Chapter 19 The Decussation of the Pyramids

4, 6, 5, 7

As corticospinal fibers descend, bundles of fibers from one side cross the midline alternately with bundles from the other side, so that a given transverse section is likely to show the decussation mainly from one side. On the diagram, fibers crossing the medulla from left to right are shown in levels numbered ________ and ______ . Fibers crossing the medulla from right to left are shown in levels numbered _______ and ________ .

anterior, left

As labeled about 10 to 30% of the pyramidal fibers enter the spinal cord uncrossed as the ________ corticospinal tract. Fibers of the right lateral corticospinal and left anterior corticospinal tracts arise from cell bodies in the cerebral cortex on the ______ side.

cross, lateral corticospinal, opposite

As the fibers of the left pyramidal tract descend, most of them ________ the midline in the lower medulla and enter the _______ _______ tract on the ______ side

midbrain, pyramidal, posterolateral, opposite

Corticospinal fibers arise from cells in the surface gray matter of the cortex, pass into the depths of the cortex, and emerge on the anterior surface of the _________ region of the brainstem. They pass inferiorly through the pons and again move toward the anterior surface while in the medulla's ___________ tract. The fibers then decussate to a ______ ero _______ position inside the cord. Interruption of corticospinal fibers on one side of the brain at the level of the rostral medulla or above is likely to produce paralysis on the _________ side of the body.

anterior, posterolateral

Corticospinal fibers cross in the pyramidal decussation from the ________ior surface of the medulla to a postero ________al position in the spinal cord.

reference the picture

Corticospinal fibers on the right side of the top diagram are encircled. Identify the level of each section.

No

Darken the nerve fibers labeled as "A" over their whole visible length on the diagram. Do all of the diagrammed fibers cross the midline within the territory included in the diagram?

reference the picture

Draw arrows to indicate the direction of impulse conduction at the sites marked as "A," "B," "C," and "D" on the diagram.

reference the picture

Draw the decussating fibers "1" and "2" inferiorly to the spinal cord.

reference the picture

Extend the fiber labeled as "1" inferiorly to the spinal cord. Let it decussate (cross) in the section indicated by the arrow.

right

If you look at a transverse section of the medulla from an inferior view (looking up from the spinal cord), the left side of the picture represents the left side of the medulla. If you view the same section of the medulla from a superior view (looking down from the midbrain), the left side of the picture represents the ________ side of the medulla

decussation, pyramidal

In passing from medulla to spinal cord, corticospinal fibers cross the midline in the _________ation of the _______tract.

reference the picture

In the gradual transition zone between the medulla and spinal cord, 70 to 90% of the corticospinal fibers cross the midline from one side to the other. At the lowest level shown on the diagram, encircle the fibers from the left hemisphere that have crossed the midline

reference the picture

Label each picture "L" or "R" to indicate, respectively, the left and right sides of the brain.

posterolateral

Label the transition area between the medulla and spinal cord. In the spinal cord, the decussated fibers take up a postero___________ position.

B; section "B" does not show the decussation of the pyramidal tract

Look at these two sections of the medulla. The more rostral of the sections is Explain your answer:

B, C, A

Of the three sections labeled "A," "B," and "C," which one shows fibers of the pyramidal tract decussating from the left to the right side of the medulla? _________ Decussating from the right to left side of the medulla? _______At a level above the pyramidal decussation? _______

reference the picture

On each picture, draw an arrow on the decussating fibers to show the direction of impulse conduction from one side of the medulla to the other.

reference the picture

Place an "X" at the midline on two sections that show pyramidal fibers decussating from the left to the right side of the specimen.

anterior

The alternating pyramidal decussations from left to right and right to left give a braided appearance to the ________ior surface of the lower medulla

medulla, spinal cord

The decussation of the pyramidal tract extends over several millimeters of the junction area between the ________ region of the brainstem and the _________ ________ . On the diagram, indicate with an arrow the level at which the decussation begins

reference the picture

The descending corticospinal fibers of the right motor cortex appear stained on the transverse sections. At the lowest level of the diagram, encircle the transverse section of the fibers that arose in the left hemisphere and have crossed the midline.

left, right

The descending fibers marked "X" are beginning to cross from the _________ side to the _______ side of the medulla.In the left hand diagram, encircle the cross section of these same fibers, both where they begin to cross and where they are completely crossed.

left, right

The fibers shown decussating in section "A" arise mainly in the _______ cerebral hemisphere and end in the _______ side of the spinal cord.

reference the picture

The lines on the left diagram represent segments of the corticospinal tract. The arrows indicate the direction of impulse conduction. Draw arrows on the right hand diagram to indicate the direction of impulse conduction.

anterior, lateral, posterior

The pyramidal fibers in level "A" occupy an anterior position in the medulla. By "D," in the lower medulla and upper spinal cord, most of the fibers reach a posterior and lateral position. Between "B" and "C" in the pyramidal decussation, the fibers pass from an ________ position on one side across the midline in a ___________ and _________ direction. Draw a decussating fiber originating in the right pyramid from "B" to "C."

medulla

The term pyramidal tract refers to fibers in the pyramids on the anterior surface of the _________ of the brainstem