How do photoreceptor cells initiate an action potential in response to light?

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A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells. The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form an image of the environment, sight. Rods primarily mediate scotopic vision (dim conditions) whereas cones primarily mediate to photopic vision (bright conditions), but the processes in each that supports phototransduction is similar. A third class of mammalian photoreceptor cell was discovered during the 1990s: the intrinsically photosensitive retinal ganglion cells. These cells are thought not to contribute to sight directly, but have a role in the entrainment of the circadian rhythm and pupillary reflex.

Photosensitivity[edit]

Normalized human photoreceptor absorbances for different wavelengths of light

Each photoreceptor absorbs light according to its spectral sensitivity (absorptance), which is determined by the photoreceptor proteins expressed in that cell. Humans have three classes of cones (L, M, S) that each differ in spectral sensitivity and 'prefer' photons of different wavelengths (see graph). For example, the peak wavelength of the S-cone's spectral sensitivity is approximately 420 nm (nanometers, a measure of wavelength), so it is more likely to absorb a photon at 420 nm than at any other wavelength. Light of a longer wavelength can also produce the same response from an S-cone, but it would have to be brighter to do so.

In accordance with the principle of univariance, a photoreceptor's output signal is proportional only to the number of photons absorbed. The photoreceptors can not measure the wavelength of light that it absorbs and therefore does not detect color on its own. Rather, it is the ratios of responses of the three types of cone cells that can estimate wavelength, and therefore enable color vision.

Histology[edit]

Anatomy of rods and cones varies slightly.

Rod and cone photoreceptors are common to almost all vertebrates. The pineal and parapineal glands are photoreceptive in non-mammalian vertebrates, but not in mammals. Birds have photoactive cerebrospinal fluid (CSF)-contacting neurons within the paraventricular organ that respond to light in the absence of input from the eyes or neurotransmitters. Invertebrate photoreceptors in organisms such as insects and molluscs are different in both their morphological organization and their underlying biochemical pathways. This article describes human photoreceptors.

Light enters the visual system through the eye and strikes the retina at the back of it. The retina is composed of specialized cells, the , which convert light energy into neural activity.

This conversion is made possible by light-sensitive pigments located on the discs in the outer segments of the rods and cones.When light strikes these pigments, they change form, causing a .

These reactions make the photoreceptors' membranes less permeable to certain ions, such as sodium. This change in permeability alters each photoreceptor's membrane potential and allows it to send a nerve signal to cells in the next layer of the retina.

How does a photoreceptor cell respond to light?

What happens in a photoreceptor cell in response to a light stimulus?