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I've been reading about brain plasticity and how the brain can "rewire" itself.
One of the things that is not clear to me - how neurons can establish new connections. Does this rewiring mean that neurons can "disconnect" from other neurons to reuse existing dendrites? Or do neurons grow new dendrites to make these new connections, adding to existing ones?
Thank you for your input!
I haven't read anything particularly about dendrites being reshaped, though I would expect them to be as flexible as other parts of the cells.
The more commonly discussed topic (in my literary experience) is reshaping of the axon's branches before forming synaptic terminals. These branches are not fixed even in adults - neurons can grow new and retract old branches, attaching (synapsing) to other cells in new places and removing old connections (see Wikipedia: Synaptogenesis).
Additionally to this actual change in the number of synapses, individual synapses can be regulated in their signal strength by adjusting the number of neurotransmitter receptors in the postsynaptic membrane (Gerrow&Triller, 2010, also see Wikipedia: Synaptic plasticity)
Under conditions where the sensory input to the cortex has been altered, large-scale changes in dendritic branching have been observed after enriched environment experience (e.g. Greenough and Volkmar, 1973) and sensory deprivation (e.g. Tailby et al. 2005). However there is a discrepancy between those post-mortem studies and more modern in vivo studies, where only small-scale changes of the dendritic tips have been observed (Schubert et al. 2013). Large-scale axonal re-arrangements have been also implicated in post-mortem studies after sensory deprivation (e.g. Darian-Smith and Gilbert, 1994) but no such effects have been convincingly described in living animals.
These somewhat different results may be due to several technical limitations: 1) post-mortem studies have utilized injectable tracers and their uptake and/or expression levels may contribute in part to the observed results, 2) in vivo studies have been confined to superficial dendrites due to optical access limitations. Furthermore, these manipulations of the animal's experience are relatively crude ways of probing cortical circuits and do not necessarily reflect the processes that the brain may use under more physiological conditions.
By far the most well described processes under enriched environment experience, sensory deprivation and sensory stimulation, and learning paradigms are microscopic changes at the level of individual synapses. These include increased synaptic turnover (Trachtenberg et al. 2002, Xu et al. 2009), synaptic stabilization (Holtmaat et al. 2006), and synaptic strength changes (Hofer et al. 2009). These microscopic changes provide a very economical way, in comparison to large-scale dendritic or axonal re-arrangements, in which neuronal circuits can be extensively rewired given the enormous potential of dendritic spines and boutons to sample different synaptic partners (Stepanyants et al. 2002).
References
Darian-Smith, C., & Gilbert, C. D. (1994). Axonal sprouting accompanies functional reorganization in adult cat striate cortex. Nature, 368(6473), 737–740. https://doi.org/10.1038/368737a0
Greenough, W. T., & Volkmar, F. R. (1973). Pattern of dendritic branching in occipital cortex of rats reared in complex environments. Experimental Neurology, 40(2), 491–504. https://doi.org/10.1016/0014-4886(73)90090-3
Hofer, S. B., Mrsic-Flogel, T. D., Bonhoeffer, T., & Hübener, M. (2009). Experience leaves a lasting structural trace in cortical circuits. Nature, 457(7227), 313–317. https://doi.org/10.1038/nature07487
Holtmaat, A., Wilbrecht, L., Knott, G. W., Welker, E., & Svoboda, K. (2006). Experience-dependent and cell-type-specific spine growth in the neocortex. Nature, 441(7096), 979–983. https://doi.org/10.1038/nature04783
Schubert, V., Lebrecht, D., & Holtmaat, a. (2013). Peripheral Deafferentation-Driven Functional Somatosensory Map Shifts Are Associated with Local, Not Large-Scale Dendritic Structural Plasticity. Journal of Neuroscience, 33(22), 9474–9487. https://doi.org/10.1523/JNEUROSCI.1032-13.2013
Stepanyants, A., Hof, P. R., & Chklovskii, D. B. (2002). Geometry and structural plasticity of synaptic connectivity. Neuron, 34(2), 275–88. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11970869
Tailby, C., Wright, L. L., Metha, A. B., & Calford, M. B. (2005). Activity-dependent maintenance and growth of dendrites in adult cortex. Proceedings of the National Academy of Sciences, 102(12), 4631–4636. https://doi.org/10.1073/pnas.0402747102
Trachtenberg, J. T., Chen, B. E., Knott, G. W., Feng, G., Sanes, J. R., Welker, E., & Svoboda, K. (2002). Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex. Nature, 420(6917), 788–94. https://doi.org/10.1038/nature01273
Xu, T., Yu, X., Perlik, A. J., Tobin, W. F., Zweig, J. a, Tennant, K., … Zuo, Y. (2009). Rapid formation and selective stabilization of synapses for enduring motor memories. Nature, 462(7275), 915–9. https://doi.org/10.1038/nature08389
