Spine Apparatus - Morphology

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.