The retina is the innermost layer of the eye. It consists of photoreceptor cells that convert light energy into nerve impulses. These electrical signals are passed via the optic nerve to the visual cortex, allowing us to visualise our surroundings.
In this article, we will focus on exploring the structure of photoreceptors and their function. We will also discuss the clinical consequences of retinal disease.
For the anatomy of the visual pathway, please see our sister article on TeachMeAnatomy.
The retina consists of layers, which can be subcategorised into retinal pigmented epithelium (RPE) and neural retina.
The RPE is a single layer of cuboidal epithelial cells and located in the outermost layer of the retina. It is responsible for the nourishment and support of the neural retina. The tight junctions between the RPE cells form part of the blood-retinal barrier, which helps to prevent molecules passing from the choroid into the retina. The RPE is also involved in a visual cycle as it regenerates photosensitive pigments.
The neural retina consists of multiple layers. The three main cells in the neural retina are (from the outermost to innermost):
- Photoreceptor cell
- Bipolar cell
- Retinal ganglion cell
The photoreceptors are involved in phototransduction, a process of converting light photons to an electrical impulse. The impulse is then relayed by the bipolar cell to the ganglion cell. The axons of the ganglion cells then form the nerve fibre layer of the retina, which exits the eye as an optic nerve.
Types of Photoreceptor
There are three main types of photoreceptors in the human eyes:
- Intrinsically photosensitive retinal ganglion cells
In this article, we will mainly focus on the rod and cones. The number of rods and cones in the human retina is around 120 million and 6 million cells, respectively.
Rods are much more sensitive to light than cones. They can signal the absorption of a single photon! Hence, they are mainly responsible for scotopic vision (in low-light levels). However, as the light levels increase their phototransduction cascades become saturated and are unable to reflect changes in light intensity.
Rods are found on the outside of the fovea and contribute to peripheral vision. Thus, patients with degenerative changes of rod cells, such as retinitis pigmentosa, may present with a symptom of night-blindness known as nyctalopia and peripheral vision loss.
In contrast, cones are concentrated in the fovea – the central part of this contains no rods. This is also the part of the retina with the highest acuity of vision. In contrast to rods, cones are much less sensitive to light. Hence, they are solely responsible for vision in the daylight.
The other main function of cones is colour vision. It is mediated by three different types of cones, which are sensitive to different ranges of light wavelengths.
|Type of cone||Alternative name||Ranges of wavelengths|
|Red||L-cone||Sensitive to long-wave light|
|Green||M-cone||Sensitive to medium-wave light|
|Blue||S-cone||Sensitive to short-wave light|
Red and green cones are much more abundant and concentrated mainly in the fovea, whilst blue cones are few and found outside the fovea. Although the different cones have a specific colour name, each cone is sensitive to a variety of colours/wavelengths. And they all act together to distinguish coloured visual input.
Structure of Photoreceptors
Structurally, photoreceptors are neuroepithelial cells that can absorb light and convert it into an electrical signal – phototransduction. Photoreceptors are tightly packed together, thus allowing a large number of photons to be absorbed across a small area of the retina.
The structure of photoreceptors is well-adapted to their function. Overall, rods and cones are structurally compartmentalised into five distinct regions:
- Outer segment – captures light and converts it to an electrical stimulus. It consists of numerous tightly stacked membrane discs that contain proteins required for phototransduction.
- Cilium – connects the outer and inner segments together.
- Inner segment – houses metabolic organelles such as lysosomes, mitochondria, and endoplasmic reticulum. It also provides the energy needed for phototransduction.
- Cell body – contains the nucleus of the cell.
- Synaptic region – allows the communication between the photoreceptor cell and the bipolar cell.
Rods vs. Cones
Rods and cones are distinguished by the shape and morphology of their outer segments.
|Outer segment||Cylindrical in shape and consists of membranous discs, which are separated from the plasma membrane.
The outer segment is generally thinner than in the cones.
|Conical/Tapered in shape and generally shorter when compared to the rods.
It consists of membranous discs, which are continuous with the plasma membrane.
|Inner segment||Contain long thin mitochondria||Contain long thin mitochondria|
|Synaptic regions||Known as spherules, which are generally smaller than cone terminals||Known as pedicles|
|Function||Scotopic vision (in low-light levels) and peripheral vision||Daytime vision, colour vision and visual acuity|
Clinical Relevance – Retinitis Pigmentosa
Retinitis Pigmentosa (RP) is a genetic disease that results in the degeneration of photoreceptors. It initially affects the rods in the mid-peripheries and later progresses towards the macula and fovea, affecting both cones and rods.
The symptoms of retinitis pigmentosa reflect its pathophysiology. Initially, patients experience night blindness and patchy loss of peripheral vision as peripheral rods are the first to be affected by this disease. Subsequently, colour perception and visual acuity deteriorate as cones slowly atrophy. At later stages of the disease, patients may experience ‘tunnel vision‘ – a loss of peripheral vision with the retention of the central visual field, leading to a constricted circular tunnel-like view.
While looking directly at the RP retina through the ophthalmoscope we can observe bone-spicule pigmentation, arteriolar attenuation and ‘waxy’ disc pallor.
There are several diagnostic tests available for RP. The most important being the electroretinogram (ERG), which measures the electrical activity of various cell types in the retina in response to light. Other useful investigations include dark adaptometry, electrooculogram and visual field test.
Treatment of Retinitis Pigmentosa involves:
- Supportive measures such as genetic counselling or helping patients with visual impairment registration.
- Pharmacological measures include carbonic anhydrase inhibitors (e.g. acetazolamide) which relives macular oedema.
- As patients with RP may have a posterior subcapsular cataract, cataract surgery can sometimes be beneficial to their vision.