The small pyramidal neuron of the rat cerebral cortex. The perikaryon,dendrites and spines

The small pyramidal neurons of layers II and III of the rat parietal cortex have been examined with both Golgi staining and electron microscopy. The cell body contains a large nucleus and only a thin rim of cytoplasm in which the Nissl substance is not well developed. A cap of Nissl substance occurs at the base of the apical dendrite, however, and a number of cisternae of the Golgi apparatus occur in this same location. A centriole is often found here too, so that this region of the neuron represents the cell center. Little Nissl substance occurs in the dendrites, which mainly contain well-ordered arrays of microtubules. Projecting from the stem of the apical dendrite and from the secondary dendrites are spines. Basically, the spines are of three types. The most common ones have long, thin stalks and small end bulbs. The least common ones are mushroom-shaped and have thick stalks ending in large bulbs. The other spines are short and stubby, and have no well-defined stalk. The distribution of these three types of spines has been determined. Although they are rare, spines may also project from the cell body, which has very few synapses on its surface.