Formation of this " spine apparatus" depends on the protein synaptopodin and is believed to play an important role in calcium handling. Stacked discs of the smooth endoplasmic reticulum (SERs) have been identified in dendritic spines. In addition to their electrophysiological activity and their receptor-mediated activity, spines appear to be vesicularly active and may even translate proteins. Overactive Rac1 results in consistently smaller dendritic spines. The actin cytoskeleton directly determines the morphology of the spine, and actin regulators, small GTPases such as Rac, RhoA, and CDC42, rapidly modify this cytoskeleton. Because spines have a cytoskeleton of primarily actin, this allows them to be highly dynamic in shape and size. tubulin Monomers and microtubule-associated proteins (MAPs) are present, and organized microtubules are present. The cytoskeleton of dendritic spines is primarily made of filamentous actin ( F-actin). These changes in shape might affect the electrical properties of the spine. The cytoskeleton of dendritic spines is particularly important in their synaptic plasticity without a dynamic cytoskeleton, spines would be unable to rapidly change their volumes or shapes in responses to stimuli. Hippocampal and cortical pyramidal neurons may receive tens of thousands of mostly excitatory inputs from other neurons onto their equally numerous spines, whereas the number of spines on Purkinje neuron dendrites is an order of magnitude larger. Dendritic spines occur at a density of up to 5 spines/1 μm stretch of dendrite. Spines are found on the dendrites of most principal neurons in the brain, including the pyramidal neurons of the neocortex, the medium spiny neurons of the striatum, and the Purkinje cells of the cerebellum. Excitatory axon proximity to dendritic spines is not sufficient to predict the presence of a synapse, as demonstrated by the Lichtman lab in 2015. The variable spine shape and volume is thought to be correlated with the strength and maturity of each spine-synapse.ĭendritic spines usually receive excitatory input from axons, although sometimes both inhibitory and excitatory connections are made onto the same spine head. Electron microscopy studies have shown that there is a continuum of shapes between these categories. The most notable classes of spine shape are "thin", "stubby", "mushroom", and "branched". Spines with strong synaptic contacts typically have a large spine head, which connects to the dendrite via a membranous neck. 2.3.2 Sustained changes in structural plasticityĭendritic spines are small with spine head volumes ranging 0.01 μm 3 to 0.8 μm 3.2.3.1 Transient changes in structural plasticity.2.3 Observed changes in structural plasticity.
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