The specimen in this figure has been stained with a silver procedure. Such procedures do not always color the specimen evenly, as do H&E. Note that the part of the molecular layer on the right is much darker than that on the left. A rectangular area on the left has been selected for examination at higher magnification in Figure 4. Even at the relatively low magnification shown here, however, the Purkinje cells can be recognized in the silver preparation because of their large size, characteristic shape, and location between an outer molecular layer (Mol ) and an inner granular layer (Gr). The main advantage of this silver preparation is that the white matter (WM ) can be recognized as being composed of fibers; they have been blackened by the silver-staining procedure. The pia mater (Pia) and cerebellar blood vessels are also evident in the preparation.
Cerebellum, brain, human, silver stain ×400.
At higher magnification, the Purkinje cell bodies (Pkj ) stand out as the most distinctive and conspicuous neuronal cell type of the cerebellum, and numerous dendritic branches (D) can be seen. Note, also, the blackened fibers within the granular layer (Gr), about the Purkinje cell bodies, and in the molecular layer (Mol) disposed in a horizontal direction (relative to the cerebellar surface). The arrow indicates a T turn characteristic of the turn made by axons of granule cells. As these axonal branches travel horizontally, they make synaptic contact with numerous Purkinje cells.
KEY BC, basket cells BV, blood vessels D, dendrites G, Golgi type II cells Gr, granular layer Mol, molecular layer Pia, pia mater Pkj, Purkinje cells WM, white matter arrows: Upper right figure, glomeruli; Lower right figure, T branching of axon in molecular layer rectangular area, areas shown at higher magnification
The spinal cord is organized into two discrete parts. The outer part, called the white matter of the cord because of its appearance in unfixed specimens, contains ascending and descending nerve fibers. Some of the fibers go to and from the brain, whereas others connect different lev-els of the spinal cord. The inner part of the spinal cord, called the gray matter because of its appearance in unfixed specimens, contains the cell bodies of neurons as well as nerve fibers. The gray matter forms an Hor butterfly-shaped pattern surrounding the central canal. The gray matter is described as having dorsal (posterior) horns and ventral (anterior) horns. The ventral horns contain the large cell bodies of motor neu-rons, whereas the dorsal horns contain neurons that receive, process, and retransmit information from the sensory neurons whose cell bodies are located in the dorsal root ganglia. The size of the gray matter (and, therefore, the size of the spinal cord) is different at different levels. Where the gray matter contains many large motor nerve cells that control the movement of the upper and lower limbs, the gray matter and the spinal cord are considerably larger than where the gray matter contains only the motor neurons for the muscle of the torso.
Spinal cord, human, silver stain ×16.
A cross-section through the lower lumbar region of the spinal cord is shown here. The preparation is designed to stain the gray matter that is surrounded by the ascending and descending nerve fibers. Although the fibers that have common origins and destinations in the physiologic sense are arranged in tracts, these tracts cannot be distinguished unless they have been marked by special techniques, such as causing injury to the cell bodies from which they arise or by using special dyes or radioisotopes to label the axons.