MIT Press (2010)
This review refers mainly to chapter 4 of this book, which deals with the internal make up of the dendritic spines, an aspect which bears on controversies relating to Hameroff’s attempt to describe an implementation of quantum consciousness.
Dendritic spines are recipients of synaptic inputs, and almost every spine has an excitatory input, while some have both excitatory and inhibitory inputs. Spiny dendrites appear to have developed to integrate inputs from as many sources as possible. Connectivity is combined with plasticity for the individual spines, which is seen as increasing flexibility and computing power. The spines project from the shaft or main body of the dendrites, with a narrow neck where they join to the main shaft and a broader head. Spines are thought to be capable of some of their own protein synthesis.
Spines are structurally flexible with a cytoskeleton based on actin rather than microtubules. Actin and Rho pathways in the spines are well placed to control their shape and function. The spines are important for inputs of glutamate and they possess all four types of glutamate receptor. The heads of spines are filled with a network of actin filaments that interact with the post synaptic density (PSD). In the spine neck, actin filaments are formed into long bundles. Only 5% of the spine’s actin is stable. The spines also contain a number of actin related proteins, and it is thought that these may be involved in functions such as vesicle and protein transport, spine shape, anchoring of membrane proteins, notably glutamate receptors. Many actin binding proteins can be regulated by Ca2+ calcium ions, and this may be important in regulating actin activity in the spines. The actin binding proteins may also influence the formation and shape of spines. The abscence of a tubulin suggests that the spines need to have limited stability. Spines have to move so as to more efficiently connect with axons. It is suggested here that changes in the shape of spines are involved in synaptic plasticity.
Dendritic spines also contain cell-adhesion molecules, linked to the actin cytoskeleton. Cadherin molecules interact with actin filaments, and may be involved in synaptic plasticity. These molecules appear to be localised in the adhesive structure around the post synaptic density. The spine membranes also contain both sodium and potassium ion channels.
In summary, dendritic spines can be viewed as nanomachines involving membrane receptors and channels, cell adhesion proteins, cytoskeleton components and a variety of other molecules. It appears that the biochemistry of each spine is independently regulated to specifically regulate the input of each synapse. The spine represents an exceptionally complex environment with a very precise regulation of numbers of molecules and their positioning. Every molecular pathway is linked to the actin cytoskeleton.