Question

describe the effect of light in pigments inside rods, how light affects the polarization of photoreceptors, and how that affects the bipolar cells and ganglionic cells

Answer 1

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 a representation of the visual world, sight. The rods are narrower than the cones and distributed differently across the retina, but the chemical process 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 do not contribute to sight directly, but are thought to support circadian rhythms and pupillary reflex.

There are major functional differences between the rods and cones. Rods are extremely sensitive, and can be triggered by a single photon.At very low light levels, visual experience is based solely on the rod signal.

Cones require significantly brighter light (that is, a larger number of photons) to produce a signal. In humans, there are three different types of cone cell, distinguished by their pattern of response to light of different wavelengths. Color experience is calculated from these three distinct signals, perhaps via an opponent process.[5] This explains why colors cannot be seen at low light levels, when only the rod and not the cone photoreceptor cells are active. The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths, so they may respectively be referred to as S-cones, M-cones, and L-cones.

In accordance with the principle of univariance, the firing of the cell depends upon only the number of photons absorbed. The different responses of the three types of cone cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths. So, for example, an L cone cell contains a photoreceptor protein that more readily absorbs long wavelengths of light (that is, more "red"). Light of a shorter wavelength can also produce the same response from an L cone cell, but it must be much brighter to do so.

The human retina contains about 120 million rod cells, and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Certain owls, such as the nocturnal tawny owl,[6] have a tremendous number of rods in their retinae. In the human visual system, in addition to the photosensitive rods & cones, there are about 2.4 million to 3 million ganglion cells, with 1 to 2% of them being photosensitive. The axons of ganglion cells form the two optic nerves.

Photoreceptor cells are typically arranged in an irregular but approximately hexagonal grid, known as the retinal mosaic.

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.

The process of phototransduction occurs in the retina. The retina has many layers of various cell types. The most numerous photoreceptor cells (rods and cones) form the outermost layer. These are the photoreceptors responsible for mediating the sense sight. The middle retinal layer contains bipolar cells, collect signals from photoreceptors and transmit them to the retinal ganglion cells of the innermost retinal layer. Retinal ganglion cell axons collectively form the optic nerve, via which they project to the brain.

Unlike most sensory receptor cells, photoreceptors actually become hyperpolarized when stimulated; and conversely are depolarized when not stimulated. This means that glutamate is released continuously when the cell is unstimulated, and stimulus causes release to stop. In the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens cGMP-gated ion channels. These channels are nonspecific, allowing movement of both sodium and calcium ions when open. The movement of these positively charged ions into the cell (driven by their respective electrochemical gradient) depolarizes the membrane, and leads to the release of the neurotransmitter glutamate.

When light hits a photoreceptive pigment within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the plasma membrane), attached to a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes opsin (a G protein-coupled receptor) to activate its G protein transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters.The entire process by which light initiates a sensory response is called visual phototransduction.

Dark currentEdit

Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because cyclic GMP (cGMP) is bound to them. Hence, positively charged ions (namely sodium ions) enter the photoreceptor, depolarizing it to about −40 mV (resting potential in other nerve cells is usually −65 mV). This depolarization current is often known as dark current.

The signal transduction pathway is the mechanism by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve. The steps, or signal transduction pathway, in the vertebrate eye's rod and cone photoreceptors are then:

The rhodopsin or iodopsin in the disc membrane of the outer segment absorbs a photon, changing the configuration of a retinal Schiff base cofactor inside the protein from the cis-form to the trans-form, causing the retinal to change shape.

This results in a series of unstable intermediates, the last of which binds stronger to a G protein in the membrane, called transducin, and activates it. This is the first amplification step – each photoactivated rhodopsin triggers activation of about 100 transducins.

Each transducin then activates the enzyme cGMP-specific phosphodiesterase (PDE).

PDE then catalyzes the hydrolysis of cGMP to 5' GMP. This is the second amplification step, where a single PDE hydrolyses about 1000 cGMP molecules.

The net concentration of intracellular cGMP is reduced (due to its conversion to 5' GMP via PDE), resulting in the closure of cyclic nucleotide-gated Na+ ion channels located in the photoreceptor outer segment membrane.

As a result, sodium ions can no longer enter the cell, and the photoreceptor outer segment membrane becomes hyperpolarized, due to the charge inside the membrane becoming more negative.

This change in the cell's membrane potential causes voltage-gated calcium channels to close. This leads to a decrease in the influx of calcium ions into the cell and thus the intracellular calcium ion concentration falls.

A decrease in the intracellular calcium concentration means that less glutamate is released via calcium-induced exocytosis to the bipolar cell (see below). (The decreased calcium level slows the release of the neurotransmitter glutamate, which excites the postsynaptic bipolar cells and horizontal cells.)

Reduction in the release of glutamate means one population of bipolar cells will be depolarized and a separate population of bipolar cells will be hyperpolarized, depending on the nature of receptors (ionotropic or metabotropic) in the postsynaptic terminal (see receptive field).

Thus, a rod or cone photoreceptor actually releases less neurotransmitter when stimulated by light. Less neurotransmitter in the synaptic cleft between a photoreceptor and bipolar cell will serve to either excite (depolarize) ON bipolar cells or inhibit (hyperpolarize) OFF bipolar cells. Thus, it is at the photoreceptor-bipolar cell synapse where visual signals are split into ON and OFF pathways.

ATP provided by the inner segment powers the sodium-potassium pump. This pump is necessary to reset the initial state of the outer segment by taking the sodium ions that are entering the cell and pumping them back out.

Although photoreceptors are neurons, they do not conduct action potentials with the exception of the photosensitive ganglion cell – which are involved mainly in the regulation of circadian rhythms, melatonin, and pupil dilation.