Synapticplasticity is regarded as the cellular mechanism underlying the refinement of
neural connections during development and learning/memory functions in adults.
Alterations in dendritic spine morphology (elongation or shrinkage) and/or
spine density occur with synaptic plasticity. This structural modification has
been proposed to enable persistent, long-term change in synapses. Here we
review spine modifications associated with synaptic plasticity and discuss
their contributions to synaptic plasticity and brain diseases.
Dendritic spines are small postsynaptic structures protruding
from dendrites and the primary site of excitatory input. About 90% of
excitatory synapses occur on spines on the excitatory neurons in the adult
cortex. Spines are usually divided into three types based on the size,
spine head shape, and spine neck length. Mushroom spines have large heads and
constricted narrow necks, thin spines have small heads and slender necks, while
stubby spines have no distinct heads and necks. This categorization is to
provide an easy classification while the actual distribution of spine shapes is
of a continuum.
The recent development of two-photon imaging allows spine
morphology and dynamics to be studied in great detail using timelapse and
repetitive imaging, and has enabled the study of spine alterations in response
to physiological or pathological events. For example, dendritic spines
are dynamic in genesis and elimination, especially during brain development. In
adolescence, spines show much higher elimination than formation which results
in net spine loss or pruning. However in the adult, the rate of spine genesis
and elimination is much reduced and roughly equivalent, which maintains the
stability of spine density.
Spines are considered a unique calcium
compartment. Spine plasticity is demonstrated by their capacity to undergo
both rapid (seconds) and persistent (months to years) alterations in response
to either physiological or pathological events. Large spines may be the storage
site of stable long-term memory, while filopodia are considered as the
immature form of spine which can be transformed into mature spines or
eliminated. Spine plasticity is exhibited in two forms: changes in spine
morphology/size and changes in spine density. Both changes reflect altered
synaptic connections and strength. In neurodegenerative and psychiatric
diseases, spine density and morphology are altered, and these changes may
partially account for alterations in brain functions associated with these
diseases. Thus a better understanding of spine pathology may provide better
therapeutic intervention.
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