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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 was originally 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, but the exact function of the spine apparatus is still considered relatively enigmatic. ==Morphology== The spine apparatus consists of membranous saccules and tubules along with wispy filamentous material and is usually associated with a large mushroom-shaped dendritic spine. The wispy filamentous material is a cytoskeletal network which is responsible for the maintenance and alteration of spine shape and controls effectiveness of axospinous synapses. The morphology of the spine apparatus is highly indicative of and similar to the morphology and structure of the smooth surfaced endoplasmic reticulum of the dendrite. Consisting of continuous parallel flattened cisternae, the spine apparatus has a large surface area which is beneficial for its function. Benefits of the large surface area of the spine apparatus include increased electronic properties of the spine and contribution to longitudinal resistance of the cytoplasm. The spine apparatus occupies a large portion of the volume of the spine stalk, which allows it to contribute significantly to the longitudinal resistance of the cytoplasm. Therefore, the spine apparatus can have a direct effect on the membrane potential of the spine plasma membrane by variation in position and volume. The spine apparatus structure allows for dynamic changes in the surface area of the spine plasma membrane. For example, calcium-dependent mechanisms, similar to ones associated with cell shape and maintenance, have been linked to dynamic changes of spine plasma membrane surface area. These calcium-dependent mechanisms have a direct correlation with dynamic changes in dendritic spines, and hence spine plasma membrane surface area. The connection to the smooth endoplasmic reticulum also suggests a potential pathway for the transfer of proteins between the spine and dendrite. Also, the similarity between the smooth endoplasmic reticulum and the spine apparatus suggests that the spine apparatus could function as a reservoir for calcium ions. The structures of spine apparatuses differ depending on their elaboration and position in the spine. Spines can be classified into four different categories: thin, stubby, mushroom, and branched. The shape of the spine apparatus differs depending upon which spine type it is associated with. For example, when the spine apparatus is associated with thin spines, the morphology is simple and consists of a very basic tubular form; however, when the spine apparatus is associated with mushroom shaped dendritic spines, the morphology is a complex laminar arrangement of saccules in the spine head and stalk. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spine apparatus」の詳細全文を読む スポンサード リンク
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