Biology Reference
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(Nimchinsky et al., 2002); moderately increased levels of excitatory synaptic activity can
induce spine formation, but that excessive and unrestrained activation can cause excitotoxic
loss of spines.
The majority of spines contain one continuous PSD, representing one synaptic contact.
Nevertheless, the PSD of some spines appears interrupted and therefore perforated, and it was
suggested that these perforated synapses represent an intermediate step during spine division
and are thus involved in the process of synaptogenesis (Geinisman, 1993).
It has been proposed that some brain processes like learning and memory may elicit the
conversion of some morphological subtypes of synapses into others (Chang and Greenough,
1984). The existence of distinct morphological subtypes of axospinous synapses is consistent
with this idea and suggests a hypothetical model of synapse restructuring that may account for
synaptic plasticity associated with the induction phase of LTP. According to this model, LTP
induction initiates a sequence of structural synaptic modifications, which initiates with the
enlargement of typical small-nonperforated synapses and their conversion into atypically
large nonperforated ones. This is followed by the consecutive formation of perforated
synapses that have initially a focal spine partition with a fenestrated post synaptic density, and
finally a complete partition with a segmented post synaptic density. Synapses of the latter
subtype have multiple transmission zones instead of only a single one, and an increase in their
number after LTP induction, may result in an augmentation of synaptic transmission
(Calverley and Jones, 1990).
Figure 4. The typical perforated synapses can be distinguished with a rupture toward the presynaptic
terminal membrane (arrow), and the presynaptic density is in association to the spine apparatus (*);
Den: dendrite; Sp: spine; At: presynaptic button or axon terminal.
Ultrastructuraly, perforated synapses are characterized by a perforation (fenestration)
extended through the paramembranous densities, from the presynaptic terminal across the
cleft and into the postsynaptic process. This type of synapses has been reported to increase in
number during development (Itarat and Jones, 1992) and under various experimental
conditions such as complex environments (Sirevaag and Greenough, 1985), behavioral
training (Vrensen and Nunez, 1981), repetitive electrical stimulation (Geinisman et al., 1988),
