Structure and Function
The retina lines the entire posterior portion of the eye, with the exception of the optic nerve head, and extends anteriorly to terminate circumferentially at the ora serrata, the junction between the retina and the ciliary body (see Image. Horizontal Section of the Human Eye). The retina exhibits a laminated structure composed of 10 distinct neuronal layers interconnected by synapses. Cells are classified into 3 primary types: photoreceptor, neuronal, and glial. The layers, arranged from anterior to posterior, are as follows (see Image. Histology of the Retina):
Inner limiting membrane (ILM)
Nerve fiber layer (NFL)
Ganglion cell layer
Inner plexiform layer (IPL)
Inner nuclear layer (INL)
Middle limiting membrane
Outer plexiform layer (OPL)
Outer nuclear layer (ONL)
External limiting membrane (ELM)
Photoreceptor layer of rods and cones
Within the retinal layers, multiple specialized cell types convert incoming photons into action potentials that are transmitted to the brain for processing into 3-dimensional vision. Six principal cell types reside in the retina:
The intricate organization of retinal layers and specialized cell types enables precise detection of light, contrast, and motion, forming the foundation of visual perception (see Image. Cellular Architecture of the Human Retina). Disruption of these structures or cellular pathways can lead to a range of visual impairments and retinal pathologies.
Rods
Rod cells trace their evolutionary origins to approximately 500 million years ago, when a piscine ancestor developed rods to supplement preexisting cones. This adaptation likely enhanced survival in low-light environments.[3] In humans, approximately 95% of retinal photoreceptors are rods, specialized for detecting low-light levels and mediating black-and-white scotopic vision (see Image. Fluorescent Micrograph of Retinal Photoreceptor Architecture).[4]
Rods are concentrated in the outer retina, with density increasing toward the peripheral retina but lacking in the central fovea. Rods differ from cones in multiple functional aspects. Response speed is slow, and spatial acuity and contrast sensitivity are low, contrasting with the rapid response, high spatial acuity, and high contrast sensitivity of cones. Rod function is limited during daylight due to photobleaching, requiring approximately 20 minutes to recover after exposure to bright light, with full scotopic function restored roughly 40 minutes after darkness onset.[5]
Despite lower spatial acuity, off-axis visual perception mediated by rods remains remarkably effective. Objects in dark environments often appear more visible when viewed just above or below the intended target.[6] Low spatial acuity results from retinal wiring rather than intrinsic inferiority. Multiple rods converge onto a single RGC, whereas cones maintain a 1:1 ratio with their corresponding RGCs. Rods exhibit higher sensitivity to single photons, whereas cones detect specific wavelengths of light.
The retinal configuration allows rods to integrate photon signals over time through convergence, reducing background noise. Rod cells release glutamate as a neurotransmitter and form synapses with 2-order bipolar cells within the OPL.
Cones
The human retina contains approximately 6 to 7 million cones, representing only 5% of total retinal photoreceptors, yet visual acuity depends on as few as 100,000 cones.[7] Cone cells exhibit lower general photon sensitivity than rods but respond selectively to 1 of 3 specific wavelengths corresponding to color perception. Cones specialize in detecting red (64%), green (32%), or blue (2%) light and are concentrated primarily in the macula, the central retinal region containing the fovea.
The fovea contains exclusively cones and lacks rods or synapses, providing an unobstructed pathway for incoming light. In the fovea, the ratio of red to green cones is approximately 2:1, whereas blue-sensitive cones are located primarily in the surrounding peripheral macula. Cone-mediated photopic vision enables color discrimination across varying light levels and underlies most human daytime visual tasks.
High cone density in the fovea allows fine spatial resolution, supporting the differentiation of closely spaced points. Synaptic connections in the macula maintain a 1:1 ratio between cones and RGCs, in contrast to the convergence observed with rods, which allows each cone to contribute maximal visual information.
Cones exhibit functional differences compared with rods. Cone cells rapidly adapt to changes in light intensity and resist saturation under constant illumination, whereas rods require photorecovery after bleaching. Membrane current recovery in cones occurs within approximately 20 milliseconds, compared with 20 minutes or longer in rods. Cone cells release glutamate onto 2nd-order bipolar cells located in the OPL.
Retinal Ganglion Cells
RGCs serve as the primary output neurons of the retina and constitute a third class of photoreceptors exhibiting intrinsic photosensitivity. These cells transmit both image-forming and non–image-forming signals that contribute to circadian rhythm regulation, melatonin modulation, and control of pupil size.[8]
Approximately 20 distinct types of RGCs exist, with 1% to 2% of all RGCs intrinsically photosensitive due to selective expression of the G-protein peptide neuromodulator melanopsin, similar to rods and cones.[9][10] RGCs receive excitatory and inhibitory inputs from 2 types of intermediate neurons: amacrine and bipolar cells. RGCs and amacrine cells form functional on–off center subunits, enabling the brain to interpret motion, such as a small dot moving at a distance.[11]
Axons of RGCs converge at the optic disc and traverse the lamina cribrosa unmyelinated to avoid obstructing incoming light. RGC axons project to multiple central targets, including the suprachiasmatic nucleus, olivary pretectal nucleus, intergeniculate leaflet, ventral division of the lateral geniculate nucleus, and preoptic area. These projections facilitate circadian rhythm synchronization and the pupillary light reflex.[12]
Amacrine Cells
Amacrine cells function as intermediate neurons that release the inhibitory neurotransmitters γ-aminobutyric acid (GABA) or glycine. Unique gap junction physiology allows these cells to exert both inhibitory and excitatory effects.
Considerable diversity exists among amacrine cells, enabling a wide range of retinal functions and establishing their role as the retina's ultimate utility cells. The diversity of RGCs contributes to an even more diverse population of amacrine cells. To date, 20 distinct RGC types have been identified, each associated with over 42 distinct amacrine cell types. This cellular diversity supports the formation of dedicated functional microcircuits capable of detecting variations in light intensity, movement, and direction.
Functional microcircuits are categorized into 3 types of amacrine cells: wide-, medium-, and narrow-field. Wide-field amacrine cells communicate horizontally within a single retinal layer, facilitating lateral integration of information. Narrow-field amacrine cells penetrate multiple retinal layers, enabling vertical integration of signals. Stratification of amacrine cell output occurs at either presynaptic or postsynaptic sites, and, in combination with gap junctions, allows both inhibitory and excitatory functions despite the release of inhibitory neurotransmitters. Presynaptic outputs inhibit bipolar cell terminals, whereas postsynaptic outputs inhibit RGC dendrites.
Recent studies demonstrate a paracrine function for specific amacrine cell subtypes. Some of these cells release dopamine.[13]
Bipolar Cells
Bipolar cells are 2nd-order long-projection neurons, named for the 180° orientation of their axons. These cells receive visual input from rod and cone photoreceptors and project axons onto RGCs.
Thirteen distinct bipolar cell types are classified as either rod or cone bipolar cells, depending on the photoreceptor type providing input. Bipolar cells form circuits with photoreceptors that establish the fundamental elements of vision, including chromatic composition, polarity, contrast, and the temporal profile of incoming visual stimuli. Each cone and rod bipolar cell is further subdivided based on its response to light: ON-bipolar cells depolarize, whereas OFF-bipolar cells hyperpolarize.
Cone bipolar cells include both ON and OFF types, whereas rod bipolar cells consist exclusively of ON types. Rods mediate scotopic vision and require only binary detection of photon presence, making ON bipolar cells sufficient. Cones contribute to photopic vision, allowing differentiation of fine detail, motion, and color, and thus require both ON and OFF bipolar cells to convey qualitative information about incoming photons.
Bipolar cells link the outer and inner retinal layers through synapses with rods and cones in the IPL. The IPL functions as a switchboard, with different bipolar cell types stratifying into 5 sublayers to transmit distinct forms of visual information to specific RGC and amacrine cell populations.
Bipolar cells form defined synaptic relationships with RGCs, amacrine cells, and horizontal cells. Amacrine cells inhibit bipolar cell terminals presynaptically within the IPL, while horizontal cells provide GABAergic inhibitory inputs to bipolar cells. Consequently, bipolar cells receive glutamatergic inputs from rods and cones and GABAergic inputs from horizontal cells, and in turn, deliver glutamatergic excitatory input to RGCs and amacrine cells. This arrangement of parallel information processing allows highly preprocessed excitatory signals to function as the elementary building blocks of vision.[14]
Horizontal Cells
Horizontal cells modulate information transfer between bipolar cells and photoreceptors, contributing to retinal adaptation under both bright and low-light conditions. These cells possess wide, diffuse horizontal projections and are coupled to neighboring horizontal cells via gap junctions.
Three distinct types of horizontal cells exist in the retina, with cell bodies concentrated in the outer retina, primarily within the INL. Horizontal cells are GABAergic interneurons that provide inhibitory inputs to bipolar cells and deliver inhibitory feedback to rods and cones. However, the mechanism of inhibition remains debated. Some evidence suggests that horizontal cells do not rely on GABA for inhibition but instead modulate the pH within the synaptic cleft to suppress bipolar cell and photoreceptor activity.
Horizontal cells form synaptic contacts within the OPL, conveying information about polarity, spectral sensitivity, signal speed, and spatial receptive field structure. Lateral inhibitory GABAergic inputs from horizontal cells amplify signals from ON-OFF centers by antagonizing surrounding bipolar cells. This antagonism enhances contrast through binary signaling and supports 2-point spatial differentiation.
Horizontal cells also interact structurally with bipolar cells, sheathing their processes. Invaginating contacts are formed with dendrites of ON cone bipolar cells, whereas basal contacts are established with OFF cone bipolar cells.[15][16]
Central and Peripheral Retina
The central retina differs from the peripheral retina in both thickness and cellular composition. The central retina is thicker and densely packed with cones, whereas the peripheral retina is thinner and composed predominantly of rods.
The ONL contains both rods and cones and maintains uniform thickness across the retina. Composition within this layer varies, with a higher concentration of cones in the center and more rods in the periphery.
Variability in thickness is more pronounced in the INL. This layer contains a greater density of synaptic connections between cones and bipolar cells, as well as elevated concentrations of horizontal cells and amacrine cells. Cones exhibit a 1:1 synaptic convergence ratio with 2nd-order neurons, supporting high-acuity visual processing. In contrast, peripheral rods converge onto 2nd-order neurons at a ratio closer to 3:1, contributing to regional differences in INL thickness.[17]
Müller Glial Cells
Radial glial cells of the retina, also known as Müller cells, contribute to the ELM by forming adherens junctions with the inner segments of rods and cones. The retinal ILM consists of laterally apposed Müller cell endfeet and associated components of the basement membrane.[18]
Inner Limiting Membrane
The ILM forms the innermost retinal surface, bordering the vitreous humor and serving as a diffusion barrier between the neural retina and the vitreous. This layer contains laterally contacting Müller cell endfeet and components of the retinal basement membrane.
Nerve Fiber Layer
The NFL is the 2nd innermost layer of the retina, situated immediately beneath the ILM. Retinal NFL thinning, as observed on optical coherence tomography, can occur in patients with retinitis pigmentosa.[19]
Ganglion Cell Layer
This layer contains RGCs and displaced amacrine cells. Smaller RGCs typically extend dendrites into the IPL, whereas larger RGCs arborize in other sublayers of the retina.
Inner Plexiform Layer
The IPL contains a dense reticulum of interlacing dendrites. Both RGCs and neurons of the INL contribute to this network.[20]
Inner Nuclear Layer
The INL contains the cell bodies of bipolar, horizontal, and amacrine cells. This layer provides lateral and vertical integration of signals between photoreceptors, bipolar cells, and RGCs.
Outer Plexiform Layer
The OPL contains synapses between rods and cones and the footplates of horizontal cells.[21] Retinal capillaries are primarily located within this layer.
Outer Nuclear Layer
Cell bodies of rods and cones are located within the ONL. This layer additionally contains rod photoreceptor granules and cone cell extensions.
External Limiting Membrane
The ELM contains the bases of rod and cone photoreceptor cell bodies. This layer forms a barrier between the subretinal space—into which the inner and outer segments of rods and cones project in close association with the underlying pigment epithelial layer—and the neural retina proper.
Retinal Pigment Epithelium
The retinal pigment epithelium (RPE) supports the retina and performs multiple essential functions. These roles include vitamin A metabolism, maintenance of the blood-retina barrier, phagocytosis of photoreceptor outer segments, production of the mucopolysaccharide matrix surrounding the outer segments, and active transport of materials into and out of the RPE.[22][23]
Blood Supply and Lymphatics
The retina exhibits the highest rate of oxygen consumption of any tissue in the human body, necessitating a constant and substantial supply of oxygenated hemoglobin. To meet this demand, the retina possesses a dual blood supply, receiving blood from both the choroid and branches of the ophthalmic artery.
Blood leaves the heart via the aorta and enters the common carotid artery, which bifurcates into the internal and external carotid arteries. The internal carotid enters the skull, and just distal to the cavernous sinus, its 1st branch is the ophthalmic artery. The ophthalmic artery gives rise to the central retinal artery and the posterior ciliary arteries, which supply the retina from distinct angles.
The central retinal artery is the 1st branch of the ophthalmic artery. This blood vessel runs inside the dura beneath the optic nerve, travels through the optic disc, and supplies the cells of the macula. The posterior ciliary arteries divide into short and long branches that penetrate the sclera to supply the posterior uveal tract.
Retinal blood flow remains constant despite variations in intraocular pressure and systemic blood pressure and operates independently of sympathetic autoregulation. Regulation occurs locally through factors such as nitric oxide, prostaglandins, endothelin, and most importantly, arterial carbon dioxide tension. Similar to the brain, retinal blood flow increases with elevated carbon dioxide levels and decreases when carbon dioxide is reduced.
The posterior ciliary arteries supply the outer and middle retinal layers, whereas the central retinal artery supplies the inner retina. The choroid, forming the posterior portion of the uveal tract, nourishes the outer retinal layers. Blood to the choroid is provided by the long and short posterior ciliary arteries.
Capillaries are present throughout the retina, from the innermost NFL to the OPL, and occasionally within the ONL. Nutrients from the choriocapillaris behind the RPE support the photoreceptor layer. The photoreceptors and most of the OPL rely indirectly on the choriocapillaris, whereas the inner retinal layers are supplied by superficial and deep capillary plexuses formed by branches of the central retinal artery. The inner retinal layers are highly sensitive to hypoxic stress, whereas the outer retina demonstrates greater resistance to hypoxia.
