Retinal Photoreceptor
Contents
Basic information

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.
Anatomy
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.
Molecular profile
- Neurotransmitter: Glutamate
Physiology
Phototransduction process
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.
Spectral sensitivity
In humans, there are three different types of cone photoreceptors with different spectral sensitivities and one type of rod photoreceptor.
Luminance sensitivity
Rods respond to lower light intensities than cones.
Synaptic Connections
Photoreceptors are presynaptic to retinal bipolar cells and horizontal cells. Horizontal cells and OFF bipolar cells are depolarized by glutamate and thus inhibited by light.
Photoreceptors receive synaptic inhibition from horizontal cells.
References
Rhoades, Rodney and Pflanzer, Richard, (2003) Human Physiology, 4th, 274-278
Jones, Bryan, June 2007, Webvision "the Organization of the Retina and Visual System" http://webvision.med.utah.edu/photo1.html
Gurney, Peter, Technical Journal 13(1):37–44 April 1999 http://www.answersingenesis.org/tj/v13/i1/retina.asp
F.J. Livesey and C.L. Cepko, Nature Reviews, Neuroscience, vol2, Feb. 2001, 109-118 http://genetics.med.harvard.edu/~cepko/pdf/downloads/Nature-Review%20Neuroscience/NatRevNe'01_Vertebrate%20neu.pdf
Molavi, Diana, Washington University School of Medicine, 1997 http://thalamus.wustl.edu/course/eyeret.html
C Brandon and D M Lam, L-glutamic acid: a neurotransmitter candidate for cone photoreceptors in human and rat retinas. Proc Natl Acad Sci U S A. 1983 August; 80(16): 5117–5121.
Additional information
Image Links
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