Rod cell
|
Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. Note that the wavelength scale is not represented linearly. |
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 are 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 100 million rod cells in the human retina.
A rod cell is sensitive enough to respond to a single
photon of light. Since rods require less light to function than cones, they are therefore the primary source of visual information at night. 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 individuals seeing something vague occur out of the corner of his or her eye.
Because they have 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 do cones, so stimuli they receive are added over about a hundred 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 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.
[ Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science, 4th ed., pp.507-513. McGraw-Hill, New York. ] 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.
[ Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science, 4th ed., pp.507-513. McGraw-Hill, New York. ]Activation of a photoreceptor cell is actually a
hyperpolarization; when they are 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 due to the fact that 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.
[ Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science, 4th ed., pp.507-513. McGraw-Hill, New York. ] 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.
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 |
| very light sensitive; sensitive to scattered light (have more pigment than cones) | at least 1/10th of the rods light sensitive; sensitive only to direct light |
| loss causes night blindness | loss constitue legal blindness |
| low visual acuity | high visual acuity; better spacial resolution |
| not present in fovea | concentrated in fovea |
| slow response to light, stimuli added over time | fast response to light, can perceive more rapid changes in stimuli |
| stacks of membrane-enclosed disks are unattached to cell membrane | disks are attached to outer membrane |
| 20 times more rods than cones in the retina |
| - | one type of photosensitive pigment (monochrom vision) | three types of photosensitive pigment in humans (color vision) |
| confer achromatic vision | confer color vision |
*
Sensory receptor
