On-OFF 방향 선택적 신경절 세포
망막에 있는 방향 선택성 세포들은 시각적 자극의 방향에 다르게 반응하는 신경세포들입니다. 이 단어는 \"자극이 수용장에서 한 방향으로 움직일 때 특히 격렬하게 신호를 발생하는” 한 무리의 신경세포를 묘사하는 데 쓰입니다[1] 생쥐의 망막에선 세 종류의 방향 선택성 세포들이 알려져 있습니다; ON/OFF 방향 선택성 신경절 세포, ON 방향 선택성 신경절 세포(밝은 자극의 이끄는 선두 가장자리에 반응) 및 OFF 방향 선택성 신경절 세포(밝은 자극의 끌리는 꼬리 부분에만 반응)입니다. 각각의 세포는 독특한 생리 및 해부학적 구조를 갖고 있습니다.[2] 이 페이지의 나머지 부분의 내용은 ON/OFF 방향 선택성 신경절 세포에만 해당되는 내용입니다.
생리(Physiology)
ON/OFF방향 선택성 신경절 세포는 국소적인 움직임 탐지기의 역할을 합니다. 만일 밝은 자극(예를 들어 빛)이 세포가 선호하는 방향으로 움직이면 세포는 이끄는 선두 가장자리와 끌리는 꼬리 쪽 가장자리 모두 에서 신호를 발사합니다. 중요한 대비로써, 밝은 자극이 선호 방향의 반대 방향(반응이 없는 방향)으로 움직이는 경우 매우 적거나 반응이 아예 없게 됩니다.Cite error: Invalid <ref>
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ON/OFF DSGCs can be divided into 4 subtypes differing in their directional preference, ventral, dorsal, nasal, or temporal. The cells of different subtypes also differ in their dendritic structure and synaptic targets in the brain.[4]
From the early experiments in the 1960s, it was shown that receptive fields are fairly large, sensitive to small changes, and direction-selective subunits are repeated many times throughout the retina.[5]
Anatomy
The ON/OFF DSGCs are commonly recognized by their bistratified dendritic arbors, which extend to two layers of the inner plexiform layer (IPL). These cell types are also known to synapse with both bipolar cells and starburst amacrine cells (SAC). As described above, there are four cell subtypes, each with own preference for direction. Each subtype of ON/OFF DSGCs has differences in dendritic patterns and axonal projections to the brain. These differences indicate that outputs from different subtypes may wire to different parts of the brain [4]
Connections
Excitation comes from both bipolar cells and starburst amacrine cells.[6] The main source of inhibition is from starburst amacrine cells. Using manual reconstruction of 6 ON/OFF DSGCs and their synaptic partners, it was found that over 90% of SAC – ON/OFF DSGC synapses were oriented in the null direction.[7]
As illustrated in the accompanying figure, light enters the retina through the photoreceptors, and excitatory inputs are transmitted to the ON/OFF DSGCs via Glutamate and Acetylcholine from the bipolar and starburst amacrine cells. Inhibitory GABA inputs, which are crucial for suppressing information in the null direction (and thereby creating a direction-selective motion detector) are received from SACs. The motion detection result is fed to higher parts of the brain for further processing.
Molecules
As described above, ON/OFF DS ganglion cells can be divided into 4 subtypes differing in their directional preference, ventral, dorsal, nasal, or temporal. Recent research has identified markers for distinguishing between the different subtypes, and for separating ON/OFF DSGCs from other retinal ganglion cells. These markers are independent of experience, and suggest a method for how these cells obtain different inputs.
Recent research has lead to the development of transgenic mouse lines that selectively mark ON/OFF DSGCs that prefer ventral or nasal motion and another line that marks ventral and dorsal preferring DSGCs. These lines were used to identify cell surface molecules (including Cadherin 6, CollagenXXV1, and Matrix metalloprotease 17), that allow each of the four types of ON/OFF DSGCs to be differentiated. A neuropeptide, CART (cocaine and amphetamine regulated transcript) has been found to differentiate ON/OFF DSGCs from all other retinal ganglion cells. Strikingly, these patterns of molecular differentiation occur before animal eye-opening, and demonstrate that these differences are experience-independent. Therefore, the molecular differences may help to explain the differing functionality between subtypes. [4]
Models
The firing pattern of On-Off Direction-Selective Ganglion cells is time-dependent and is supported by the Reichardt- Hassenstain model, which detects spatiotemporal correlation between two adjacent cells [8].
As applied to the visual system, this model considers the processed stimulus(i.e., light) inputs to two adjacent cells. After a time delay, each delayed input is multiplied by the original signal from the other cell. The resulting signals are subtracted, and the positive outcome indicates the preferred direction [8].
This behavior was validated in the visual system using calcium imaging in the fly [9]. However, this model correspondence has only been completed at a high-level (input-output), rather than at an anatomical or physiological level.[6]
History
Direction Selective units were first explored in cats by Hubel and Wiesel in 1959. Levick and Barlow performed many of the seminal early experiments related to direction selectivity during the 1960s using rabbit retina [5]. In these experiments, they measured action potentials generated from a black-white grating with a small slit [8]. Many additional experiments have been performed during the past fifty years in organisms as diverse as the turtle (e.g., Marchiafava 1979) and the mouse (Briggman 2011).
References
- ↑ H. B. Barlow and W. R. Levick (1965) The Mechanism of Directionally Selective Units in Rabbit's Retina J. Physiol. 178: 477-504
- ↑ \"Motion Sensing in Vision.\" Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/wiki/Motion_Sensing_in_Vision (Accessed April 02, 2012).
- ↑ D. I. Vaney, B. Sivyer, and W. R. Taylor (2012). Direction selectivity in the retina: symmetry and asymmetry in structure and function. Nature Neuroscience 13 (3): 194-208
- ↑ 4.0 4.1 4.2 4.3 J. N. Kay et al. (2011) Retinal ganglion cells with distinct directional preferences differ in molecular identity, structure, and central projections. J. Neurosci. 31 (21): 7753-7762 doi: 10.1523/JNEUROSCI.0907-11.2011
- ↑ Cite error: Invalid
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- ↑ 6.0 6.1 6.2 6.3 A. Borst and T. Euler (2011). Seeing Things in Motion: Models, Circuits, and Mechanisms. Neuron 71 (6): 974-994 doi:10.1016/j.neuron.2011.08.031
- ↑ 7.0 7.1 K. L. Briggman, M. Helmstaedter, and W. Denk (2011). Wiring specificity in the direction-selectivity circuit of the retina Nature 471: 183–188
- ↑ Cite error: Invalid
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- ↑ J. Haag (2004). Fly Motion Vision Is Based on Reichardt Detectors Regardless of the Signal-to-noise Ratio. Proc. Natl. Acad. Sci. 101 (46): 16333-16338 doi: 10.1073/pnas.0407368101
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