Everything about Rod Cell totally explained
Rod cells, or
rods, are
photoreceptor cells in the
retina of the
eye that can function in less intense
light than can the other type of photoreceptor,
cone cells. Since they're more light-sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in
peripheral vision. There are about 120 million rod cells in the human retina.
A rod cell is sensitive enough to respond to a single
photon 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're therefore the primary source of visual information at night (
scotopic vision). 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) since the pooled information from multiple cells is less distinct than it would be if the
visual system received information from each rod cell individually. The convergence of rod cells also tends to make peripheral vision very sensitive to movement, and is responsible for the phenomenon of an individual seeing something vague occur out of the corner of his or her eye.
Because they've only one type of light sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in
color vision.
Rod cells also respond more slowly to light than cones do, so stimuli they receive are added over about 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 However, if multiple flashes of sub-threshold light occurs during the 100 millisecond period, the energy of the flashes of light would summate to produce a light which will reach threshold and send a signal to the brain.
Experiments by
George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm (red). This fact is responsible for the
Purkinje effect, in which blue colors appear more intense relative to reds in darker light, when rods take over as the cells responsible for vision.
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a
synapse with another neuron, for example a
bipolar cell. The inner and outer segments are connected by a
cilium. The inner segment contains
organelles and the cell's
nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials.
Response to light
Activation of a photoreceptor cell is actually a
hyperpolarization; when they're not being stimulated, rods and
cones depolarize and release a
neurotransmitter spontaneously, and activation of photopigments by light sends a signal by preventing this. Depolarization 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, 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 (iodopsin is found in cone cells) consists of a large protein called opsin (situated in the plasma membrane), attached to which is 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 it to activate a regulatory protein called
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 (Kandel et al., 2000). Though cone cells primarily use the transmitter substance
acetyl choline, rod cells use a variety. The entire process by which light initiates a sensory response is called
visual phototransduction.
Activation of a single molecule 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 activate 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 vitamin A, 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.
Revert 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 then binds to the phosphorylated rhodopsins to further inhibit the rhodopsin's activity.
While arrestin shuts off rhodopsin, an RGS protein (functioning as a GTPase-activiating proteins(GAPs)) drives the transducin (G-protein) into an "off" state by increasing the rate of hydrolysis of the bounded GTP to GDP.
Also as the cGMP sensitive channels allow not only the influx of sodium ions, but also calcium ions, with the decrease in concentration of cGMP, cGMP sensitive channels are then closed and reducing the normal influx of calcium ions. The decrease in the concentration of calcium ions stimulates the calcium ion-sensitive proteins, which would then activiate the guanylyl cyclase to replenish the cGMP, rapidly restoring its original concentration. The restoration opens the cGMP sensitive channels and causes a depolarization of the plasma membrane.
Densitization
When the rods are exposed to a high concentration of photons for a prolonged period, they become densitized (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 arrestin. The bound arrestin can contribute to the densitization 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).
Table
Comparison of rod and
cone cells, from Kandel Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000).
Principles of Neural Science, 4th ed., pp.507-513. McGraw-Hill, New York.
| Rods |
Cones |
| Used for night vision |
Used for day vision |
| Highly sensitive to light; sensitive to scattered light (they have more pigment than cones) |
At least 1/10th of the rods' light sensitivity; sensitive only to direct light |
| Loss causes night blindness |
Loss constitutes legal blindness |
| Low spatial resolution with higher noise |
High spatial resolution with lower noise |
| Not present in the fovea |
Concentrated in the fovea |
| Slower response to light; rods need to be exposed to light over time |
Quicker response to light; can perceive more rapid changes in stimuli |
| Stacks of membrane-enclosed disks are unattached to the cell membrane |
Disks are attached to the outer membrane |
| 22 times as numerous as cones in the retina |
| One type of photosensitive pigment (monochromatic stimulus) |
Three types of photosensitive pigment in humans (trichromatic stimulus) |
| Confer achromatic vision, with more emphasis on detecting motion |
Confer colour vision, with more emphasis on detecting fine details |
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