Figure 1. Pyramidal cell in hippocampal area CA1 of the rat brain showing the typical appearance of the principal excitatory neurons that occur throughout the brain. (a) The low-magnification view of these cells shows an apical dendrite projecting towards the upper left quadrant of the figure. Many lateral dendrites emerge from the large apical dendrite. Several basillar dendrites project from the base of the cell soma. Dendritic spines are the tiny projections that stud the surface of these dendrites in both the apical and basillar dendritic arbour. (b) A higher magnification view of the dendrites reveals the tiny spines a thin axon (arrow) passing through the dentritic arbour.

Figure 2. (a) Electron micrograph of a section through dendritic spines in stratum radiatum of hippocampal area CA1 that has been colour coded to identify dendrites (yellow), axons (green) and astroglia (purple). In this fortuitous section, three spines were sectioned parallel to their longitudinal axis revealing spines of the stubby (S), mushroom (m) and thin (t) morphologies. The postsynaptic density (PSD) occurs on the spine head (see t) immediately adjacent the synaptic cleft (c) and to a presynaptic axonal bouton that is filled with round vesicles (v). This t spine contains a small tube of smooth endoplasmic reticulum (ser) in its neck. In the m spine a spine apparatus (sa) is visible. A perforated postsynaptic density (perf) is evident on the head of another mushroom spine. Near to this spine is a large astrocytic process (ast) identified by the glycogen granules and clear cytoplasm. (b) A three-dimensional reconstruction of a dendrite showing a variety of spine and synapse shapes and the presence of polyribosomes (black spheres) at the base of the spines. Scale cube=1 m³.

Figure 3. (a) Spines increase the packing density of synapses. Schematic illustrates a cross-section through two dendrites, one without and one with dendrites spines. Convolution and interdigitation of dendrite, axon and spine membranes support more synapses. (b) The presence of spines also allows for an increase in synaptic density without increasing the overall volume of the brain. (c) Spines exist to amplify electrical potential at the synapse and promote associativity among neighbouring synapses. Spine shape and resistance of the spine neck may influence potential (V) generated by synaptic activation. (d) Spines exist as molecular compartments. Smooth endoplasmic reticulum (tubules), calcium and a myriad of other signalling mechanisms (stippling) are recruited in response to synaptic activation (asterisk).