In general terms, spines seem to be maintained by an 'optimal' level of synaptic activity: spine density increases when there is insufficient activity, and decreases when stimulation is excessive. Indeed, changes in spine density have been observed in response to changes in the efficacy of neurotransmission. Regulated changes in spine number might reflect mechanisms for converting transient changes in synaptic activity into long-lasting alterations. So, the filopodium–spine transition is unlikely to be a predestined process, but instead one that is reversible and regulated by factors such as synaptic activity. However, a simple developmental relationship between filopodia and spines does not seem to exist. What is the significance of dendritic spines? There is no definitive answer to this question, but the prevailing view is that their primary function is to provide a microcompartment for segregating postsynaptic chemical responses, such as elevated calcium.Äendritic filopodia are widely believed to be the precursors of dendritic spines. In addition, most spines exhibit a single, continuous postsynaptic density (PSD), but some PSDs are discontinuous or perforated. However, spine morphology is not static spines change size and shape over variable timescales. Spines have been classified by shape as thin, stubby, mushroom- and cup-shaped. So, spines represent the main unitary postsynaptic compartment for excitatory input. Most excitatory synapses in the mature mammalian brain occur on spines. Dendritic spines are morphological specializations that protrude from the main shaft of dendrites.
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