AII amacrine cells

Last updated January 04, 2025

AII (A2) amacrine cells are a subtype of amacrine cells. Amacrine cells are neurons that exist in the retina of mammals to assist in interpreting photoreceptive signals. AII amacrine cells serve the critical role of transferring light signals from rod photoreceptors to the retinal ganglion cells (which contain the axons of the optic nerve).

The AII amacrine cells are unique because they work primarily with the vertical transmission of information, meaning they connect the bipolar and ganglion cells. Other amacrine cells primarily assist with horizontal pathways, meaning they connect similar types of neurons.^[1]^[2] Amacrine II cells also work very similarly to rod photoreceptors in terms of threshold (amount of stimulation needed to begin performing), saturation level (how densely they exist, and where), and spectral sensitivities (how sensitive the cell is to changes in stimulation levels). However, the Amacrine II cell works faster than the rod photoreceptors. ^[2]

A photograph of the retina of the human eye. (d.mSTf~ ljzr, License link) Retina1.jpg — A photograph of the retina of the human eye. (د.مصطفى الجزار, License link)

Morphology

AII amacrine cells are round or oval, and include dendrites which connect together to create a systematic mosaic. They have two main forms, which differ in their dendritic trees (dendritic formations). The first form is made of one dendrite with multiple short, thin arms that end in circular appendages. The second form has multiple thin dendrites with attached spines, and extensive branching. Amacrine II cells are found most densely in the central retina, but are found in the surrounding retinal areas as well. ^[2]

Development

The study of the development of amacrine cells is relatively recent. A recent study found that amacrine cells develop during the Peri-Eye-Opening Period (during 7-28 days post birth). During this time, the cells are developing dendrites and dendritic spines, shifting resting membrane potentials (RMPs), developing synaptic activity, and developing Potassium currents (K+). ^[3]

The Retina ^[4]

The retina is a region of tissue at the back of the eye. It is light sensitive, which is why the mentioned light activated cells are located here. It is responsible for collecting light information through the eye, turning it to electric information, and transmitting it to the optic nerve.

The Optic Nerve

The optic nerve is a bundle of nerves that transmits one-way electrical information from the retina to the brain. Each eye has an optic nerve. ^[5]

Classical Rod Pathway

To understand the role of AII amacrine cells in the mammalian retina, we must understand the Classical Rod Pathway. This can be summarized as follows: ^[2]

Scotopic lighting is dim lighting. The mammalian eye primarily utilizes its rod shaped photoreceptors to interact with this light.^[6]
- Photoreceptors contain photopigments. These units contain one protein, known as opsin, and one molecule, known as a chromophore. The chromophore of vertebrate animals is retinal, a form of Vitamin A.
  - Humans have four types of opsin. One exists in rod photoreceptors and is responsible for low light vision (as in the classical rod pathway). The rest exist in cone photoreceptors and work together to provide colored vision. The amino acid composition of each opsin causes them to be used specifically when the environment calls for them. ^[6]
    
    The conformation change of retinal, caused by light.
When photons of light reach the retina, they are absorbed by the chromophore (retinal). This causes a change of configuration in the retinal, which causes a signaling pathway that results in the closure of membrane sodium channels. This change triggers the cell membrane to become charged negatively through the process of hyperpolarization. ^[6]
- As soon as a photon of light stimulates a rod, the AII amacrine cells are stimulated.
The chromophore and opsin detach, reverting the chromophore to its original conformation.
- This step is the main difference between this vertebrate pathway and the invertebrate pathway. In invertebrates, the chromophore is rarely reused. Instead, new chromophores are generated for the next use. ^[6]
  
  The three-layered network that carries photoreceptor information to the optic nerve. Photoreceptors are shown at the top, and the optic nerve is shown at the bottom.
Now, the information must travel from the photoreceptors to the optic nerve. This process takes place in a three layered network of synapsing retinal cells. The photoreceptors exist at the back of the retina. The photoreceptors synapse (connect) with bipolar cells . The signals are transmitted along the network through the processes of depolarization and hyperpolarization. The photoreceptors first depolarize, meaning the cell becomes less negatively charged than its surroundings. This causes them to release glutamate, a neurotransmitter. Bipolar cells possess glutamate receptors, meaning they recognize the neurotransmitter and respond to the stimulus. ^[7] They respond by hyperpolarizing, meaning their cells become more negative than their surroundings (opposite of photoreceptors). The bipolar cells then synapse with the ganglion cells that make up the optic nerve in the region known as the inner plexiform layer (IPL). When the system is 'ON', signals are interpreted by the inner half of the IPL. When the system is 'OFF', signals are interpreted by the outer half of the IPL. Bipolar, amacrine, and ganglion cells interact in both regions. ^[8]
- This is where the Amacrine II cells come into play. They assist in the transfer of information between the rod photoreceptors and the ganglion cells. These cells create a complex system that controls the connections between each layer, and ensures that the activity of these connections stays within acceptable bounds. This control system ensure that even when light is varying strongly, the system is not overwhelmed.
  - The Amacrine II cells also create contrast between signals, allowing them to be differentiated. ^[6]
  - This system allows for small, but important, signals to be amplified, while large, but irrelevant signals, may be muted. ^[2]
  - Multiple types of amacrine cells exist here, but only the Amacrine II cells form vertical pathways. ^[1]
  - The amacrine cells were originally thought to be methodically placed in this system. However, it has been discovered that the placement of amacrine cells, especially amacrine II cells, to each other and to bipolar cells is largely random in most vertebrates. It is not yet known why this is. It is hypothesized that amacrine cells are able to migrate within the retina, causing a random formation. It is also hypothesized that the reasoning for the randomness is to promote more even coverage and connectivity that a rigid system would not allow. ^[9]
- The amacrine II cells transfer signals bidirectionally, allowing for impressive synchronization of responses from this network. ^[2]
- From this network, the bipolar cells each turn on or off to control the intensity of light perceived, and to contrast between colors. ^[6]

Interconnectivity between Amacrine II cells ^[2]

Dopamine decreases the interconnectivity of Amacrine II cells.
The base level of interconnectivity between Amacrine II cells is currently unknown, as different methods of study have produced different results. One model has suggested that each Amacrine II cell connects to an average of three others, while other models have suggested that each cell may be connected to between 20-300 others. Connectivity may depend on the region of the retina, or the level of light the region is accustomed to.

Note: A small proportion of rods contact the cone bipolar cells directly.

Footnotes

Related Research Articles

The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.

The visual system is the physiological basis of visual perception. The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system and the neural system.

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

<span class="mw-page-title-main">Rod cell</span> Photoreceptor cells that can function in lower light better than cone cells

Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 92 million rod cells in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.

<span class="mw-page-title-main">Retinal ganglion cell</span> Type of cell within the eye

A retinal ganglion cell (RGC) is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within the ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain.

<span class="mw-page-title-main">Retina bipolar cell</span> Type of neuron

As a part of the retina, bipolar cells exist between photoreceptors and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.

<span class="mw-page-title-main">Electroretinography</span> Medical intervention

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors, inner retinal cells, and the ganglion cells. Electrodes are placed on the surface of the cornea or on the skin beneath the eye to measure retinal responses. Retinal pigment epithelium (RPE) responses are measured with an EOG test with skin-contact electrodes placed near the canthi. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions can elicit stronger response from certain components.

Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells in the vertebrate retina. A photon is absorbed by a retinal chromophore, which initiates a signal cascade through several intermediate cells, then through the retinal ganglion cells (RGCs) comprising the optic nerve.

<span class="mw-page-title-main">Amacrine cell</span> Interneuron cells in the retina of the eye

In the anatomy of the eye, amacrine cells are interneurons in the retina. They are named from Greek a– 'non' makr– 'long' and in– 'fiber', because of their short neuronal processes. Amacrine cells are inhibitory neurons which project their dendritic arbors onto the inner plexiform layer (IPL). They interact with retinal ganglion cells and bipolar cells.

<span class="mw-page-title-main">Retina horizontal cell</span>

Horizontal cells are the laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizontal cells are believed to be responsible for increasing contrast via lateral inhibition and adapting both to bright and dim light conditions. Horizontal cells provide inhibitory feedback to rod and cone photoreceptors. They are thought to be important for the antagonistic center-surround property of the receptive fields of many types of retinal ganglion cells.

Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of an additional photoreceptor was first suspected in 1927 when mice lacking rod and cone cells still responded to changing light levels through pupil constriction; this suggested that rods and cones are not the only light-sensitive tissue. However, it was unclear whether this light sensitivity arose from an additional retinal photoreceptor or elsewhere in the body. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.

<span class="mw-page-title-main">Outer plexiform layer</span>

The outer plexiform layer is a layer of neuronal synapses in the retina of the eye. It consists of a dense network of synapses between dendrites of horizontal cells from the inner nuclear layer, and photoreceptor cell inner segments from the outer nuclear layer. It is much thinner than the inner plexiform layer, where amacrine cells synapse with retinal ganglion cells.

<span class="mw-page-title-main">Bipolar neuron</span> Neuron with only one axon and one dendrite

A bipolar neuron, or bipolar cell, is a type of neuron characterized by having both an axon and a dendrite extending from the soma in opposite directions. These neurons are predominantly found in the retina and olfactory system. The embryological period encompassing weeks seven through eight marks the commencement of bipolar neuron development.

In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells (ipRGC), which contain the photopigment melanopsin. The axons of the ipRGCs belonging to the retinohypothalamic tract project directly, monosynaptically, to the suprachiasmatic nuclei (SCN) via the optic nerve and the optic chiasm. The suprachiasmatic nuclei receive and interpret information on environmental light, dark and day length, important in the entrainment of the "body clock". They can coordinate peripheral "clocks" and direct the pineal gland to secrete the hormone melatonin.

Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin. While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, rhodopsin in the broad sense refers to any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle.

The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. When the photon is absorbed, the 11-cis retinal photoisomerizes into all-trans retinal as it is ejected from the opsin protein. Each molecule of retinal must travel from the photoreceptor cell to the RPE and back in order to be refreshed and combined with another opsin. This closed enzymatic pathway of 11-cis retinal is sometimes called Wald's visual cycle after George Wald (1906–1997), who received the Nobel Prize in 1967 for his work towards its discovery.

<span class="mw-page-title-main">OPN1LW</span> Protein-coding gene in humans

OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum. The gene contains 6 exons with variability that induces shifts in the spectral range. OPN1LW is subject to homologous recombination with OPN1MW, as the two have very similar sequences. These recombinations can lead to various vision problems, such as red-green colourblindness and blue monochromacy. The protein encoded is a G-protein coupled receptor with embedded 11-cis-retinal, whose light excitation causes a cis-trans conformational change that begins the process of chemical signalling to the brain.

Mammals normally have a pair of eyes. Although mammalian vision is not as excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.

Retinal precursor cells are biological cells that differentiate into the various cell types of the retina during development. In the vertebrate, these retinal cells differentiate into seven cell types, including retinal ganglion cells, amacrine cells, bipolar cells, horizontal cells, rod photoreceptors, cone photoreceptors, and Müller glia cells. During embryogenesis, retinal cells originate from the anterior portion of the neural plate termed the eye field. Eye field cells with a retinal fate express several transcription factor markers including Rx1, Pax6, and Lhx2. The eye field gives rise to the optic vesicle and then to the optic cup. The retina is generated from the precursor cells within the inner layer of the optic cup, as opposed to the retinal pigment epithelium that originate from the outer layer of the optic cup. In general, the developing retina is organized so that the least-committed precursor cells are located in the periphery of the retina, while the committed cells are located in the center of the retina. The differentiation of retinal precursor cells into the mature cell types found in the retina is coordinated in time and space by factors within the cell as well as factors in the environment of the cell. One example of an intrinsic regulator of this process is the transcription factor Ath5. Ath5 expression in retinal progenitor cells biases their differentiation into a retinal ganglion cell fate. An example of an environmental factor is the morphogen sonic hedge hog (Shh). Shh has been shown to repress the differentiation of precursor cells into retinal ganglion cells.

References

1 2 "Visual Processing: Eye and Retina (Section 2, Chapter 14) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston". nba.uth.tmc.edu. Archived from the original on 2023-12-02. Retrieved 2024-11-06.
1 2 3 4 5 6 7 Farsaii, Mahnoosh; Connaughton, Victoria P. (1995), Kolb, Helga; Fernandez, Eduardo; Jones, Bryan; Nelson, Ralph (eds.), "AII Amacrine Cells", Webvision: The Organization of the Retina and Visual System, Salt Lake City (UT): University of Utah Health Sciences Center, PMID 21413403, archived from the original on 2024-06-13, retrieved 2024-11-06
↑ Zhang, Xuhong (Feb 5, 2024). "Characterization of Retinal VIP-Amacrine Cell Development During the Critical Period". Cellular and Molecular Neurobiology. 44 (1): 19. doi:10.1007/s10571-024-01452-x. PMC 10844409 . PMID 38315298.
↑ "Retina". www.cancer.gov. 2011-02-02. Retrieved 2024-12-05.
↑ "Optic Nerve". Cleveland Clinic. 2024-04-11. Retrieved 2024-12-04.
1 2 3 4 5 6 Land, Michael (June 1, 2020). "Photoreception". Britannica. Archived from the original on 2024-05-22. Retrieved 2024-11-29.
↑ Marc, Robert (2014-09-04). "The AII amacrine cell connectome: a dense network hub". Frontiers in Neural Circuits. 8: 104. doi: 10.3389/fncir.2014.00104 . PMC 4154443 . PMID 25237297.
↑ Ichinose, Tomomi (Aug 26, 2022). "On and Off Signaling Pathways in the Retina and the Visual System". Frontiers in Ophthalmology. 2. doi: 10.3389/fopht.2022.989002 . PMC 10016624 . PMID 36926308.
↑ Rockhill, Rebecca (Feb 25, 2000). "Spatial order within but not between types of retinal neurons". The Proceedings of the National Academy of Sciences. 97 (5): 2303–2307. Bibcode:2000PNAS...97.2303R. doi: 10.1073/pnas.030413497 . PMC 15796 . PMID 10688875.

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[:1-1] 1 2 "Visual Processing: Eye and Retina (Section 2, Chapter 14) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston". nba.uth.tmc.edu. Archived from the original on 2023-12-02. Retrieved 2024-11-06.

[:2-2] 1 2 3 4 5 6 7 Farsaii, Mahnoosh; Connaughton, Victoria P. (1995), Kolb, Helga; Fernandez, Eduardo; Jones, Bryan; Nelson, Ralph (eds.), "AII Amacrine Cells", Webvision: The Organization of the Retina and Visual System, Salt Lake City (UT): University of Utah Health Sciences Center, PMID 21413403, archived from the original on 2024-06-13, retrieved 2024-11-06

[3] Zhang, Xuhong (Feb 5, 2024). "Characterization of Retinal VIP-Amacrine Cell Development During the Critical Period". Cellular and Molecular Neurobiology. 44 (1): 19. doi:10.1007/s10571-024-01452-x. PMC 10844409 . PMID 38315298.

[4] "Retina". www.cancer.gov. 2011-02-02. Retrieved 2024-12-05.

[5] "Optic Nerve". Cleveland Clinic. 2024-04-11. Retrieved 2024-12-04.

[:0-6] 1 2 3 4 5 6 Land, Michael (June 1, 2020). "Photoreception". Britannica. Archived from the original on 2024-05-22. Retrieved 2024-11-29.

[7] Marc, Robert (2014-09-04). "The AII amacrine cell connectome: a dense network hub". Frontiers in Neural Circuits. 8: 104. doi: 10.3389/fncir.2014.00104 . PMC 4154443 . PMID 25237297.

[8] Ichinose, Tomomi (Aug 26, 2022). "On and Off Signaling Pathways in the Retina and the Visual System". Frontiers in Ophthalmology. 2. doi: 10.3389/fopht.2022.989002 . PMC 10016624 . PMID 36926308.

[9] Rockhill, Rebecca (Feb 25, 2000). "Spatial order within but not between types of retinal neurons". The Proceedings of the National Academy of Sciences. 97 (5): 2303–2307. Bibcode:2000PNAS...97.2303R. doi: 10.1073/pnas.030413497 . PMC 15796 . PMID 10688875.

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