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Synapse Web | |||||
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The difference between macular and perforated synapses can be seen in electron micrographs in which the postsynaptic densities have been stained (Figure 3).
Most synapses in stratum radiatum (>90%) occur on spines. As shown in Figure 3, spines come in a variety of shapes. A thin spine (T) has a small head with a macular postsynaptic density. The length of the spine neck is much greater than its diameter. The mushroom spine (M) has a large head, typically greater than 0.6 microns in diameter. An elaboration of the endoplasmic reticulum, called a spine apparatus (sa) is often visible within the neck of a mushroom spine. Mushroom spines also tend to have perforated (perf) postsynaptic densities on the spine head. The stubby spine (S) does not have a constricted neck, and its overall length is roughly equal to its diameter. The stubby spine illustrated above possesses a macular postsynaptic density.
Occasionally synapses occur directly on the shaft of a dendrite (shaft) without the participation of a dendritic spine. Symmetric (inhibitory) synapses in stratum radiatum tend to be shaft synapses. All of the symmetric synapses of the volume of Figure 1 are shaft synapses.
Figure 4 shows the distribution of synapses among the various locations and shapes in stratum radiatum as measured over many adult rats. Synapses on thin spines are the most common. Most synapses have a macular shape.
| Last Updated: 06/01/07 |