptic terminal - contains the synapse which communicates with the next layer     of cells in the visual pathway, the Bipolar Neurons.

Click on the Rods and Cones in the above diagram for more information

Visual information going into the Pupil and passing through the Lens is projected upside down and backwards on the Retina. The inverted projection is due to the properties of Lens and the way light is bent while passing through it. Once light lands on the Retina it is absorbed by the Rods and Cones photoPigments located in the outer segment.

There can be as many as a billion photoPigment molecules located in a single receptor. It is these photoPigments that makeup the differences between each of the 3 types of Cones. The rod also contains a distinct photoPigment. Two proteins make up the photoPigments - retinene and an opsin. The retinene, a derivative of vitamin A, is common to all the receptor types, and the opsin is different in each.

Through a process known as phototransduction light energy is converted into electrical impulses that pass along the visual pathway. Impulses are passed onto bipolar neurons by hyperpolarization of the synaptic terminal, resulting in a reduction of transmitter. Transmitter release from the synaptic terminal is inversely proportional to the stimulus (light). The greater the light source, the greater the receptor is hyperpolarized which leads to a greater reduction in transmitter release. This means Photoreceptors, Rods and Cones, are inhibited by their adequate stimulus.

The Bipolar Neurons are actually inhibited by the Photoreceptors. Therefore a reduction in transmitter release from the receptor results in excitation of the bipolar neuron. The excitation of the bipolar neuron results in another graded potential, similar to the receptors, and this graded potential passes onto the Ganglion Cells. The Ganglion Cells propagate an action potential down its axons to the brain. An action potential is necessary at this point because the signal must be carried a longer distance.

The axons of the Ganglion Cells form the Optic Nerve.