Myelin is lipid-rich, and on gross inspection appears white. Recall that Schwann cells are the glial cells responsible for myelination in the peripheral nervous system. Astrocytes in the CNS provide metabolic support for neurons and play an important role in maintenance of the blood-brain barrier whereas oligodendrocytes (another type of glial cell) are responsible for myelination of CNS axons. Both neurons and glia have fine processes projecting from the cell body, which generally cannot be resolved in the light microscope without special staining techniques. Nervous tissue contains two basic categories of cells: neurons and support cells ( glia). These granules are of little pathological significance, but they generally increase with age. Another feature commonly found in nervous tissue are starchlike granules known as "corpora amylacea" (amylon = starch, Greek) which are aggregates of dead cells and/or proteinaceous secretions that may be found in either white or gray matter. The white matter contains nerve fibers ( axons) entering and exiting the gray matter, and traveling up and down the spinal cord, linking it to the brain. The delicate meshwork of dendritic processes and nerve fibers ( axons) lying between cells in the gray matter is called the neuropil. Cells that are well preserved show features characteristic of most neurons: large cell body, large pale nucleus, Nissl substance, and cell processes (most of which are dendrites). Many neurons in the spinal cord may appear shrunken and surrounded by an empty space due to poor fixation. Note that sacral levels of the cord (levels S2-4) also contain visceral motor neurons in the lateral horn, but these are parasympathetic motor neurons. celiac, aorticorenal, and superior/inferior mesenteric ganglia). The cells here are preganglionic sympathetic motor neurons whose axons terminate in either sympathetic chain ganglia or the "visceral" (or "pre-aortic") ganglia associated with the major branches of the abdominal aorta (e.g. The lateral horn contains relatively large, multipolar visceral motor neurons of the intermediolateral cell column that extends from levels T1 through L2 of the spinal cord. The cells of Clarke's nucleus then relay this information via axonal projections that extend all the way up into the cerebellum (hence the reason why the cells are so large) where it is processed to allow for coordinated movement. The dorsal nucleus of Clarke is in the dorsal horn and contains relatively large, multipolar neurons that receive proprioceptive information from dorsal root ganglion cells innervating muscle spindles in the trunk and lower limb. In addition to the dorsal and ventral horns, two structures especially obvious in the thoracic cord are the dorsal nucleus of Clarke and the lateral horn. Slide 66a shows a section of thoracic spinal cord. This marginal zone is white primarily because of the abundance of myelinated axons, which you can observe in your sections, although the axons are very faint and therefore difficult to see. The white matter fills most of this cross-section external to the central butterfly-shaped region of gray matter. The neurons of the mantle layer produce processes that grow outward to form the fiber tracts of white matter, which carry parallel myelinated axons for long-range interneuronal contacts in CNS. Neuropil refers to the regions in gray matter that lie between cell bodies, devoid of nuclei but complexly crowded with neuronal cell processes and synapses. Note also the smaller nuclei of neuroglial cells. Depending on how the cells are cut, you may see the nucleus, nucleolus, and axon hillock, as well as the Nissl bodies (rough ER) filling the cytoplasm. In the gray matter, examine the large motor neuron cell bodies carefully. This gray matter occupies a butterfly-shaped cross-section in this mature spinal cord.
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Recall during development, newly formed neurons proliferate adjacent to the canal to form a mantle layer, which becomes the gray matter in the central region of the mature spinal cord. The perikarya of large somatic motor neurons located in the ventral horn of the cord innervate the skeletal muscles of the limbs and trunk of the body (soma="body" in Greek, hence "somatic"). In these slides, dorsal happens to be "up," but you should be able to tell dorsal and ventral horns based on morphology and the cells present rather than the orientation. At low magnification, differentiate inner gray matter from outer white matter and identify dorsal and ventral horns of the gray matter. Review the organization of the spinal cord using an anatomy atlas, then examine the cross section of the lumbar spinal cord in slide #65-2. Slide 66a (thoracic spinal cord, luxol blue & cresyl violet)