Dendritic Spines

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Certain dendrites possess fingerlike extensions, 1-2 mm in length, called dendritic spines. In low magnification images, a high density of dendritic spines gives dendrites a fuzzy appearance (Figs. 1 and 2). Spiny protrusions are also found on the cell soma and on the axon initial segment, but the dendritic spines are more numerous and have received the most study. Neurons that have large numbers of spines on their dendrites are referred to as spiny neurons, whereas those with smooth dendrites or few dendritic spines are termed aspiny. Dendritic spines are one of the most intensively studied structures in the central nervous system because they receive the bulk of excitatory synaptic input and are thought to be key sites of synaptic plasticity, the process by which neurons are able to modify their properties in response to activity.

Dendritic spines vary widely in their size and morphology between classes of neurons and even within the same neuron (Fig. 7). The prototypical spine, seen in the hippocampus, neostriatum, cerebral cortex, and cerebellar cortex, consists of a thin stalk or neck, ~100nm in diameter, which arises from the main dendritic shaft, and a larger, rounder head. Dendritic spines can be distinguished from the main dendrite by the lack of mitochondria and microtubules, although some of the larger spines found in the olfactory bulb and hippocampal area CA3 contain both of these structures. Dendritic spines have very high concentrations of microfilaments, in particular filamentous actin. The majority of spines receive only a single synaptic contact onto the spine head, which usually uses the neurotransmitter glutamate. In the cerebral cortex and the neostriatum, about 20% of the spines receive a second synapse onto the spine neck, which is inhibitory or modulatory in action. The only other organelle seen with any regularity in the spine is the smooth endoplasmic reticulum, although coated and uncoated vesicles, multivesicular bodies, and single ribosomes have also been reported. Larger spines in the hippocampus, cerebral cortex, and neostriatum also contain a curious structure called the spine apparatus, consisting of lamellar plates of endoplasmic reticulum alternating with dense, filamentous material. The spine apparatus has

Figure 7 High-voltage electron micrograph of a spiny dendrite from the rat neostriatum showing the variation in size and shape of spines, even on a single dendrite. Spines can exhibit many morphologies, ranging from short stumpy spines to long thin spines without an obvious spine head (long thin arrow) to larger spines with an obvious head (short thick arrow) and neck (short thin arrow). The high-voltage electron microscope allows the microscopist to use much thicker sections than is possible in a conventional electron microscope. The thicker sections permit the viewing of dendritic spines in their entirety. The spiny dendrite was first injected with a fluorescent dye, which was then converted into a label that is visible under the electron microscope.

Figure 7 High-voltage electron micrograph of a spiny dendrite from the rat neostriatum showing the variation in size and shape of spines, even on a single dendrite. Spines can exhibit many morphologies, ranging from short stumpy spines to long thin spines without an obvious spine head (long thin arrow) to larger spines with an obvious head (short thick arrow) and neck (short thin arrow). The high-voltage electron microscope allows the microscopist to use much thicker sections than is possible in a conventional electron microscope. The thicker sections permit the viewing of dendritic spines in their entirety. The spiny dendrite was first injected with a fluorescent dye, which was then converted into a label that is visible under the electron microscope.

filamentous links with the postsynaptic site and may be involved in the shuttling of proteins to the synapse. As is the case with the SER in the dendritic shaft, proteins involved in the the uptake, storage, and release of intracellular calcium have been localized to the spine SER and spine apparatus. Release of calcium from these intracellular stores is thought to be an important signal transduction mechanism for synaptic transmission and may be involved in spine dynamics.

The most densely spiny cell in the CNS is the Purkinje neuron, which has been estimated to have upward of 150,000 spines per neuron. Precise counts of dendritic spines are difficult to obtain because they are too small to be resolved sufficiently in the light microscope, but they are too large to be contained completely within the thin sections typically employed for electron microscopy. To gain accurate spine counts, it is necessary to perform serial section reconstruction at the electron microscopic level or use high-voltage electron microscopy (Fig. 7). However, electron microscopic analysis is typically performed over a small portion of the entire dendritic tree, and the density of dendritic spines tends to vary over the length of the dendrite. Thus, a review of the literature can result in wildly disparate estimates of the total spine number on a given neuron type. Quantification of spine density is important because it gives an estimate of the total synaptic input to a given neuron.

Dendritic spines are labile structures in that neurons can change the size, shape, and number of their dendritic spines in response to developmental, environmental, pathological, and experimental influences. For example, researchers have shown that spine numbers are very sensitive to reproductive hormones such as estrogen. During the estrous cycle of the rat, the number of spines on a hippocampal pyramidal neuron can fluctuate by as much as 20%. Spine dynamics can be visualized directly by observing living neurons in culture or in brain slices. In these preparations, individual spines were observed to change their shape within seconds after a drug or electrical stimulus was applied to the culture dish. By using time lapse microscopy, researchers have observed new spines growing or established spines disappearing over the course of minutes.

The function of dendritic spines has been a matter of debate since their first description by Cajal. Their function clearly goes beyond simply increasing the surface area available for synaptic contact, because few synapses are established on the dendritic shafts of spiny neurons and only ~ 10% of the total spine surface is taken up by a synaptic contact. Many theories have focused on their possible role in synaptic plasticity. One method by which neurons are thought to store information long term is by structural or biochemical modification of certain synapses. Because each spine usually receives only a single synapse, dendritic spines serve to isolate individual synapses. Rapid changes in spine shape could alter the signaling properties of a particular synapse, and the spine itself may serve as a local compartment whereby modification of a given synapse can occur in isolation. Dendritic spines may also serve to protect the main dendrite from any negative effects associated with synaptic stimulation. As a consequence of synaptic activation, levels of calcium are raised inside the cell. These high concentrations of intracellular calcium are necessary for signal transduction but are also deleterious to the cell. Dendritic spines may allow local increases in intracellular calcium in the vicinity of the synapse while restricting its spread to other parts of the neuron.

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