Role of ipRGCs in Visual Processing

Abstract

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are cells which take part in the formation of an image as well as a non–image creation visual response. Mure et al (2019), performed extracellular electrophysiological studies on freshly harvested human eye retinas. The study discovered three functional ipRGC subtypes. This review summarises the findings of the Mure et al (2019) study, providing valuable insights for those seeking biology dissertation help.

Summary of Mure et al., (2019) and how it Addresses Current Thinking in its Research Field

Mure et al (2019) reveal that, ipRGCs in mammals are an element of retinal ganglion cells. The retinal ganglion cells take part in a set of light responses which include; circadian entrainment as well as pupillary light reflex (PLR) and modulation of alertness or sleep. The cells also participate in mood and some aspects of vision. This argument is fully supported by Lall et al (2012), who argue that photic information is conveyed to the brain via certain retinal ganglion cell populations which contains the retinaldehyde-based photopigment, melanopsin. To achieve this, these cells signal retinal output though a multisynaptic circuitry of the retinohypothalamic tract (RHT) as well as monosynaptic projections which extend to the olivary pretectal nucleus and intergeniculate leaflet (IGL).

To scholars, this means that, there is need to understand the intrinsically photosensitive retinal ganglion cells and the image-formation, non–image-formation and visual responses. This understanding is very critical to understand the retinal cell and cover the gap of knowledge concerning these cells and vision. Lall et al (2012), also suggest that this light-evoked innervation drives the non-image-forming physiological and behavioural mechanisms of PLR, sleep propensity as well as pineal melatonin suppression and circadian photoentrainment.

Mure et al (2019), also reveal that the ipRGCs also function as mediate rod- and cone-initiated photo-responses whose role is to expand sensitivity for ipRGCs ranges to dim light. Melanopsin has been discovered in the human retina (Allen et al 2019). In the human eye, it takes the role of suppressing PLR and nocturnal melatonin. However, the study by Mure et al (2019), reveals limited knowledge of the ipRGC role in the human eye. This clearly brought up the research gap in the literature, hence creating the need and basis of this study. Therefore, to contribute knowledge to this field, the researchers performed extracellular electrophysiological assessments as well as recordings of freshly harvested retinas from five human donors (two males and three females). The outcome showed that photoresponsive cells were present in retinas from the five donors. Moreover, the outcome indicated that intrinsic light responses were slow and sustained past the 30-stimulation. The scholars also suggest that in each sample from the five donors, photoresponsive cells were identified at an average density of 2.47 cells/mm2. This is very close to the lower range of the earlier observed melanopsinimmuno-positive RGCs density in the human eye retinas (Allen et al 2019). Therefore, this indicate that, the average density observed can be concluded and be used to further advance studies in the same field. However, the study suggest that, retinal intrinsic photo-responses were reversibly limited by opsinamide.

The article also estimated if human ipRGCs can sustain photo-responses in a prolonged light exposure like that of rodents. The study discovered that all ipRGCs had a positive light response to several minutes. These results were similar to the results by Wong, (2012) which show the validity and reliability of the results. This discovery to scholars is very useful in understanding the ability of ipRGCs to sustain responses and its correspondence time. Additionally, the functional diversity of human intrinsically photosensitive retinal ganglion cells by Mure et al (2019) is a background to fully understanding human eye and its function. This study is also a step to advancing treatment and rectification of human eye problems. The retina was also stimulated for 30 seconds to ascertain its sensitivity. The scholars discovered similar profiles of sensitivity in all donors, however, the cells' responses were not easily detectable. This resulted in the characterisation of the human retina as slow activation, delayed deactivation, low sensitivity and sustained response during light stimulation. This conclusion matches earlier findings by Detwiler (2018) who performed studies in mice and Karnas et al (2013) who performed studies in Arvicanthis rodents. According to Mure et al (2019) the principal analysis components of response parameters such as latency, sensitivity as well as duration were independent of the donor. However, the cell's response seemed to be clustered into two groups which were named Type 1and Type 2. Type 1 had higher sensitivity and a half-maximal response which means that, these category can be very critical to advancing research in the same field. These cells also sustained a light response which prolonged after turning off the lights. On the other hand, Type 2 cell categories were less sensitive. Their response to light exposure only terminated shortly after turning off the lights. Studying these cells would help improve studies of ganglion cells and eye related disorders. In the study, another category, Type 3 arose based on the fact that they responded only to higher irradiances in a stronger way. However, the response was almost similar to those of Types 1 and 2. In contrast, Type 3 had a lower response which rapidly extinguished after turning off the lights. Additionally, to estimate whether the Type 3 11-cis-retinal– dependent cells were designed for the human eye, the scholars recorded retinal photo-responses from retinal degeneration (RD) mice. The results indicated that the mice had an extensive degeneration of cone and rod photoreceptors. This is a background to understand light response from ipRGCs.

Lastly, Mure et al (2019), came to the conclusion that human ipRGCs can be categorised into three subtypes based on their relationships with cones and rods, chromophore supply and responses to light. Moreover, various chromophore regeneration modes within the varying ipRGCs populations are closely related to heterogeneity in transduction of signals. This conclusion to general science is very critical to understanding the rods and cones of the eye and development of more sophisticated eye management strategies.

Mure et al (2019), assessed the intrinsic photoresponses of human retinal ganglion cells. The study discovered three subtypes of cells based on responses to light. This methodology is very critical and valid as it is also used to generate reliable results similar to a study by Reifler et al (2015). However, this study discovered that the rat retina had five subtypes of RGC photoreceptors. This methodology gave the study validity. Additionally, the study presents it’s results in chronological order which makes the study presentable and easy for the reader to follow. This resulted in the characterisation of the human retina as slow activation, delayed deactivation, low sensitivity and sustained response during light stimulation. This indicates that, similar studies in the same field, which are systematic and chronologically presented can be very critical to bridging the gap of knowledge and also assist in faster improving of human health and full understanding of human eye. The study also challenges the existing knowledge that the major ipRGCs major function is to count photons hence means that, there is need for more research even to existing studies to asserting their presented knowledge.

However, research ethics are important as they advocate for harm avoidance. Therefore, failure to consider ethical consideration significantly causes harm to human participants as per Ritchie et al (2013). To highlight the importance of ethics, the researchers suggest that, while collecting data, unruly researchers may conduct unethical studies, hence creating harm to the society. Regardless of many studies which highlight the importance of ethical consideration in research, Mure et al., (2019), does not consider or highlight the ethical consideration in their research. Additionally, the study is limited since it only utilises tissue from five donors. Low study populations used to create conclusion, may result in serious, unpleasant and life-threatening side effects or even result in uninformed results. This is an indication to researchers that, scientific studies require to fully abide by the ethical considerations and most importantly, adhere to standard study population to minimize cases of biased knowledge and ineffective conclusions. This would help in quick progress in this field of study.

Conclusion

Mure et al., (2019) investigated the ipRGCs role in the human eye. The study identified three subtypes of these cells and discovered that these cells may be related to heterogeneity in the mechanism of signal transduction. The study expands the existing knowledge and provides direct quantitative data on the human ipRGC role. The study is significant since it presents its results chronologically, brings new knowledge to the study area and effective for application in the area of study which will help in the development of light-based interventions for improving human health. On the other hand, the study utilises a limited study population and does not highlight the ethical consideration put in place hence, limiting the study. Considering ethical consideration and study population can help increase the progress of knowledge in this field.

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References

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Detwiler, P.B., 2018. Focus: Sensory Biology and Pain: Phototransduction in Retinal Ganglion Cells. The Yale Journal of Biology and Medicine, 91(1), p.49.

Karnas, D., Hicks, D., Mordel, J., Pévet, P. and Meissl, H., 2013. Intrinsic photosensitive retinal ganglion cells in the diurnal rodent, Arvicanthis ansorgei. PLoS One, 8(8).

Lall, G.S., Atkinson, L.A., Corlett, S.A., Broadbridge, P.J. and Bonsall, D.R., 2012. Circadian entrainment and its role in depression: a mechanistic review. Journal of neural transmission, 119(10), pp.1085-1096.

Reifler, A.N., Chervenak, A.P., Dolikian, M.E., Benenati, B.A., Meyers, B.S., Demertzis, Z.D., Lynch, A.M., Li, B.Y., Wachter, R.D., Abufarha, F.S. and Dulka, E.A., 2015. The rat retina has five types of ganglion-cell photoreceptors. Experimental eye research, 130, pp.17-28.

Ritchie, J., Lewis, J., Nicholls, C.M. and Ormston, R. eds., 2013. Qualitative research practice: A guide for social science students and researchers. sage.

Wong, K.Y., 2012. A retinal ganglion cell that can signal irradiance continuously for 10 hours. Journal of Neuroscience, 32(33), pp.11478-11485.