A dendritic spine is a small (sub-micrometre) membranous extrusion that protrudes from a dendrite and forms one half of a synapse. Typically spines have a bulbous head (the spine head) which is connected to the parent dendrite through a thin spine neck. Dendritic spines are found on the dendrites of most principal neurons in the brain including cortical pyramidal neurons, medium spiny neurons of the striatum and Purkinje cells in the cerebellum. 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.
Spines come in a variety of shapes and have been categorized accordingly, e.g. mushroom spines, thin spines and stubby spines. Electron microscopy studies have shown that there is a continuum of shapes between these categories. There is some evidence that differently shaped spines reflect different developmental stages and also strengths of a synapse. Using two-photon laser scanning microscopy and confocal microscopy, it has been shown that the volume of spines can change depending on the types of stimuli that are presented to a synapse. Also using the same technique, time-lapse studies in the brains of living animals have shown that spines come and go, with the larger mushroom spines being the most stable over time.
Dendritic spines as biochemical compartments
Spines are believed to restrict diffusion of ions and second messengers from the synapse to the dendrite. As such, they form biochemical compartments that can encode changes in the state of an individual synapse without necessarily affecting the state of other synapses of the same neuron.
Role in synaptic plasticity
Changes in dendritic spine density underlie many brain functions, including motivation, learning, and memory. In particular, long-term memory is mediated in part by the growth of new dendritic spines (or the enlargement of pre-existing spines) to reinforce a particular neural pathway. By strengthening the connection between two neurons, the ability of the presynaptic cell to activate the postsynaptic cell is enhanced. This type of synaptic regulation forms the basis of synaptic plasticity.
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