A photoreceptor is a specialized neuron in the retina that transduces light. There are two types of photoreceptors: cones and rods. They differ in morphology, sensitivity, and function.
A photoreceptor can be divided into 3 parts: outer segment, inner segment and synaptic terminal. The outer segment is formed by cytoplasmic space, Discs and plasma membrane. The inner segment contains mitochondria. In between inner segment and synaptic terminal, a nucleus is located. The names for Cones and Rods reflect the different shapes of their outer segments.
- Neurotransmitter: Glutamate
Sodium channels in the plasma membrane of the cell are opened by the presence of cGMP when there is no light. And this causes sodium ions move into the cell. When there is light, the photons enter the cell. And it causes pigment (rhodopsin molecule in rods) to activate transducin, which activates phosphodiesterase. This enzyme hydrolyzes cGMP to 5’-GMP. The reduction of cGMP causes sodium channels in the plasma membrane to close, and decreasing in the amount of sodium ions entering the cell causes a hyperpolarization. The hyperpolarization of the cell membrane causes less neurotransmitter to be released.
In humans, there are three different types of cone photoreceptors with different spectral sensitivities and one type of rod photoreceptor.
Rods respond to lower light intensities than cones.
Photoreceptors receive synaptic inhibition from horizontal cells.
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This link is broken and need to be fixed --Pkatz 13:23, 6 September 2008 (EDT)