PHOTORECEPTORS; RETINAL GANGLION CELLS

 

The retina and the optic nerve form the proximal part of the visual pathway.

The visual pathway is made up of the retina, optic nerves (ON), optic chiasma, optic tracts, lateral geniculate nucleus (LGN), optic radiations and visual cortex.

Some other areas of the cortex are also associated with vision such as the frontal eye fields.

The retina is composed of three superimposed neurons that establish a connection with each other. The outer-most neuron is the photoreceptor. The second neuron, the bipolar cell, is in the nuclear layer. The third or internal neuron is the retinal ganglion cell (RGC).

The cell bodies (soma) of the RGCs are located in the ganglion cell layer (GCL), between the retinal nerve fibre layer (NFL) and the inner plexiform layer.

Their axons form the retinal NFL and synapse with neurons in the LGN of the thalamus.

There are up to seven layers of RGC cells in the central retina or fovea (60–80 μm thickness) and a few as one cell layer in the peripheral retina (10–20 μm).

There are between 500.000 and 1.2 million RGCs per retina and approximately 100 rods and 4–6 cones per RGC.

The axons form criss-crossed bundles which are separated and ensheathed by glial cells. The bundles leave the eye to form the optic nerve (ON). Upon existing through the lamina cribrosa, the axons become myelinated with oligodendrocytes.

The ganglion cell complex (GCC) represents the combination of three layers: the NFL, GCL and inner plexiform layer. These layers contain, respectively, the axons, the cell bodies and the dendrites of the ganglion cells.

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction.

The photoreceptors convert light (visible electromagnetic radiation) into signals that can stimulate biological processes.

There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells.

Rods primarily mediate scotopic vision (dim conditions) whereas cones primarily mediate photopic vision (bright conditions).

A third class of mammalian photoreceptor cell was discovered during the 1990s, known as the intrinsically photosensitive retinal ganglion cell. These cells are do not contribute to visual impulses directly, but have a role in circadian rhythm and pupillary reflex.

The photoreceptors contain certain proteins to enable phototransduction.

The membranous photoreceptor protein opsin contains a pigment molecule called retinal. In rod cells, these together are called rhodopsin.

In cone cells, there are different types of opsins that combine with retinal to form pigments called photopsins. Three different classes of photopsins in the cones react to different ranges of light frequency, a differentiation that allows the visual system to calculate color.

The function of the photoreceptor cell is to convert the light information of the photon into a form of information communicable to the nervous system and readily usable to the organism. This conversion is called signal transduction.

The opsin found in the intrinsically photosensitive ganglion cells of the retina is called melanopsin. These cells are involved in various reflexive responses of the brain and body to the presence of (day)light, such as the regulation of circadian rhythms, pupillary reflex and other non-visual responses to light. Melanopsin functionally resembles invertebrate opsins.

The distribution of rods and cones (and classes thereof) in the retina is called the retinal mosaic. Each human retina has approximately 6 million cones and 120 million rods.

The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal.