SLIDE 75 - SPINAL CORD WITH SPINAL GANGLION AND NERVE ROOTS

Look first at low magnification for overall orientation to the spinal cord, cut here in X-section.  Determine dorsal (posterior) versus ventral (anterior) surfaces; the anterior median fissure, containing prominent blood vessels and connective tissue, is the main clue.  Locate the centralcanal, lined by a simple columnar epithelium known as the ependyma. (Figure 7-24 Wheater) What does the canal contain in life?

Surrounding the cord are three connective tissue coats, the meninges: (1) the outermost dense collagenic layer, the dura mater; (2) the intermediate, loose C.T. meshwork, the arachnoid; and (3) the thin, innermost C.T. layer, the pia mater, which is closely applied to the surface of the cord.  Blood vessels run deep in the arachnoid meshwork and are closely applied to the pia mater.  Their small branches penetrate the neural tissue of the spinal cord.

Also outside of the cord, but within the meninges, are sections of dorsal and ventralroots and the dorsal root (spinal) ganglia.  Which roots are motor and which Asensory?  Which are afferent in relation to the spinal cord?  Which are efferents? Which carry information from receptor endings?  Or relay information to effectors?

Inside the cord, look for the butterfly-shaped gray matter with its dorsal and ventral horns.  Note the large, multipolar neurons lying within the gray matter, as well as a background feltwork (neuropil) of processes running in all directions.  The small nuclei scattered throughout the neuropil belong to supporting glial cells.  These take the place of ordinary connective tissue within the central nervous system.  Their cytoplasm is not visible in this preparation, but is stellate in shape with many fine processes.  Their processes invade every free space between neuronal cell bodies and processes. (Notice the compactness of neural tissue in EM).  Look for three types of nuclei: the larger, more open-faced nuclei of astrocytes; the smaller, denser, round nuclei of oligodendroglia, and the elongate, slender dark nuclei of microglia.  What is the relation of astrocytes to blood vessels and the blood-brain barrier?  Which glia have phagocytic activity?  Which glia have a myelin-producing function?

Now look, with higher magnification, at some anterior horn cells.  These are multipolarmotorneurons.  Note dendrites, clumped Nissl substance, vesicular nucleus, prominent single nucleolus, and possibly a pale axon hillock.  What organelle comprises Nissl substance?  On what part of the cell are synaptic contacts made?  Learn to recognize a synapse in EM. (Fig. 7-9 and 7-10 Wheater)

Surrounding the gray matter is a mass of white matter, which consists mostly of myelinated neuronal axons cut in cross-section.  Neither the myelin nor the axons proper are well preserved here.

NOW LOOK AT THE SPINAL GANGLION. Notice the rounded neurons of variable sizes, many of them quite large.  These are pseudounipolar sensory neurons.  Note their finely granular Nissl substance, vesicular nuclei, prominent nucleoli and well-defined capsules of satellite cells surrounding the neurons.  These satellite cells are comparable to the glial cells of the central nervous system or the Schwann cells of peripheral nerve.  Notice nerve processes passing into and out of the ganglion, separating the cell bodies into rather tight clusters.  In what direction do impulses pass through the ganglion in relation to the spinal cord?  Remember: these processes are not synapsing with neurons in the spinal ganglion; they are simply the afferent and efferent processes of pseudounipolar cells that reside here.

Note the loose areolar C.T. which surrounds the satellite cell capsules and lies between the individual nerve processes of the spinal ganglion, which is part of the peripheral nervous system.  Connective tissue supports the cells and processes of the peripheral nervous system, i.e., all ganglia and nerves lying outside of the brain and spinal cord.

Finally, in this section, look at the dorsal and ventral roots entering and leaving the cord, and the short segment of spinal nerve just lateral to the ganglion.