Abstract
It is still unclear to what extent structural plasticity in terms of synaptic rewiring is the cause for cortical remapping after a lesion. Recent two-photon laser imaging studies demonstrate that synaptic rewiring is persistent in the adult brain and is dramatically increased following brain lesions or after a loss of sensory input (cortical deafferentation). We use a recurrent neural network model to study the time course of synaptic rewiring following a peripheral lesion. For this, we represent axonal and dendritic elements of cortical neurons to model synapse formation, pruning and synaptic rewiring. Neurons increase and decrease the number of axonal and dendritic elements in an activity-dependent fashion in order to maintain their activity in a homeostatic equilibrium. In this study we demonstrate that synaptic rewiring contributes to neuronal homeostasis during normal development as well as following lesions. We show that networks in homeostasis, which can therefore be considered as adult networks, are much less able to compensate for a loss of input. Interestingly, we found that paused stimulation of the networks are much more effective promoting reorganization than continuous stimulation. This can be explained as neurons quickly adapt to this stimulation whereas pauses prevents a saturation of the positive stimulation effect. These findings may suggest strategies for improving therapies in neurologic rehabilitation.
Highlights
The theoretical standard model for developing cortical representations are self-organizing maps (SOMs, Kohonen, 1982)
We joined the concepts of these two models to create a novel neural network model for activity-dependent structural plasticity
We model synaptic rewiring in the cortical lesion projection zone (LPZ) after peripheral somatosensory (Merzenich et al, 1983, 1984) or focal retinal lesion (Keck et al, 2008)
Summary
The theoretical standard model for developing cortical representations are self-organizing maps (SOMs, Kohonen, 1982). 2. Updating the number of axonal and dendritic elements and synapse deletion As we extensively reviewed in Butz et al (2009), neuronal activity by changing intra-cellular calcium concentration has considerable impact on neuronal morphology (Mattson et al, 1988; Wolff and Wagner, 1983; reviewed in Lipton and Kater, 1989; Mattson, 1988): On the postsynaptic side, neurons adapt their input connectivity in a homeostatic manner – with respect to changes in synaptic weight (Turrigiano et al, 1998) but they increase and decrease the number of excitatory (Kirov et al, 2004) and inhibitory spines (Knott et al, 2002).
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