Spine Anatomy And Biochemical Compartmentalization

So far we have discussed the synapse in abstract terms related largely to its function. However, the synapse is also a physical entity and the structural attributes of this entity confer some interesting properties. In this last section, we will look at certain physical aspects of the synapse that will be important to consider before moving on to the next chapter, where we begin to talk about molecular mechanisms for LTP. In brief, there are three points we will highlight in this section. First, most synapses in the CNS and almost all excitatory synapses in the hippocampus are at specialized structures called dendritic spines. Second, spines are small, well-circumscribed biochemical compartments that localize proteins and signaling molecules to a specific postsynaptic compartment. Third, spines are of course contiguous with dendrites and thus continuously sense the local dendritic membrane potential.

A picture of part of the dendritic region of an area CA1 pyramidal neuron is shown in Figure 10. The fuzzy appearance of the

CA1 dendritic tree in this picture is caused by the abundance of small dendritic spines protruding at right angles to the dendritic shaft. Almost all (about 95%) of the Schaffer-collateral synapses we have been discussing in the abstract are actually physically present at spines. Most spines have a fairly simple elongated mushroom-like (i.e., chicken drumstick) shape, although there is clearly great diversity of their morphology. For example, a low percentage (about 2%) of CA1 pyramidal neuron spines are bifurcated and actually have two synapses on them. Spines have an actin-based cytoskeleton, and most have both smooth endoplasmic reticulum that can contribute to local calcium release and polyribosomes where local protein synthesis occurs. In hippocam-pal pyramidal neurons microtubules and mitochondria are limited to the dendritic shaft.

A distinguishing feature of the area of synaptic contact at the spine is the post-synaptic density, or PSD. This is a highly compact biochemical structure containing scaffolding proteins, receptors, and signal transduction components. The calcium/ calmodulin sensitive protein kinase CaMKII is particularly enriched at the PSD, as is a

A mCDS B

A mCDS B

FIGURE 10 Dendrites with spines in a hippocampal pyramidal neuron. This figure illustrates the presence and shapes of dendritic spines on pyramidal neurons in the hippocampus. The spines are the small mushroom-shaped lateral projections containing synaptic contacts. (A) Courtesy of Liqun Lou, Stanford University. (B) Courtesy of E. Korkotian, The Weizmann Institute.

FIGURE 10 Dendrites with spines in a hippocampal pyramidal neuron. This figure illustrates the presence and shapes of dendritic spines on pyramidal neurons in the hippocampus. The spines are the small mushroom-shaped lateral projections containing synaptic contacts. (A) Courtesy of Liqun Lou, Stanford University. (B) Courtesy of E. Korkotian, The Weizmann Institute.

structural protein called PSD-95, a name based on its molecular weight. We will return to these molecules in much more detail in Chapters 6 and 7.

The dendritic spine membrane surrounds the PSD and the area immediately below it and thus circumscribes a discrete biochemical compartment. The spine neck, however, is open to the dendritic shaft so there is still considerable diffusion of soluble spine contents (such as calcium and second messengers) into the local dendritic

Was this article helpful?

0 0
Eliminating Stress and Anxiety From Your Life

Eliminating Stress and Anxiety From Your Life

It seems like you hear it all the time from nearly every one you know I'm SO stressed out!? Pressures abound in this world today. Those pressures cause stress and anxiety, and often we are ill-equipped to deal with those stressors that trigger anxiety and other feelings that can make us sick. Literally, sick.

Get My Free Ebook


Post a comment