Rod cells in the eye are photoreceptor cells that are located in the retina of the eye and have the ability to function with a lower light intensity than the other photoreceptor cells called cone cells. They are usually on the outer edges of the retina and they are mostly used in peripheral vision.

A characteristic of rod cells is that they are more sensitive than cone cells, and are the ones that allow us to see in the dark, or night vision. On the other hand, they have a small role when it comes to color vision, explaining why when we are in dark situations, colors are very dim or non-existent.

Structure of rod cells

While they have the same basic structure as cone cells, they differ in a few things, like their shape; rod cells are longer and leaner. However, their basic structure consists of opsin-containing disks that lie at the end of the cell that s next to retinal pigment epithelium which is also attached to the inside of the eye ball. The way the cells are stacked in the detector portion of the cell is what gives it its high efficiency.

In comparison, rod cells are much more abundant than cone cells, with approximately 100 million rod cells while cone cells amount to around 7 million.

Rod cells have a synaptic terminal, an inner segment and an outer segment. The inner and outer segment and joined by a cilium, that lines the distal segment.The inner segment also has organelles and the nucleus, while the outer segment is what houses the light-absorbing materials, and it is pointed towards the back of the eye. The synaptic terminals forms a synapse with another neuron, normally a bipolar or a horizontal cell.

In humans, the rod cells are approximately 2 microns in diameter and 100 microns long.

What do they do?

Their main function involves photoreception, and the way it happens is by hyperpolarization of the cell. When they’re not stimulated, they depolarize and spontaneously release a neurotransmitter. This neurotransmitter hiperpolarizes a bipolar cell, therefore the cell doesn’t release its transmitter so the synapse is not excited.

When the photopigments send a signal by hyperpolarizing the rod cell, it doesn’t send it neurotransmitter, so the bipolar cell then releases it transmitter and excites the synapse.

The photoreceptor cell changes shape when the photoreceptive pigment receives light. This pigment is known as rhodopsin and it made up of a protein called opsin and a prosthetic group called retinal.

A rod cell is highly sensitive and it can respond to a single photon of light, compared to a cone cell, it’s about 100 times more sensitive. Because they need much less light to function, they are the main source of visual information in low light or at night. They can also converge on a single interneuron, when multiple rod cells unite to collect and amplify the signals. However, when this occurs, visual acuity is compromised since the information being pooled from several cells is usually less distinct than when the information is received from each rod individually. They are also slower at sensing temporal changes, such as quickly changing images, compared to cone cells.