Rod cell

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 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 are 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 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 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 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. 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