Spine apparatus
Encyclopedia
The spine apparatus is a specialized form of endoplasmic reticulum
(ER) that is found in a subpopulation of dendritic spines in central neurons. It was discovered by E. G. Gray in 1959 when he applied electron microscopy to fixed cortical tissue. The SA consists of a series of stacked discs that are thought to be connected to each other and to the dendritic system of ER-tubules. The actin binding protein synaptopodin (which has originally been described in podocytes of the kidney) is an essential component of the SA. Mice that lack the gene for synaptopodin do not form a spine apparatus. The SA is believed to play a critical role in learning and memory.
, a spine apparatus is found in about 20% of all dendritic spines. The SA is mostly found in large mushroom-shaped spines which are thought to carry strong synapses. Not all spiny cells form a SA: Purkinje cells of the cerebellum
, for example, have many dendritic spines, but no spine apparatus.
s and IP3 receptors). Therefore, it has long been speculated that the spine apparatus might be involved in calcium signaling inside the spine. A knockout mouse
lacking synaptopodin provided the first evidence for an involvement of the spine apparatus in synaptic plasticity. These mice, which had no spine apparatus, had impaired long-term potentiation
in the hippocampus and also deficits in spatial learning. Recent studies have shown that synapses on spines with a spine apparatus are stronger and display enhanced long-term plasticity. A specific form of long-term depression
(mGluR-dependent LTD) is only supported by spines with spine apparatus and seems to be triggered by calcium release from this structure. In summary, an important function of the spine apparatus is the regulation of plasticity at individual synapses, a process known as metaplasticity
.
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
(ER) that is found in a subpopulation of dendritic spines in central neurons. It was discovered by E. G. Gray in 1959 when he applied electron microscopy to fixed cortical tissue. The SA consists of a series of stacked discs that are thought to be connected to each other and to the dendritic system of ER-tubules. The actin binding protein synaptopodin (which has originally been described in podocytes of the kidney) is an essential component of the SA. Mice that lack the gene for synaptopodin do not form a spine apparatus. The SA is believed to play a critical role in learning and memory.
Morphology
In CA1 pyramidal cells of the hippocampusHippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, a spine apparatus is found in about 20% of all dendritic spines. The SA is mostly found in large mushroom-shaped spines which are thought to carry strong synapses. Not all spiny cells form a SA: Purkinje cells of the cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, for example, have many dendritic spines, but no spine apparatus.
Function
Dendritic spines in cortical neurons contain ion channels that are capable of releasing calcium from the endoplasmic reticulum (ryanodine receptorRyanodine receptor
Ryanodine receptors form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons...
s and IP3 receptors). Therefore, it has long been speculated that the spine apparatus might be involved in calcium signaling inside the spine. A knockout mouse
Knockout mouse
A knockout mouse is a genetically engineered mouse in which researchers have inactivated, or "knocked out," an existing gene by replacing it or disrupting it with an artificial piece of DNA...
lacking synaptopodin provided the first evidence for an involvement of the spine apparatus in synaptic plasticity. These mice, which had no spine apparatus, had impaired long-term potentiation
Long-term potentiation
In neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...
in the hippocampus and also deficits in spatial learning. Recent studies have shown that synapses on spines with a spine apparatus are stronger and display enhanced long-term plasticity. A specific form of long-term depression
Long-term depression
Long-term depression , in neurophysiology, is an activity-dependent reduction in the efficacy of neuronal synapses lasting hours or longer. LTD occurs in many areas of the CNS with varying mechanisms depending upon brain region and developmental progress...
(mGluR-dependent LTD) is only supported by spines with spine apparatus and seems to be triggered by calcium release from this structure. In summary, an important function of the spine apparatus is the regulation of plasticity at individual synapses, a process known as metaplasticity
Metaplasticity
Metaplasticity is a term originally coined by W.C. Abraham and M.F. Bear to refer to the plasticity of synaptic plasticity. Until that time synaptic plasticity had referred to the plastic nature of individual synapses. However this new form referred to the plasticity of the plasticity itself, thus...
.