Learning: A permanent reorganization of neuronal maps

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Learning: A permanent reorganization of neuronal maps Structure Morphogenesis Homoestasis Funktion Learning Brain constantly changes under the influence of experiences. The overall activity should remain constant (homeostasis). So when new connections are shaped, others disappear, when some connections become stronger others become weaker. Experiences

Experience dependent plasticity Experiment: Rats try to retrieve food pellets from container (difficult). Results: Accuracy increases if the rats are successful, i.e. if they reach and eat the food. Otherwise there is no significant change. Kleim et al. (2004)‏

Experience dependent plasticity Cortical mapping by electrical recordings: Representation of the pawn (green) only increases if performance increases. If only motor activity increases but not performance, the representation of the upper arm increases (blue) instead. When the pawn representation (blue) increases, the representation of the upper arm shrinks and vice versa. SRC = skilled reaching conditions (animal succesful) URC = unskilled reaching conditions (animal not succesful)‏ Kleim et al. (2004)‏

Weight changes or rewiring? Changes in cortical representations and mappings can arise from either changes in synaptic strengths or from synaptic rewiring by forming and deleting entire synapses – or even both on different timescales. Therefore, weight changes rather contribute to short-term plasticity and wiring changes to long-term plasticity. Chklovskii et al. (2004) Nature

Structural plasticity of axon terminals In-vivo 2 Photon-Laser Imaging from the cortex of living mice reveals a permanent axonal remodelling even in the adult brain leading to synaptic rewiring Top: Axonal outgrowth/retraction Red arrow: Axonal outgrowth Yellow: Remains Blue: Retracts Middle: Rewiring Blue: Retracts and looses synapse Red: Grows and creates new syn. Bottom: 3D-reconstruction of middle. Synapses shown in red. The PND93 synapse is deleted and at PND113 a new one is created.

Structural plasticity of dendritic spines 2μm Dendritic spines on an appical dendrite on a LIV-neuron in V1 of the rat Arellano et al. (2007) Neurosci. Spine growth precedes synapse formation Knott et al., 2006 Spine formation via filopodia-shaped spines (see arrow, top figure) precedes synapse formation. Spines in synapses are rather mushroom-shaped and carry receptor plates (active zones, red, top figure). Spines contact axonal terminals or axonal varicosities in reach and form synapses (left).

Structural plasticity of dendritic spines Stable and transient spines In-vivo imaging of dendritic trees within the barrel cortex of living rats Trachtenberg et al. (2002) Nature Spines are highly flexible structures that are responsible (together with axonal varicosities) for synaptic rewiring. Only one third of all spines are stable for more than a month. Another third is semistable, meaning that it is present for a couple of days. Transient spines appear and disappear within a day.

Activity shapes neuronal form and connectivity Neuronal activity Neuronal morphology Network connectivity Morphology Connectivity . Neuronal activity changes the intracellular calcium. Via changes in intra- cellular calcium, neurons change their morphology with respect to their axonal and dendritic shape. This leads to changes in neuronal connectivity which, in turn, adapts neuronal activity. The goal is that by these changes neurons achieve a homeostatic equilibrium of their activity.

Application to lesion-induced plasticity Modelling the rewiring of cortical maps: Cortical remapping substantially appears after cortical lesions and changes in neural input (deafferentation). I.e. monkeys show pronounced remapping following finger amputation. Representation of the remaining fingers enlarge so that they fill the unused representations. 1-5: cortical representations of digits 1-5

Models for structural plasticity The model is... a recurrent neural network with simple spiking neurons each synapse consists of a presynaptic and a postsynaptic element. We assume that... Neighbouring neurons have a higher chance to form synapses than distant neurons Neurons change the number of their synaptic elements in an activity-dependent manner Synapses are formed in a recombination step The network yields towards homeostasis by changing its connectivity Connectivity control Sorted connectivity (Connectivity matrix)‏ Inhibition 1 2 3 ………………80 81…100 Excitation Neuron number 1 2 3 ………………80 81…100 Excitation Inhibition -10 10

Synapse representation and synaptic rewiring Synapse formation Synapse deletion Synapse turnover after synapse formation 1 2 3 4 5 6 7 8 9 10 5 7 3 1 10 9 after synapse deletion 4 5 6 7 8 3 2 1 10 9 x Butz et al. (2008) Hippocampus The algorithm proceeds in three steps: Update in network activity (on a short, functional time scale) Update in the number of free synaptic elements (and eventually synapse deletion) (on a longer, morphogenetic time scale)‏ Update in connectivity (return to step 1)‏

Activity-dependent changes in synaptic elements Step 1) Update in neuronal activity: Firing rate Membrane potential Firing state input rate Average firing rate Homeostatic rule The firing rate is given by a standard sigmoidal function that defines the activity of a poisson-spiking neuron. The firing rate depends on the synaptic input each neuron recieves. We compute the average firing rate of each neuron over a certain time frame. The deviation from a desired value drives morphogenetic changes of the neuron (next slide).

Activity-dependent changes in synaptic elements Step 2) Update in synaptic elements: and Activity-dependent changes: Homeostatic rule mV Low activity Ai Diexc (Axonal) presynaptic element (A)‏ (Dendritic) exc. postsynaptic element (Dexc) (Dendritic) inh. postsynaptic element (Dinh)‏ mV t High activity Diinh Ai Diexc When the activity of a neuron is low (left diagram), it expresses many postsynaptic excitatory elements (green) to increase the chance to get more (incoming) excitatory synapses and more activity. On the other hand it will have only few axons leaving. If activity increases (right), the number of postsynaptic elements is decreased and instead inhibitory postsynaptic elements are formed (red). More axons will sprout. This grounds the homeostatic behaviour of the neurons.

Activity-dependent changes in synaptic elements Step 3) Update in connectivity After reorganization Excitation Inhibition Changes Excitation Inhibition Before reorganization Inhibition Excitation Excitation Inhibition A few changes in synaptic connectivity (above) are sufficient to balance neuronal activity of all cells homeostatically. 0.5 1 si NE N #neuron Activity, late 1 0.5 si NE N Activity, early

Application to lesion-induced plasticity control si #neuron It was found that following deafferentation, cortical rewiring accompanies cortical remapping. I.e. neurons bordering the deafferented area (also called lesion projection zone, LPZ) become disinhibited because local lateral inhibition gets lost by the lesion, too. They respond with axonal sprouting as activity promotes axonal outgrowth. The consequence is that neurons bordering the LPZ reoccupy vacant dendritic targets within the LPZ and rebalance the activity of the deafferented neurons. (Darian-Smith & Gilbert, 1994, 1995)‏ deafferentation disinhibition si #neuron balanced activity si #neuron

Compensatory network rewiring after deafferentation disinhibition si #neuron When we reduce the input to a subset of neurons (Deafferentation), synapses are formed that would normally not occur during development but arise from a compensatory network rewiring. We clearly see, an axonal sprouting of neighbouring neurons. After reorganization Before reorganization Changes Excitation Inhibition

Developing network activity Average activity During development, the network periodically shows phases of strong fluctuations in activity that arise from strong synaptic rewiring ('critical periods') until network activities have reached a homeostatic equilibrium. 1 Inhibitory neurons 0,5 Excitatory neurons 0,1 T developing stable

Reorganizatzion after Deafferentation Average activity 1 Synaptic rewiring leads to a reinnervation of deafferented neurons and rebalance their activity levels. Inhibitory neurons 0,5 Excitatory neurons 0,1 T reorganizing stable Deafferented neurons