In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength. There are several underlying mechanisms that cooperate to achieve synaptic plasticity, including changes in the amount of neurotransmitter released into a synapse and changes in how effectively cells respond to those neurotransmitters (Gaiarsa et al., 2002). Since memories are postulated to be stored in synapses of the brain, synaptic plasticity is one of the important neurochemical foundations of learning and memory (see Hebbian theory). Neuroscience is a field of study that deals with the structure, function, development, genetics, biochemistry, physiology, pharmacology, and pathology of the nervous system, divided into the central nervous system (the brain and spinal cord), and the peripheral nervous system, consisting of the myriad nerve pathways running throughout the body. ... Illustration of the major elements in a prototypical synapse. ... Drawing by Santiago Ramón y Cajal of cells in the pigeon cerebellum. ... Illustration of the major elements in a prototypical synapse. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. ... Memory is the ability of the brain to store, retain, and subsequently recall information. ... Comparative brain sizes In animals, the brain, or encephalon (Greek for in the head), is the control center of the central nervous system. ... A supervised child learning the countries of Asia on the floor of the central hall of the Field Museum, Chicago, Illinois Learning is the process of acquiring knowledge, skills, attitudes, or values, through study, experience, or teaching, that causes a change of behavior that is persistent, measurable, and specified or... Memory is the ability of the brain to store, retain, and subsequently recall information. ... Hebbian theory describes a basic mechanism for synaptic plasticity wherein an increase in synaptic efficacy arises from the presynaptic cells repeated and persistent stimulation of the postsynaptic cell. ...

Two known molecular mechanisms for synaptic plasticity were revealed by research in laboratories such as that of Eric Kandel. The first mechanism involves modification of existing synaptic proteins (typically protein kinases) resulting in altered synaptic function (Shi et al., 1999). The second mechanism depends on second messenger neurotransmitters regulating gene transcription and changes in the levels of key proteins at synapses. This second mechanism can be triggered by protein phosphorylation but takes longer and lasts longer, providing the mechanism for long-lasting memory storage. Long-lasting changes in synaptic connectivity (long-term potentiation, or LTP), between two neurons can involve the making and breaking of synaptic contacts. Eric Richard Kandel (born November 7, 1929) is a neuroscientist who won a Nobel Prize in the year 2000 for his research on the physiological basis of memory storage in neurons. ... A protein kinase is an enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation). ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ... Transcription is the process through which a DNA sequence is enzymatically copied by an RNA polymerase to produce a complementary RNA. Or, in other words, the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately... An example of long-term potentiation (LTP). ...

A synapse's strength also depends on the number of ion channels it has (Debanne et al., 2003). Several facts suggest that neurons change the density of receptors on their postsynaptic membranes as a mechanism for changing their own excitability in response to stimuli. In a dynamic process that is maintained in equilibrium, NMDA and AMPA receptors are added to the membrane by exocytosis and removed by endocytosis (Shi et al., 1999; Song and Huganir, 2002; Pérez-Otaño and Ehlers, 2005). These processes, and by extension the number of receptors on the membrane, can be altered by synaptic activity (Shi et al., 1999; Pérez-Otaño and Ehlers, 2005). Experiments have shown that AMPA receptors are delivered to the membrane due to repetitive NMDAR activation (Shi et al., 1999; Song and Huganir, 2002). The postsynaptic density (PSD) is a cytoskeletal specialization at neuronal synapses that was originally identified as an electron-dense region at the membrane of a postsynaptic neuron, as viewed by electron microscopy. ... The NMDA receptor (NMDAR) is an ionotropic receptor for glutamate (NMDA (N-methyl d-aspartate) is a name of its selective specific agonist). ... The AMPA receptor (AMPAR) is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system. ... Exocytosis is the process by which a cell is able to get rid of large molecules or materials including wastes through its membrane. ... Endocytosis is a process whereby cells absorb material (molecules such as proteins) from outside by engulfing it with their cell membrane. ...

If the strength of a synapse is only reinforced by stimulation or weakened by its lack, a positive feedback loop will develop, leading some cells never to fire and some to fire too much. But two regulatory forms of plasticity, called scaling and metaplasticity, also exist to provide negative feedback (Pérez-Otaño and Ehlers, 2005). Synaptic scaling serves to maintain the strengths of synapses relative to each other, lowering amplitudes of small excitatory postsynaptic potentials in response to continual excitation and raising them after prolonged blockage or inhibition (Pérez-Otaño and Ehlers, 2005). This effect occurs gradually over hours or days, by changing the numbers of NMDA receptors at the synapse (Pérez-Otaño and Ehlers, 2005). Metaplasticity, another form of negative feedback, reduces the effects of plasticity over time (Pérez-Otaño and Ehlers, 2005). Thus, if a cell has been affected by a lot of plasticity in the past, metaplasticity makes future plasticity less effective. Since LTP and LTD rely on the influx of Ca²⁺ through NMDA channels, metaplasticity may be due to changes in NMDA receptors, for example changes in their subunits to allow the concentration of Ca²⁺ in the cell to be lowered more quickly (Pérez-Otaño and Ehlers, 2005). Postsynaptic potentials are changes in the membrane potential of the neuron that receives information at a synapse. ... The NMDA receptor (NMDAR) is an ionotropic receptor for glutamate (NMDA (N-methyl d-aspartate) is a name of its selective specific agonist). ... Calcium plays a vital role in the anatomy, physiology and biochemistry of organisms and of the cell, particularly in signal transduction pathways. ...

See also

• Hebbian theory
• Long-term potentiation (LTP)
• Spike timing dependent plasticity (STDP)

Hebbian theory describes a basic mechanism for synaptic plasticity wherein an increase in synaptic efficacy arises from the presynaptic cells repeated and persistent stimulation of the postsynaptic cell. ... An example of long-term potentiation (LTP). ... Spike timing dependent plasticity (STDP) is a form of synaptic plasticity naturally occurring in neurons. ...

References

• Debanne D., Daoudal G., Sourdet V., and Russier M. 2003. Brain plasticity and ion channels. Journal of Physiology-Paris, 97(4-6), 403-414.
• Gaiarsa J.L., Caillard O., and Ben-Ari Y. 2002. Long-term plasticity at GABAergic and glycinergic synapses: mechanisms and functional significance. Trends in Neurosciences, 25(11), 564-570.
• Pérez-Otaño I., Ehlers M.D. 2005. Homeostatic plasticity and NMDA receptor trafficking. Trends in Neurosciences, 28(5) 229-238. <http://www.psychiatry.wustl.edu/zorumski/journal%20club/Perez-Otano%20and%20Ehlers%209_23.pdf>
• Shi S.H., Hayashi Y., Petralia R.S., Zaman S.H., Wenthold R., Svoboda K., Malinow R. 1999. Rapid spine delivery and redistribution of AMPA receptors after synaptic NMDA receptor activation. Science, 284(5421), 1811-1816. <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10364548&dopt=Citation>
• Song, I., Huganir R.L. 2002. Regulation of AMPA receptors during synaptic plasticity. Trends in Neurosciences, 25(11), 578-589. <http://www.ingentaconnect.com/content/els/01662236/2002/00000025/00000011/art02270>

External link

• Overview