Dendritic Spines

Abstract

Dendritic spines are small protrusions from dendrites that host the majority of excitatory synapses in the central nervous system. Structurally, excitatory synapses located on dendritic spines are asymmetric, with a presynaptic bouton that contains round clear vesicles apposed to a postsynaptic density where glutamate receptors are anchored. Spines form biochemical compartments that can isolate activated signalling pathways that occur at one synapse from those at other synapses, thereby providing a way to enhance the specificity of connections between neurons in the brain. Structural changes of dendritic spines in response to stimulation facilitate changes in synaptic strength, and these changes are likely to underlie important higher brain functions such as learning and memory. Dysregulation of spine morphology is seen in several neurological disorders such as Alzheimer's disease and fragile X syndrome.

Key Concepts

  • Spines are the primary site of excitatory synapses.
  • Smooth endoplasmic reticulum can form complex structures localized to large spines.
  • The cytoskeleton of spines is mostly composed of actin.
  • Trans‐synaptic adhesion molecules as well as proteins secreted from astroglia help stabilise synapses.
  • Spines can act as small biochemical compartments.
  • Spine structure is responsive to activity.

Keywords: brain; neuron; dendrite; synapse; ultrastructure; plasticity; spine

Figure 1. Pyramidal cell in hippocampal area CA1 of the rat brain showing the typical appearance of the principal excitatory neurons that occur throughout the brain. (a) The low‐magnification view of these cells shows an apical dendrite projecting towards the upper right quadrant of the figure. Many lateral dendrites emerge from the large apical dendrite. Several basilar dendrites project from the base of the cell soma. Dendritic spines are the tiny projections that stud the surface of these dendrites in both the apical and basilar dendritic arbour. (b) A higher magnification view of the dendrites reveals the tiny spines. A thin axon (arrow) is passing through the dendritic arbour. Reproduced with permission from Bell et al. (2014)©John Wiley and Sons Ltd.
Figure 2. (a) Electron micrograph of a section through dendritic spines in stratum radiatum of hippocampal area CA1 that has been colour coded to identify dendrites (yellow), axons (green) and astroglia (purple). In this fortuitous section, three spines were sectioned parallel to their longitudinal axis revealing spines of the stubby (S), mushroom (m) and thin (t) morphologies. The postsynaptic density (PSD) occurs on the spine head (see t) immediately adjacent the synaptic cleft (c) and to a presynaptic axonal bouton that is filled with round vesicles (VES). This t spine contains a small tube of smooth endoplasmic reticulum (SER) in its neck. In the m spine, a spine apparatus (SA) is visible. A perforated postsynaptic density (PERF) is evident on the head of another mushroom spine. Near to this spine is a large astrocytic process (AST) identified by the glycogen granules and clear cytoplasm. (b) A three‐dimensional reconstruction of a dendrite showing a variety of spine and synapse shapes and the presence of polyribosomes (black spheres) at the base of the spines. Scale cube = 1 µm3. Reproduced with permission from Watson et al. (2016) © John Wiley and Sons Ltd.
Figure 3. (a) Spines increase the packing density of synapses. Schematic illustrates a cross‐section through two dendrites, one without and one with dendrites spines. Convolution and interdigitation of dendrite, axon and spine membranes support more synapses. (b) The presence of spines also allows for an increase in synaptic density without increasing the overall volume of the brain. (c) Spines exist to amplify electrical potential at the synapse and promote associativity among neighbouring synapses. Spine shape and resistance of the spine neck may influence potential (V) generated by synaptic activation. (d) Spines exist as molecular compartments. Smooth endoplasmic reticulum (tubules), calcium and a myriad of other signalling mechanisms (stippling) are recruited in response to synaptic activation (asterisk). Reproduced with permission from Bourne and Harris (2010) © John Wiley and Sons Ltd.
Figure 4. (a) Long‐term potentiation (LTP) demonstrated by enhanced synaptic response following a brief, high‐frequency stimulation (arrow) in the stratum radiatum of CA1 in the hippocampus. (b) Hippocampal slice preparation with stimulating electrode activating CA3 Schaeffer collaterals and recording electrode in the stratum radiatum of CA1. (c) LTP in the adult results in larger spines. Total synaptic area is balanced by a loss in small spines. (d) LTP in young animals (P15) results in an increase in spine density and an overall increase in synaptic area. (e) Nascent zones (blue) are decreased initially after induction of LTP (30 min) but at 2 h are increased.
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References

Bell ME, Bourne JN, Chirillo MA, et al. (2014) Dynamics of nascent and active zone ultrastructure as synapses enlarge during long‐term potentiation in mature hippocampus. The Journal of Comparative Neurology 522: 3861–3884.

Bourne JN and Harris KM (2010) Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP. Hippocampus 21: 354–373.

Bourne JN and Harris KM (2012) Nanoscale analysis of structural synaptic plasticity. Current Opinion in Neurobiology 3: 372–382.

Buffington SA, Huang W and Costa‐Mattioli M (2014) Translational control in synaptic plasticity and cognitive dysfunction. Annual Review of Neuroscience 37: 17–38.

Cao G and Harris KM (2014) Augmenting saturated LTP by broadly spaced episodes of theta‐burst stimulation in hippocampal area CA1 of adult rats and mice. Journal of Neurophysiology 112: 1916–1924.

Singh SK, Stoqsdill JA, Pulimood NS, et al. (2016) Astrocytes assemble thalamocortical synapses by bridging NRX1α and NL1 via hevin. Cell 164: 183–196.

Watson DJ, Ostroff L, Cao G, et al. (2016) LTP enhances synaptogenesis in the developing hippocampus. Hippocampus 26 (5): 560–576.

Further Reading

Bailey CH, Kandel ER and Harris KM (2015) Structural components of synaptic plasticity and memory consolidation. Cold Spring Harbor Perspectives in Biology 7 (7): a021758.

Dzyubenko E, Gottschling C and Faissner A (2016) Neuron‐Glia interactions in neural plasticity: contributions of neural extracellular matrix and perineuronal nets. Neural Plasticity 2016: Article ID 5214961.

Granger AJ and Nicoll RA (2013) Expression mechanisms underlying long‐term potentiation: a postsynaptic view, 10 years on. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 369 (1633): 20130136.

Harris KM and Stevens JK (1988) Dendritic spines of rat cerebellar Purkinje cells: serial electron microscopy with reference to their biophysical characteristics. Journal of Neuroscience 8: 4455–4469.

Harris KM and Kater SB (1994) Dendritic spines: cellular specializations imparting both stability and flexibility to synaptic function. Annual Review of Neuroscience 17: 341–371.

Harris KM and Stevens JK (1989) Dendritic spines of CA1 pyramidal cells in the rat hippocampus: serial electron microscopy with reference to their biophysical characteristics. Journal of Neuroscience 9: 2982–2997.

Nimchinsky EA, Sabatini BL and Svoboda K (2002) Structure and function of dendritic spines. Annual Review of Physiology 64: 313–353.

Peters A, Palay SL and deF Webster H (1991) The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd edn. New York: Oxford.

Sorra KE and Harris KM (2000) Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines. Eichenbaum HB (ed.) with Harris KM and Sorra KE (special issue eds) Dendritic Spines of the Hippocampus. Hippocampus 10: 501–511.

Stuart G, Spruston N and Hausser M (eds) (2001) Dendrites. Oxford: New York.

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Kirk, Lyndsey M, and Harris, Kristen M(Nov 2016) Dendritic Spines. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000093.pub3]