Plate 27 Spinal Cord

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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 levels 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 H-or 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 neurons, 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 appendages, the arms and legs, 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.

Figure 1, spinal cord, human, silver stain x16.

A cross section through the 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.

The gray matter of the spinal cord appears roughly in the form of a butterfly. The anterior and posterior prongs are re ferred to as ventral horns (VH) and dorsal horns (DH), respectively. The connecting bar is called the gray commissure (GC). The neuron cell bodies that are within the anterior horns (anterior horn cells) are so large that they can be seen even at this extremely low magnification (arrows). The pale-staining fibrous material that surrounds the spinal cord is the pia mater (Pia). It follows the surface of the spinal cord intimately and clips into the large ventral fissure (VF) and into the shallower sulci. Blood vessels (BV) are present in the pia mater. Some dorsal roots (DR) of the spinal nerves are included in the section.

Figure 2, spinal cord, human, silver stain x640.

This preparation shows a region of a ventral horn. The nucleus (N) of the ventral horn cell is the large, spherical, pale-staining structure within the cell body. It contains a spherical, intensely staining nucleolus. The ventral horn cell has many processes, two of which are obvious. A num-

Figure 3, spinal cord, human, toluidine blue x640.

This preparation of the spinal cord is from an area comparable to that of Figure 2. The toluidine blue reveals the Nissl bodies (NB) that appear as the large, dark-staining ber of other nuclei (NN) belong to neuroglial cells. The cytoplasm of these cells is not evident. The remainder of the field consists of nerve fibers and neuroglial cells whose organization is hard to interpret. This is called the neuropil (Np). A capillary crosses through the field below the cell body.

bodies in the cytoplasm. Nissl bodies do not extend into the axon hillock. The axon leaves the cell body at the axon hillock. The nuclei of neuroglial cells (NN) are also evident here, but their cytoplasm is not. The neuropil stains very faintly.

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