Photoreceptors and their function

Photoreceptors

The light-sensitive cells of the retina are cells that generate nerve stimulation by absorbing a photon arriving at the retina. These cells are of two types: rods and cones. Cones are sensitive to light of different colours, or different wavelengths, different intensities and different colour saturation. They are the first neurons of the retina. They provide photopic vision and are responsible for visual acuity. They are most abundant in the central pit (fovea centralis), which is a small pit in the macula. Towards the periphery of the retina, their density gradually decreases. In total, we find 6 million cones on the retina. There are 3 types of cones, which can be distinguished only by the pigment in the cytoplasm, not by the shape of the cell.

Rod	Type	Sensitivity	Highest differential sensitivity
S (short)	β	400–500 nm	420–440 nm
M (medium)	γ	450–630 nm	534–555 nm
L (long)	ρ	500–700 nm	564–580 nm

Rods are light-sensitive cells that respond to lower light intensity than cones, but are unable to distinguish colours. They provide scotopic vision.

Structure of cones and rods

Structure of rods and cones

The cones are usually thicker compared to the rods, but in the central hole they are as long and wide as the rods. On a light-sensitive cell, we distinguish the outer part - it is the photosensitive part of the cell, the nucleus region and the synapse region. The outer part is divided into the inner process, the outer process and the transition zone. The photosensitive pigment rhodopsin (in rods), iodopsin in cones is located in the disc-shaped membrane structures of the outer process. These structures are bottle-shaped in cones and longer in rods. The inner protuberance serves as an energy reservoir for the vision process, which is why numerous mitochondria are found there, as well as ribosomes producing proteins for the vision process.

Function

The function of light-sensitive cells can be summarized to convert the light energy of incident light into the energy of atomic motion and to induce the release of neurotransmitter into the synaptic space between the light-sensitive cell and the bipolar neuron to which these cells are connected by a synapse. Channels transporting Na+ ions inside the cell and channels transporting K+ ions outside the cell. The opening of Na+ channels is dependent on cGMP (cyclic guanosine monophosphate). The K+ channels are open permanently and maintain a negative transmembrane potential (-40 mV) at the cell membrane. The light-sensitive rod pigment rhodopsin is composed of a protein part, opsin, and a non-protein part, 11-cis-retinal. In cones a similar light-sensitive pigment iodopsin is found. Different species of cones have slightly different opsin parts of the pigment, so they are sensitive to light of different wavelengths.

Stimulation of the photoreceptor after illumination time

Stimulation of the photoreceptor cell

1. When a light-sensitive cell is stimulated with a photon, the light-sensitive 11-cis-retinal undergoes isomerization to the trans form
2. The trans form of retinal is a longer molecule, therefore it does not sit at the fixation site on opsin and rhodopsin conformationally changes to metarhodopsin II. However, metarhodopsin is unstable, so retinal is released from opsin.
3. Opsin activates the regulatory protein transducin. Transducin is composed of three subunits alpha, beta, gamma and in the cytoplasm binds the GDP molecule to the alpha subunit.
4. Activation of transducin causes GTP to bind instead of GDP.
5. The transducin breaks down into its subunits.
6. The complex of the alpha subunit and GTP activates the enzyme phosphodiesterase, which converts cGMP to 5'GMP.
7. The decrease in cGMP concentration causes the closure of Na+ channels which causes hyperpolarization of the cell (membrane potential changes from -40 mV to -70 mV).
8. Hyperpolarization causes closure of voltage-gated Ca2+ ion channels.
9. The decrease in the concentration of Ca2+ in the cytoplasm stops the release of neurotransmitter into the synaptic space in the synapse region.

Restoration of rhodopsin

1. GAP (GTPase activating protein) interacts with the alpha subunit of transducin and causes hydrolysis of the bound GTP molecule to GDP. This results in a decrease in phosphodiesterase activity, therefore the transformation of cGMP to 5'GMP is slowed down.
2. As the Ca2+ concentration decreases, the enzyme guanylate cyclase is activated, which stimulates the transformation of GTP to cGMP. At higher cGMP concentrations, Na+ channels reopen and the transmembrane potential returns to normal (-40 mV).
3. The enzyme rhodopsin kinase and protein arrestin deactivate metarhodopsin II.
4. The trans form of retinal is transported into pigment cells where it is reduced to retinol. Retinol is transported back to the rods where it is converted to 11-cis-retinal capable of binding to opsin.
5. Opsin and 11-cis-retinal bind to each other to form rhodopsin.

The above describes how the sticks work. For suppositories, this method differs in the following:

• cones contain another pigment instead of rhodopsin - iodopsin;
• the neurotransmitter for communication with the bipolar cell in rods is glutamate, while in cones it is acetylcholine.