Transverse Sections of the Spinal Cord





The spinal cord is perhaps the most simply arranged part of the central nervous system (CNS). Its basic structure, indicated in a schematic drawing of the eighth cervical segment ( Fig. 2.1 ), is the same at every level—a butterfly-shaped core of gray matter surrounded by white matter. An often indistinct central canal in the middle of the butterfly is the remnant of the lumen of the embryonic neural tube.




Figure 2.1


Schematic drawing of the spinal cord at the level of the eighth cervical segment.

(Modified from Nolte J: The Human Brain , ed 6, Philadelphia, 2009, Elsevier.)


The extensions of the gray matter posteriorly and anteriorly are termed the posterior and anterior ( dorsal and ventral ) horns , respectively. The zone where the two horns meet is the intermediate gray . At every level, the posterior horn is capped by a zone of closely packed small neurons, the substantia gelatinosa . Beyond this, there are level-to-level variations in the configuration of the spinal gray ( Fig. 2.2 ). For example, the motor neurons that innervate skeletal muscle are located in the anterior horns, so these horns expand laterally in lumbar and lower cervical segments to accommodate the many motor neurons required for the muscles of the lower and upper extremities. Other examples are pointed out in Fig. 2.2 . When studied in microscopic detail, the spinal gray matter can be partitioned into a series of 10 layers (Rexed laminae) , as indicated on the right side of Fig. 2.1 . Some of these laminae have clear functional significance. For example, lamina II corresponds to the substantia gelatinosa, which plays an important role in regulating sensations of pain and temperature.


Figure 2.2



(A–H) Cross sections of a spinal cord at eight different levels, all shown at about the same magnification. (A) The fourth sacral segment (S4). Several features common to all spinal levels can be seen. The substantia gelatinosa (4) caps the posterior horn (5). Also, afferent fibers entering through dorsal rootlets (2) sort themselves into small-diameter fibers that move laterally and enter Lissauer’s tract (3) and large-diameter fibers that enter more medially (1) at the edge of the posterior funiculus. (This sorting occurs at all spinal levels and can be seen in all of the sections in this series.) Little white matter is present in any of the funiculi because most fibers either have already left descending pathways or have not yet entered ascending pathways. The anterior spinal artery (6) is cut in cross section as it runs longitudinally near the anterior median fissure of the cord. Shown enlarged in Fig. 2.3 .





(A–H) Cross sections of a spinal cord at eight different levels, all shown at about the same magnification. (A) The fourth sacral segment (S4). Several features common to all spinal levels can be seen. The substantia gelatinosa (4) caps the posterior horn (5). Also, afferent fibers entering through dorsal rootlets (2) sort themselves into small-diameter fibers that move laterally and enter Lissauer’s tract (3) and large-diameter fibers that enter more medially (1) at the edge of the posterior funiculus. (This sorting occurs at all spinal levels and can be seen in all of the sections in this series.) Little white matter is present in any of the funiculi because most fibers either have already left descending pathways or have not yet entered ascending pathways. The anterior spinal artery (6) is cut in cross section as it runs longitudinally near the anterior median fissure of the cord. Shown enlarged in Fig. 2.3 .





(C) The second lumbar segment (L2). The posterior funiculus (1) is larger because ascending fibers carrying touch and position information from the lower limb have been added. The lateral funiculus is also larger, reflecting increased numbers of descending fibers in the lateral corticospinal tract (2) and ascending fibers in the spinothalamic tract (4). This section is at the rostral end of the lumbar enlargement, so the anterior horn (5) no longer is enlarged laterally. Clarke’s nucleus (3), which extends from about T1 to L3 and contains the cells of origin of the posterior spinocerebellar tract, makes its appearance. The anterior white commissure (6), the principal route through which axons can cross the midline in the spinal cord, is present at this and all other spinal levels. Shown enlarged in Fig. 2.4 .

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Dec 29, 2019 | Posted by in NEUROLOGY | Comments Off on Transverse Sections of the Spinal Cord
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