|M.Sc Student||Gordon Urit|
|Subject||Different Plasticity Mechanisms in Neocortical Pyramidal|
Neurons: Role of NMDA Spike
|Department||Department of Medicine||Supervisor||Professor Jackie Schiller|
|Full Thesis text|
The cortex is the largest structure of the brain responsible for basic sensory and motor processing and for higher cognitive functions. Understanding the basic integrative and plasticity rules in the single neuron level bares importance to understanding of the cortical network function. Memory and learning are elementary capabilities of the brain; both involve long term changes in the neuronal network as result of previous meaningful activity. It is widely accepted that the mechanisms responsible for acquisition of new memories involve changes in the synapse level (strengthening or weakening of synaptic contacts). This research focuses on mechanisms of synaptic strength changes in cortical pyramidal neurons.
Most current common models of neuronal plasticity are based on the Hebbian postulate stating that if the activity of a neuron helps consistently to fire a target neuron than these active connections will strengthen. This general idea received a "modern" interpretation in the form of Spike Timing Dependent Plasticity rules (STDP). We show here that in addition to STDP other mechanisms prevail in pyramidal neurons of the cortex.
The main questions this research raise are; first, what is the role of synaptic morphological location on rules governing its plasticity? And second, to what extent do local dendritic spikes are involved in plasticity mechanisms in the basal dendritic tree.
This work describes two main findings: 1. Plasticity rules in neocortical pyramidal neurons are location dependent. We find that while synapses contacting the proximal basal tree changes easily using STDP rules, synapses contacting the distal basal tree are resistant to change using the global activity of the neuron. This implies that different pathways may have different plasticity rules. 2. We find that the distally located synapses preserve their ability to change but under special conditions. We find that in order for distally located synapses to change local spiking mechanism (NMDA-spike) has to coincide with a neuromodulator molecule-BDNF. Thus this kind of plasticity may belong to family of supervised learning mechanisms. This plasticity form is different form STDP not only by the electrical signal needed and the necessity in gating molecule but in additional aspects such as the temporal characteristics, the flexibility and/or stability under ongoing neural background activity, and more.
In conclusion we describe location dependent synaptic plasticity rules in neocortical pyramidal neurons. This newly-found form of plasticity, together with its special characteristics, adds a new aspect to the field of synaptic plasticity and neural network dynamics.