112 Neuroanatomy: Draw It to Know It
Ascending Pathways
Here, we will draw the ascending spinal cord pathways in
axial cross-section. Draw an outline of a spinal cord in
axial cross-section and include the gray matter horns. We
start with the posterior column pathway, which com-
prises large sensory fi bers that carry vibration, two-point
discrimination, and joint position sensory information.
Within the posterior column, draw the posterior median
septum, which divides the posterior white matter into
right and left posterior columns. Th en, label the poste-
rior intermediate septum, which further subdivides each
posterior column into medial and lateral segments. Label
the medial segment as the gracile fasciculus and the lat-
eral segment as the cuneate fasciculus. Write thoracic
level 6 (T6) over the posterior intermediate septum to
indicate that the gracile fasciculus carries large fi ber sen-
sory information from the lower body (from below T6)
and the cuneate fasciculus carries sensory information
from the upper body (from T6 and above).^4 S e n s o r y
aff erents from the face travel via the trigeminal sensory
system. A mnemonic to remember the function of the
gracile fasciculus is that ballerinas must have good sen-
sory input from their feet to twirl grace fully.
Now, label the anterolateral system as a long pathway
that lies just outside of the anterior gray matter horn and
extends between the anterior and lateral white matter
funiculi. It comprises small fi ber sensory pathways that
carry pain, itch, and thermal sensory information. Th e
anterolateral system includes the spinothalamic tract
and the spinal-hypothalamic and spinal-brainstem path-
ways. In regards to the anterolateral system’s somatotopic
organization, the lower-most spinal levels comprise its
outermost somatotopic layer and the upper-most spinal
levels comprise its innermost somatotopic layer; thus,
indicate that the arms lie along the inner aspect of the
anterolateral system and the legs lie along the outer
aspect. As a helpful mnemonic, consider that in cervical
syringomyelia, a fl uid-fi lled cavity within the central
cord (see Drawing 7-8 ), there is oft en a suspended
sensory level wherein small fi ber sensation is lost in a
cape-like distribution in the arms and upper trunk but is
preserved in the legs. We can imagine that as the central
fl uid collection expands outward, it aff ects the arms and
upper trunk fi rst, and only later the legs. In regards to the
sensory information carried by the anterolateral system,
the lateral regions subserve pain and temperature and
the anterior regions subserve tactile and pressure sensa-
tion. Indeed, texts oft en further subdivide the anterolat-
eral system into separate ventral and lateral pathways.
Next, label the ventral commissure, which lies in between
the anterior horns; it is the white matter pathway through
which the anterolateral system fi bers decussate.
Lastly, along the posterior lateral wall of the spinal
cord, label the posterior spinocerebellar tract, and along
the anterior lateral wall, label the anterior spinocerebel-
lar tract. Th ese tracts comprise large sensory fi bers that
carry joint proprioception to the cerebellum for the
coordination of movement; we address the spinocerebel-
lar pathways in Drawing 7-5.
Now, show that the cell bodies for spinal sensory
fi bers lie within dorsal root ganglia, which are situated
just proximal to where the anterior and posterior roots
merge, within the intervertebral foramina. Indicate that
spinal sensory axons are pseudo-unipolar, meaning they
contain a single short axon that emanates from the cell
body and divides into processes that extend peripherally
(eg, to the skin’s surface) and centrally (eg, to the spinal
cord).
Finally, let’s draw the anterolateral system and poste-
rior column projections. Label Lissauer’s tract along the
dorsal edge of the dorsal horn. Indicate that the central
processes of the anterolateral system pass through
Lissauer’s tract, synapse within the ipsilateral dorsal
horn, and cross within the ventral commissure to ascend
the spinal cord within the anterolateral system. Th en,
indicate that the posterior column pathway fi bers enter
the spinal cord medial to Lissauer’s tract and directly
ascend the spinal cord within the ipsilateral posterior
column.^2 , 4 – 12