Neural circuit reconstruction
Neural circuit reconstruction is the reconstruction of the detailed circuitry of the nervous system (or a portion of the nervous system) of an animal. It is sometimes called EM reconstruction since the main method used is the electron microscope (EM).[1] This field is a close relative of reverse engineering of human-made devices, and is part of the field of connectomics, which in turn is a sub-field of neuroanatomy.
Model systems
Some of the model systems used for circuit reconstruction are the fruit fly,[1] the mouse,[2] and the nematode C. elegans.[3]
Sample preparation
The sample must be fixed, stained, and embedded in plastic.[4]
Imaging
The sample may be cut into thin slices with a microtome, then imaged using transmission electron microscopy. Alternatively, the sample may be imaged with a scanning electron microscope, then the surface abraded using a focused ion beam, or trimmed using an in-microscope microtome. Then the sample is re-imaged, and the process repeated until the desired volume is processed.[5]
Image processing
The first step is to align the individual images into a coherent three dimensional volume.
The volume is then annotated using one of two main methods. The first manually identifies the skeletons of each neurite.[6] The second techniques uses computer vision software to identify voxels belonging to the same neuron, which are then corrected in the process of proofreading.[7]
Notable examples
- The connectome of C. elegans was the seminal work in this field.[3] This circuit was obtained with great effort using manually cut sections and purely manual annotation on photographic film. For many years this was the only circuit reconstruction available.
Limitations and future work
Understanding the detailed operation of the reconstructed networks also requires knowledge of gap junctions (hard to see with existing techniques), the identity of neurotransmitters and the locations and identities of receptors. In addition, neuromodulators can diffuse across large distances and still strongly affect function.[8] Currently these features must be obtained through other techniques. Expansion microscopy may provide an alternative method.
References
- 1 2 Chklovskii, Dmitri B; Vitaladevuni, Shiv; Scheffer, Louis K (2010). "Semi-automated reconstruction of neural circuits using electron microscopy". Current Opinion in Neurobiology. 20 (5): 667–75. doi:10.1016/j.conb.2010.08.002. PMID 20833533.
- ↑ Bock, Davi D.; Lee, Wei-Chung Allen; Kerlin, Aaron M.; Andermann, Mark L.; Hood, Greg; Wetzel, Arthur W.; Yurgenson, Sergey; Soucy, Edward R.; et al. (2011). "Network anatomy and in vivo physiology of visual cortical neurons". Nature. 471 (7337): 177–82. doi:10.1038/nature09802. PMC 3095821
. PMID 21390124. - 1 2 White, John G., Eileen Southgate, J. Nichol Thomson, and Sydney Brenner (1986). "The structure of the nervous system of the nematode Caenorhabditis elegans". Philos Trans R Soc Lond B Biol Sci. 314 (1165): 1–340.
- ↑ Hayat, M. Arif (2000). Principles and techniques of scanning electron microscopy. Biological applications, fourth edition. Cambridge University Press. ISBN 978-0521632874.
- ↑ Briggman, Kevin L., and Davi D. Bock (2012). "Volume electron microscopy for neuronal circuit reconstruction". Current opinion in neurobiology. 22 (1): 154–161.
- ↑ Saalfeld, Stephan, Albert Cardona, Volker Hartenstein, and Pavel Tomančák (2009). "CATMAID: collaborative annotation toolkit for massive amounts of image data". Bioinformatics. 25 (15): 1984–1986.
- ↑ Chklovskii, Dmitri B., Shiv Vitaladevuni, and Louis K. Scheffer. (2010). "Semi-automated reconstruction of neural circuits using electron microscopy." Current opinion in neurobiology" (PDF). 20 (5): 667–675.
- ↑ Bargmann, Cornelia I. (2012). "Beyond the connectome: how neuromodulators shape neural circuits". Bioessays. 34 (6): 458–465.