Rod cells are photoreceptor cells in the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

of the eye that can function in lower

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in

peripheral vision Peripheral vision, or ''indirect vision'', is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in (or out of) the "corner of one's eye". The vast majority of the area in th ...

. On average, there are approximately 92 million rod cells (vs ~6 million cones) in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.

Structure

Rods are a little longer and leaner than cones but have the same basic structure.

Opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...

-containing disks lie at the end of the cell adjacent to the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ce ...

, which in turn is attached to the inside of the eye. The stacked-disc structure of the detector portion of the cell allows for very high efficiency. Rods are much more common than cones, with about 120 million rod cells compared to 6 to 7 million cone cells. Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, usually a

bipolar cell A bipolar neuron, or bipolar cell, is a type of neuron that has two extensions (one axon and one dendrite). Many bipolar cells are specialized sensory neurons for the transmission of sense. As such, they are part of the sensory pathways for smel ...

or a horizontal cell. The inner and outer segments are connected by a

cilium The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike project ...

, which lines the distal segment. The inner segment contains organelles and the cell's nucleus, while the rod outer segment (abbreviated to ROS), which is pointed toward the back of the eye, contains the light-absorbing materials. A human rod cell is about 2 microns in diameter and 100 microns long. Rods are not all morphologically the same; in mice, rods close to the outer plexiform synaptic layer display a reduced length due to a shortened synaptic terminal.

Function

Photoreception

In vertebrates, activation of a photoreceptor cell is a hyperpolarization (inhibition) of the cell. When they are not being stimulated, such as in the dark, rod cells and cone cells depolarize and release a neurotransmitter spontaneously. This neurotransmitter hyperpolarizes the

. Bipolar cells exist between photoreceptors and ganglion cells and act to transmit signals from the photoreceptors to the ganglion cells. As a result of the bipolar cell being hyperpolarized, it does not release its transmitter at the bipolar-ganglion synapse and the synapse is not excited. Activation of photopigments by light sends a signal by hyperpolarizing the rod cell, leading to the rod cell not sending its neurotransmitter, which leads to the bipolar cell then releasing its transmitter at the bipolar-ganglion synapse and exciting the synapse. Depolarization of rod cells (causing release of their neurotransmitter) occurs because in the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely sodium channels, though calcium can enter through these channels as well). The positive charges of the ions that enter the cell down its electrochemical gradient change the cell's membrane potential, cause

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

, and lead to the release of the neurotransmitter glutamate. Glutamate can depolarize some neurons and hyperpolarize others, allowing photoreceptors to interact in an antagonistic manner. When light hits photoreceptive pigments within the photoreceptor cell, the pigment changes shape. The pigment, called rhodopsin (conopsin is found in cone cells) comprises a large protein called

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...

(situated in the plasma membrane), attached to which is a covalently bound prosthetic group: an organic molecule called

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use reti ...

(a derivative of

vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably ...

). 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 an increased affinity for the regulatory protein called transducin (a type of G protein). Upon binding to rhodopsin, the alpha subunit of the G protein replaces a molecule of GDP with a molecule of GTP and becomes activated. This replacement causes the alpha subunit of the G protein to dissociate from the beta and gamma subunits of the G protein. As a result, the alpha subunit is now free to bind to the cGMP phosphodiesterase (an effector protein). The alpha subunit interacts with the inhibitory PDE gamma subunits and prevents them from blocking catalytic sites on the alpha and beta subunits of PDE, leading to the activation of cGMP phosphodiesterase, which hydrolyzes cGMP (the second messenger), breaking it 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 the neurotransmitter glutamate. Though cone cells primarily use the neurotransmitter substance acetylcholine, rod cells use a variety. The entire process by which light initiates a sensory response is called visual phototransduction. Activation of a single unit of rhodopsin, the photosensitive pigment in rods, can lead to a large reaction in the cell because the signal is amplified. Once activated, rhodopsin can activate hundreds of transducin molecules, each of which in turn activates a phosphodiesterase molecule, which can break down over a thousand cGMP molecules per second. Thus, rods can have a large response to a small amount of light. As the retinal component of rhodopsin is derived from

, a deficiency of vitamin A causes a deficit in the pigment needed by rod cells. Consequently, fewer rod cells are able to sufficiently respond in darker conditions, and as the cone cells are poorly adapted for sight in the dark, blindness can result. This is night-blindness.

Reversion to the resting state

Rods make use of three inhibitory mechanisms (negative feedback mechanisms) to allow a rapid revert to the resting state after a flash of light. Firstly, there exists a rhodopsin kinase (RK) which would phosphorylate the cytosolic tail of the activated rhodopsin on the multiple serines, partially inhibiting the activation of transducin. Also, an inhibitory protein -

arrestin Arrestins (abbreviated Arr) are a small family of proteins important for regulating signal transduction at G protein-coupled receptors. Arrestins were first discovered as a part of a conserved two-step mechanism for regulating the activity of ...

then binds to the phosphorylated rhodopsins to further inhibit the rhodopsin activity. While arrestin shuts off rhodopsin, an RGS protein (functioning as a

GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are ...

s(GAPs)) drives the transducin (G-protein) into an "off" state by increasing the rate of hydrolysis of the bounded GTP to GDP. When the cGMP concentration falls, the previously open cGMP sensitive channels close, leading to a reduction in the influx of calcium ions. The associated decrease in the concentration of calcium ions stimulates the calcium ion-sensitive proteins, which then activate the guanylyl cyclase to replenish the cGMP, rapidly restoring it to its original concentration. This opens the cGMP sensitive channels and causes a depolarization of the plasma membrane.Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter (2008). ''Molecular Biology of The Cell'', 5th ed., pp.919-921. Garland Science.

Desensitization

When the rods are exposed to a high concentration of photons for a prolonged period, they become desensitized (adapted) to the environment. As rhodopsin is phosphorylated by rhodopsin kinase (a member of the GPCR kinases(GRKs)), it binds with high affinity to the

. The bound arrestin can contribute to the desensitization process in at least two ways. First, it prevents the interaction between the G protein and the activated receptor. Second, it serves as an adaptor protein to aid the receptor to the clathrin-dependent endocytosis machinery (to induce receptor-mediated endocytosis).

Sensitivity

A rod cell is sensitive enough to respond to a single

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

of light and is about 100 times more sensitive to a single photon than cones. Since rods require less light to function than cones, they are the primary source of visual information at night (

scotopic vision In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...

). Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, collecting and amplifying the signals. However, this convergence comes at a cost to visual acuity (or

image resolution Image resolution is the detail an image holds. The term applies to digital images, film images, and other types of images. "Higher resolution" means more image detail. Image resolution can be measured in various ways. Resolution quantifies how ...

) because the pooled information from multiple cells is less distinct than it would be if the

visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...

received information from each rod cell individually. Cone-response-en

Rod cells also respond more slowly to light than cones and the stimuli they receive are added over roughly 100 milliseconds. While this makes rods more sensitive to smaller amounts of light, it also means that their ability to sense temporal changes, such as quickly changing images, is less accurate than that of cones.Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). ''Principles of Neural Science'', 4th ed., pp. 507–513. McGraw-Hill, New York. Experiments by George Wald and others showed that rods are most sensitive to wavelengths of light around 498 nm (green-blue), and insensitive to wavelengths longer than about 640 nm (red). This is responsible for the

Purkinje effect The Purkinje effect (; sometimes called the Purkinje shift, often incorrectly pronounced ) is the tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of da ...

: as intensity dims at twilight, the rods take over, and before color disappears completely, peak sensitivity of vision shifts towards the rods' peak sensitivity (blue-green).

References

External links

NIF Search - Rod Cell
via the

Neuroscience Information Framework The Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources and provides many auth ...

{{DEFAULTSORT:Rod Cell Human eye anatomy Histology Photoreceptor cells Human cells Neurons