Retinal Photoreceptor

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The layers and cell types in the retina and the connections (Ramón y Cajal, Santiago, 1899. Instituto Santiago Ramón y Cajal, Madrid, Spain.)

Basic information

Dubuc, Bruno, Canadian Institutes of Health Research: Institute of Neurosciences, Mental Health and Addiction 2002,

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.

Diagram of a rod photoreceptor. (Modified from Young R.W., The Bowman Lecture: Biological renewal: Applications to the eye, Trans. Ophthalmol. Soc. UK 102:60, 1982, by permission of Royal College of Ophthalmologists, London.)
EM image of rods and cone receptors, Anderson, D.H. and Fisher, S.K. (1976) The photoreceptors of diurnal squirrels: outer segment structure, disc shedding, and protein renewal. J. Ultrastruct. Res. 55, 119-141.

Molecular profile

  • Neurotransmitter: Glutamate


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.


Rhoades, Rodney and Pflanzer, Richard, (2003) Human Physiology, 4th, 274-278

Jones, Bryan, June 2007, Webvision "the Organization of the Retina and Visual System"

Gurney, Peter, Technical Journal 13(1):37–44 April 1999

F.J. Livesey and C.L. Cepko, Nature Reviews, Neuroscience, vol2, Feb. 2001, 109-118'01_Vertebrate%20neu.pdf

Molavi, Diana, Washington University School of Medicine, 1997

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

This link is broken and need to be fixed --Pkatz 13:23, 6 September 2008 (EDT)