Generality
The retina is a tissue of nervous origin, which covers almost the entire inner wall of the eye. This delicate structure contains the photoreceptors, which are two types of cells sensitive to light waves: the rods are involved in monochromatic vision in conditions of soft or crepuscular light; the cones are instead responsible for color vision, but they are active only when the light is intense (day vision). The retina works, therefore, like a phototransducer, ie it captures light stimuli and converts them into bioelectric signals, which in turn are sent to the brain through the fibers of the optic nerve.
In addition to cones and rods, in the retina there are other types of cells (horizontal, bipolar, amacrine and ganglionary), which establish different contacts between them and, overall, contribute to making a first visual signal processing .
The retina can be affected by various types of pathological conditions that have different visual repercussions depending on the area concerned. This eye structure can also be affected by vascular or degenerative diseases resulting from general pathologies of the body, such as arterial hypertension, diabetes or vascular sclerosis.
Structure
The retina is the innermost of the three layers that make up the wall of the eyeball. Taken as a whole, this membrane engages posteriorly to the stem of the optic nerve, while anteriorly it is grafted onto the pupillary margin of the iris.
Note : the retina derives from an extroflexion of the diencephalon, to which it remains connected by means of the optic nerve.
In all its extension, the retina is structurally constituted by two overlapping sheets: one external in contact with the choroid ( pigmented epithelium ) and the other internal in relation with the vitreous body ( sensory retina ).
The border between these two sheets is a line now called serrato (in this point, the nervous leaflet merges with the pigmented leaflet and the vascular habit).
The sensory retina is the largest portion, consisting of a system of neurons with laminar organization (9 superimposed layers) and, being provided with photoreceptors and other neurons, represents the optical part. The pigmented epithelium, on the other hand, has a very simple structure, free of nerve cells and insensitive to light.
Retinal layers
The retina is made up of multiple layers of cells, each with a specific function.
Proceeding from the external surface (applied to the choroid) to the inner portion (applied to the vitreous body), they are distinguished:
- Pigmented epithelium : it is the most external layer, interposed between the basement membrane of the choroid and the first nervous layer of the retina formed by cones and rods. The pigmented epithelium consists of a single layer of epithelial cells containing a dark colored (fuscina) pigment. These elements absorb light, preventing it from spreading (for instance, they create the conditions of a "dark room"). The pigmented epithelium has several other functions: it guarantees the exchange of oxygen and nutrients (glucose, amino acids, etc.) and waste metabolites between the photoreceptors and the choroid; it absorbs the membranes of the outer disks, ensuring a renewal of the receptor structures and constitutes the blood-retinal barrier, which modulates the exchanges between the blood and the retinal tissues. The pigmented layer of the retina also participates in the metabolism of photoreceptors, storing and releasing vitamin A (retinal) for the renewal of visual pigments (note: without the pigmented epithelium, cones and rods would not be able to regenerate photopigments).
Curiosity . The pigmented epithelium is tenaciously adherent to the choroid on the external side, but it can be easily separated from the sensory retina. Therefore, when retinal detachment occurs, the two retinal sheets (inner side) are always involved.
- Photoreceptor layer : it is composed of the outer and inner segments of cones and rods. In their outer segment, the light stimulus causes a reversible chemical modification of the visual pigment and the creation of an electrical potential, which is transmitted to the bipolar cells and, subsequently, to the ganglion cells.
- External limiting : it is a very thin connective membrane located on the border between the receptor portion of the photoreceptors and their nuclei.
- External granular layer : it consists of the cellular bodies of cones and rods, with their nuclei and their expansions.
- External plexiform layer : it is the first synaptic zone interposed between the final ends of the photoreceptors (spheres in the rods and pedicels in the cones) and the dendrites of the bipolar cells; in this region there are also horizontal cells and Müler cells. The latter are connective elements that have a nutritional and supportive function.
- Internal granular layer : it consists of the cellular bodies of bipolar cells; there are also Müller cells, horizontal and amacrine.
- Internal plexiform layer : it is the second synaptic zone that connects bipolar cells and ganglion neurons.
- Ganglion layer : it consists of the cellular bodies of ganglion cells (or multipolar); the bodies and expansions of part of the astrocytes are also found there.
- Layer of optical fibers : it is represented by the axons of ganglion cells that are preparing to flow into the optic nerve.
- Internal limiting : it is the boundary line between the nervous retinal leaflet and the vitreous body, constituted by the base surface of the Müller cells, with the interposition of a cementing component.
The layers of the nerve leaflet in the retina, which go from the photoreceptors to the ganglion cell layer, are indispensable for correctly activating the vision, as they give rise to the transformation of the light impulses into the images that we actually see when we open our eyes. Therefore, their main function is to begin the visual sensory process.
Vascularization
The retina is nourished by two independent vascular beds:
- On the inner face, the central artery system of the retina supplies the ganglion and bipolar cells and the layer of nerve fibers through the Müller cells and the astrocytes, which wrap the capillaries in a sleeve, since there are no perivascular spaces in the retina . The central artery of the retina enters the eye at the level of the optic papilla and is divided into 4 branches that are directed towards the periphery. The wastewater goes through 4 venous branches towards the papilla and emerges from the globe through the central vein of the retina.
- On the external face, on the other hand, the blood reaches the pigmented epithelium and, through this, the photoreceptors through the choro-capillary system . Venous drainage occurs thanks to the vortex veins.
Central and peripheral area
The retina is divided into two areas: a central one (rich in cones) and a peripheral area (where the rods prevail).
Two regions are of considerable importance: the macula lutea and the optic disk.
- The optic disc (or papilla of the optic nerve) corresponds to the point where the nerve fibers which originated in the retina converge and which constitute the optic nerve.
- The macula is a small elliptical area, located at the back of the retina, laterally with respect to the posterior pole of the bulb. This region has some particular characteristics: it is, in fact, the retinal area with the highest density of cones, responsible for the so-called "fine vision" (that is, it allows reading smaller characters, recognizing objects and distinguishing colors). Inside the macula, there is a depression, called fovea. This represents the area of best visual definition, in which the greatest quantity of light rays is concentrated and which allows the most distinct and precise vision.
Functions
The retina is the structure of the eyeball used for capturing light stimuli that come from the outside and to their transformation into nerve signals to be sent, through the optic nerve, to the brain structures responsible for visual interpretation.
From the functional point of view, the retinal layers can be reduced schematically to three:
- Pigment epithelium layer and photoreceptors;
- Bipolar, horizontal and amacrine cell layer;
- Ganglion cell layer.
The initial site of the light-nerve impulse conversion process is represented by the photoreceptors: when the light radiation reaches the retina, photochemical reactions are activated which convert the information received into electrical impulses to be sent to retinal neurons (phototransduction). Cones and rods, when exposed to light or darkness, in fact, undergo conformational changes, which modulate the release of neurotransmitters (chemical signal). These neurotransmitters perform an excitatory or inhibitory action on the bipolar cells of the retina, which, in turn, transmit potential graduation to the ganglion cells. The axonal extensions of the latter constitute the optic nerve and ensure the conduction of action potentials to the cerebral structures of the optical pathways, in response to the retinal receptor transduction.
The task of conveying the signal out of the retina to the lateral geniculate body and to the cortical areas of the brain, where the visual information is processed, rests with the optic nerve.
The amacrine and horizontal cells modulate the communication in the retinal nervous tissue (for example, by lateral inhibition).
Retinal diseases
The retina is affected by numerous pathologies, which affect the sight with a different level of severity.
Retinopathies are divided into acquired and hereditary. The former are in turn distinguished in vascular, inflammatory, degenerative retinal pathologies associated with systemic diseases of the body (such as diabetes and hypertension).
The most common retinal diseases are:
- Diabetic retinopathy : ocular complication that affects about 80% of people with diabetes mellitus for over 15 years;
- Vascular retinopathy : is due to alteration of blood vessels; includes arterial and venous occlusions, hypertensive and arteriosclerotic retinopathy.
- Retinal detachment : consists in raising the nerve retina (the inner section of the retina) from the pigment epithelium (the outermost part); it can be partial (involving only some sectors of the retina) or total.
Furthermore, degenerative-senile diseases and retinal cancers (such as retinoblastoma) are possible.
Note . Retinopathies are linked by the absence of pain, except for other ocular complications. This characteristic depends on the fact that the retina is free of receptors sensitive to pain sensations.
To assess the possible presence of a retinopathy, the ophthalmologist first examines the ocular fundus and, to confirm or deepen the diagnosis, a series of more complex diagnostic tests, such as coherent optical radiation tomography (OCT) and l 'electroretinogram.
