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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). 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. celiac, aorticorenal, and superior/inferior mesenteric ganglia). The cells here are preganglionic sympathetic 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 extension of the ventral horn slide 065-2 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 slide 066a View Image is in the dorsal horn and contains relatively large, multipolar neurons that receive proprioceptive information from dorsal root ganglion cells that are innervated by 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 extension of the ventral horn. Slide 66a View Virtual Slide shows a section of thoracic spinal cord. Remember that the perikaryon is the metabolic support center for each neuron, so, therefore, motor neurons require much larger perikarya. Neurons in the dorsal horn are essentially interneurons that project to other regions of the CNS (e.g., motor neurons in the spinal cord or sensory input to the brain), so they have much smaller overall volume and therefore much less metabolic demand compared to motor neurons which project to target muscles that may be more than a meter away. Why are perikarya of dorsal horn neurons smaller than those in the ventral horn? The perikarya of large somatic motor neurons slide 065-2 View Image located in the ventral horn of the cord innervate the skeletal muscles of the limbs and trunk, which are embryologically derived from somites (hence, somatic muscles).
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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. You should also identify the dorsal and ventral horns in slide 65-1N stained with Masson trichrome. At low magnification, differentiate inner gray from outer white matter and identify dorsal and ventral horns of the gray matter. Examine the cross section of the lumbar spinal cord in slide 065-2. Review the organization of the spinal cord using your atlas. Slide 066a thoracic spinal cord thoracic spinal cord luxol blue cross View Virtual Slide Slide 065-1 spinal cord lumbar H&E cross View Virtual Slide Slide 065-2 spinal cord lumbar H&E cross View Virtual Slide
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Slide 065-1N spinal chord Masson cross View Virtual Slide