Dendritic spine
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[[Image:Dendritic spines.jpg|thumb|''Spiny dendrite of a [[striatum|striatal]] medium spiny neuron.'']]
<!-- Deleted image removed: [[Image:Dendritic spine 1.jpg|thumb|Dendritic spines (some colored) protruding from a dendritic stalk.]] -->
[[Image:Spinetypeswikipedia.png|thumb|''Common types of dendritic spines.(click to enlarge)'']]
A '''dendritic spine''' (or spine) is a small membranous protrusion from the central stalk of a neuron's [[dendrite]] that typically receives input from a single axon. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron's cell body. Most spines have a bulbous head (the spine head), and a thin neck that connects the head of the spine to the stalk of the dendrite. The dendrites of a single neuron can contain from thousands up to a few hundred thousand spines. In addition to spines providing an anatomical substrate for memory storage and synaptic transmission, they may also serve to increase the number of possible contacts between neurons.
==Distribution==
Dendritic spines usually receive excitatory input from axons (e.g. via the neurotransmitter glutamate), although sometimes both inhibitory (e.g. via the neurotransmitter GABA) and excitatory connections are made onto the same spine head.
Spines are found on the [[dendrite]]s of most principal [[neuron]]s in the brain, including the [[Pyramidal cell|pyramidal neurons]] of the [[neocortex]], the medium spiny neurons of the [[striatum]], and the [[Purkinje cell]]s of the [[cerebellum]].
Dendritic spines occur at a density of up to 20 spines/10 [[micrometer]] stretch of dendrite. [[Hippocampus|Hippocampal]] and cortical [[pyramidal cell|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.
==Morphology==
Dendritic spines are small with spine head volumes ranging 0.01 micrometer<sup>3</sup> to 0.8 micrometer<sup>3</sup>. Spines with strong synaptic contacts typically have a large spine head, which connect to the dendrite via a membranous neck. The most notable classes of spine shape are "thin", "stubby", "mushroom", and "branched". [[Electron microscopy]] studies have shown that there is a continuum of shapes between these categories. The variable spine shape and volume is thought to be correlated with the strength and maturity of each spine-synapse.
==Biochemistry==
===Receptor activity===
Dendritic spines express glutamate receptors (e.g. [[AMPA receptor]] and [[[NMDA receptor]]) on their surface. The [[TrkB]] receptor for [[BDNF]] is also expressed on the spine surface, and is believed to play a role in spine survival. The tip of the spine contains an electrondense region referred to as the "[[postsynaptic density]]" (PSD). The PSD directly apposes the [[active zone]] of its synapsing axon and comprises ~10% of the spine's membrane surface area; neurotransmitters released from the active zone bind receptors in the postsynaptic density of the spine. One-half of the synapsing axons and dendritic spines are physically tethered by [[calcium]]-dependent [[cadherin]], which forms cell-to-cell adherent junctions between two neurons.
Glutamate receptors (GluRs) are localized to the postsynaptic density, and are anchored by cytoskeletal elements to the membrane. They are positioned directly above their signaling machinery, which is typically tethered to the underside of the plasma membrane, allowing signals transmitted by the GluRs into the [[cytosol]] to be further propagated by their nearby signaling elements to activate [[signal transduction cascade]]s. The localization of signaling elements to their GluRs is particularly important in ensuring signal cascade activation, as GluRs would be unable to effect particular downstream effects without nearby signalers.
Signaling from GluRs is mediated by the presence of an abdundance of proteins, especially kinases, that are localized to the postsynaptic density. These include [[calcium]]-dependent [[calmodulin]], CaMKII (calmodulin-dependent protein kinase II), [[Protein Kinase C|PKC]] (Protein Kinase C), [[Protein Kinase A|PKA]] (Protein Kinase A), Protein Phosphatase-1 (PP-1), and Fyn tyrosine kinase. Certain signalers, such as CaMKII, are upregulated in response to activity.
[[smooth endoplasmic reticulum|Smooth endoplasmic reticulae]] present in the dendritic spine are responsible for the release of calcium, which is important in signaling transduction cascades in response to receptor activity.
Spines are particularly advantageous to neurons by compartmentalizing biochemical signals; Electrical activation of one spine causes compartmentalized biochemical signal transduction, that usually does not spread to neighboring synapses. This can help to encode changes in the state of an individual synapse without necessarily affecting the state of other synapses of the same neuron. The length and width of the spine neck has a large effect on the degree of compartmentalization, with thin spines being the most biochemically isolated spines.
===Cytoskeleton===
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. These changes in shape results in changing the electrical properties of the spine. The cytoskeleton of dendritic spines is primarily made of filamentous actin ([[F-actin]]). While [[tubulin]] monomers and [[microtubule-associated protein]]s (MAPs) are present, organized [[microtubule]]s are not present. Because spines have a cytoskeleton of primarily actin, this allows them to be highly dynamic in shape and size. The actin cytoskeleton directly determines the morphology of the spine, and actin regulators, small [[GTPase]]s such as [[Rac]], [[Rho]], and [[CDC42]], rapidly modify this cytoskeleton. Overactive [[Rac1]] results in consistently smaller dendritic spines.
In addition to their electrophysiological activity and their receptor-mediated activity, spines appear to be vesicularly active and may even translate [[protein]]s. Stacks of [[smooth endoplasmic reticulum|smooth endoplasmic reticulae]] (SERs) bound by an unknown amorphous material, named the "spine apparatus", have been identified in dendritic spines, and are believed to be implicated in [[Vesicle (Biology)|vesicular trafficking]] in spines. "Smooth" [[vesicle]]s have also been identified in spines, supporting the vesicular activity in dendritic spines. The presence of [[polyribosome]]s in spines also suggests protein translational activity in the spine itself, not just the dendrite's stalk.
==Plasticity==
{{Seealso|Synaptic plasticity}}
As aforementioned, dendritic spines are very "plastic", that is, spines change significantly in shape, volume, and number in small time courses. Because spines have a primarily [[actin]] [[cytoskeleton]], they are dynamic, and the majority of spines change their shape within seconds to minutes because of the dynamicity of actin remodelling. Furthermore, spine number is very variable and spines come and go; in a matter of hours, 10-20% of spines can spontaneously appear or disappear on the pyramidal cells of the cerebral cortex, although the larger "mushroom"-shaped spines are the most stable.
Spine maintenance and plasticity is activity-dependent and activity-independent. [[BDNF]] partially determines spine levels, and low levels of [[AMPA receptor]] activity is necessary to maintain spine survival, and synaptic activity involving [[NMDA receptor]]s encourages spine growth. Furthermore, [[Two-photon excitation microscopy|two-photon laser scanning microscopy]] and confocal microscopy have shown that spine volume changes according depending on the types of stimuli that are presented to a synapse.
Spine plasticity is implicated in [[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]].
==Electrotonic properties==
[[electrotonic potential|Electrotonic conduction]] refers to the passive conduction of current. Dendritic spines have a number of specific electrotonic properties. A dendritic spine has high input [[electrical resistance|resistance]], the resistance increases with smallness of headsize and narrowness of stemsize. The [[capacitance]] of the [[cell membrane|membrane]]s of spines is relatively small with the result that synaptic potentials can be relatively fast. The capacitance of the whole dendrite however becomes higher as the number of spines increases. Because there is an [[impedance matching|impedance mismatch]] between the dendritic spine and the dendrite, it is necessary with active signal boosting. The impedance mismatch also causes the spine to follow the potential of the parent dendrite.
==Modelling==
Theoreticians have for decades hypothesized about the potential electrical function of spines, yet our inability to examine their electrical properties, has until recently stopped theoretical work from progressing too far. Recent advances in imaging techniques along with increased use of two-photon glutamate uncaging have lead to a wealth of new discoveries; we now know that there are voltage-dependent sodium<ref>R. Araya, V. Nikolenko, K.B. Eisenthal, and R. Yuste, Sodium channels amplify spine potentials</ref>, potassium<ref>T.J. Ngo-Anh, B.L. Bloodgood, M. Lin, B.L. Sabatini, J. Maylie, and J.P. Adelman, SK channels and NMDA receptors form a Ca2+-mediated feedback loop in dendritic spines</ref> and calcium<ref>R. Yuste, and W. Denk, Dendritic spines as basic functional units of neuronal integration, Nature, vol. 375, pp. 682-684, 1995</ref> channels in the spine heads.
[[Cable theory]] provides the theoretical framework behind the most "simple" method for modelling the flow of electrical currents along passive neural fibres. Each spine can be treated as two compartments, one representing the neck, the other representing the spine head. The compartment representing the spine head alone should carry the active properties.
===Baer and Rinzel's Continuum Model===
To facilitate the analysis of interactions between many spines, Baer & Rinzel formulated a new cable theory for which the distribution of spines is treated as a continuum<ref>S.M. Baer, and J. Rinzel, Propagation of dendritic spikes mediated by excitable spines: a continuum theory, Journal of Neurophysiology, vol. 65, pp. 874-890, 1991</ref>. In this representation, spine head voltage is the local spatial average of membrane potential in adjacent spines. The formulation maintains the feature that there is no direct electrical coupling between neighboring spines; voltage spread along dendrites is the only way for spines to interact.
===The Spike-Diffuse-Spike Model===
The SDS model was intended as a computationally simply version of the full Baer and Rinzel model<ref>S.Coombes, and P.C.Bressloff, Solitary Waves in a Model of Dendritic Cable with Active Spines, SIAM Journal on Applied Mathematics</ref>. It was designed to be analytically tractable and have as few free parameters as possible while retaining those of greatest significance, such as spine neck resistance. The model drops the continuum approximation and instead uses a passive dendrite coupled to excitable spines at discrete points. Membrane dynamics in the spines are modelled using integrate and fire processes. The spike events are modelled in a discrete fashion with the wave form conventionally represented as a rectangular function.
==Development==
Dendritic spines are believed to develop from [[filopodia]]. During [[synaptogenesis]], dendrites rapidly sprout and retract filopodia, small membrane organelle-lacking membranous protrusions. During the first week of birth, the brain is predominated by filopodia, which eventually develop synapses. However, after this first week, filopodia are replaced by aspiny dendrites but also small, stubby spines that protrude from spiny dendrites. In the development of certain filopodia into spines, filopedia recruit presynaptic contact to the dendrite, which encourages the production of spines to handle specialized postsynaptic contact with the presynaptic protrusions.
Spines, however, require maturation after formation. Immature spines have impaired signaling capabilities, and typically lack "heads" (or have very small heads), only necks, while matured spines maintain both heads and necks.
== Pathology ==
Cognitive disorders such as [[autism]], [[mental retardation]], and [[Fragile X Syndrome]], may be resultant from abnormalities in dendritic spines, especially the number of spines and their maturity. The ratio of matured to immature spines is important in their signaling, as immature spines have impaired synaptic signaling. Fragile X Syndrome is characterized by an overabundance of immature spines that have multiple filopodia in cortical dendrites.
== References ==
===General Sources===
*Sudhof TC, Stevens CF, Cowan WM. ''Synapses''. The Johns Hopkins University Press, Baltimore (2001). ISBN 0-8018-6498-4
*Levitan IB, Kaczmarek LK. ''The Neuron: Cell and Molecular Biology, Third Edition''. Oxford University Press, New York (2002). ISBN 0-19-514522-4
*Plummer M, Page C, Hsu SC, Firestein B, Davis R. ''Advanced Neurobiology I/Neuroscience Lecture Notes'' (unpublished).
===External links===
* {{cite journal | author = Nimchinsky E, Sabatini B, Svoboda K | title = Structure and function of dendritic spines | journal = Annu Rev Physiol | volume = 64 | issue = | pages = 313–53 | year = 2002 | pmid = 11826272 | doi = 10.1146/annurev.physiol.64.081501.160008}}
* {{cite journal | author = Matsuzaki M, Honkura N, Ellis-Davies G, Kasai H | title = Structural basis of long-term potentiation in single dendritic spines | journal = Nature | volume = 429 | issue = 6993 | pages = 761–6 | year = 2004 | pmid = 15190253 | doi = 10.1038/nature02617}}
* {{cite journal | author = Yuste R, Majewska A, Holthoff K | title = From form to function: calcium compartmentalization in dendritic spines | journal = Nat Neurosci | volume = 3 | issue = 7 | pages = 653–9 | year = 2000 | pmid = 10862697 | doi = 10.1038/76609}}
* {{cite journal | author = Lieshoff C, Bischof H | title = The dynamics of spine density changes | journal = Behav Brain Res | volume = 140 | issue = 1-2 | pages = 87–95 | year = 2003 | pmid = 12644282 | doi = 10.1016/S0166-4328(02)00271-1}}
* {{cite journal | author = Kasai H, Matsuzaki M, Noguchi J, Yasumatsu N | title = Dendritic spine structures and functions | journal = Nihon Shinkei Seishin Yakurigaku Zasshi | volume = 22 | issue = 5 | pages = 159–64 | year = 2002 | pmid = 12451686}}
* {{cite journal | author = Lynch G, Rex CS, Gall CM | title = LTP consolidation: substrates, explanatory power, and functional significance | journal = Neuropharmacology | volume = 52 | issue = 1 | pages = 12–23 | year = 2007 | pmid = 16949110 | doi = 10.1016/j.neuropharm.2006.07.027}}
{{Nervous tissue}}
[[Category:Neurons]]
[[Category:Computational neuroscience]]
[[Category:Neuroscience]]
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[[ru:Дендритный шипик]]
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