Spine apparatus

The spine apparatus (SA) 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.

Morphology
In CA1 pyramidal cells of the hippocampus, 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.

Function
Dendritic spines in cortical neurons contain mainly ryanodine receptors, ion channels that are capable of releasing calcium from the endoplasmic reticulum. 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. However a mechanistic link between the SA and synaptic plasticity has not yet been established.