292 Neuroanatomy: Draw It to Know It
Cerebral White Matter — Overview
Here, we will draw an overview of the white matter path-
ways in accordance with the organization described in
the 2006 white matter textbook Fiber Pathways of the
Brain. Begin with a coronal section through the brain,
brainstem, and upper spinal cord. Include a sulcal inden-
tation on the surface of the cerebral cortex as a point of
reference and also include a thalamus and head of the
caudate. Th ree white matter pathway types exist: associa-
tion fi bers, which connect areas within a hemisphere;
cord fi bers, which either directly connect areas on oppo-
site sides of the neuroaxis or provide an important step
in that cross-axis connection; and striatal fi bers, which
provide communication between the cerebral cortex and
the basal ganglia.
First, we will draw the association fi bers, which are
subdivided in accordance with the distance they travel.
Begin with a short association fi ber (aka U-fi ber or arcu-
ate bundle). Short association fi bers travel between g yri
just underneath the innermost cerebral cortical gray
matter layer (layer 6). Certain white matter diseases,
such as subtypes of multiple sclerosis, spare the short
association fi bers. Next, draw a mid-range association
fi ber, called a neighborhood association fi ber. Neigh-
borhood association fi bers extend into the deep white
matter to connect areas a mid-distance away from one
another. Finally, draw a long-distance association fi ber,
called a long association fi ber. Long association fi bers
extend deep into the brain and connect distant ipsi-
hemispheric regions. Note that we will draw the specifi c,
named long association fi bers in our sagittal diagram,
separately.
Next, draw a prototypical striatal fi ber projection
from the cerebral cortex to the basal ganglia. Projections
from throughout the cortex synapse in the basal gan-
glia and are evidentiary of the widespread role of the
basal ganglia in behavioral as well as motor functions.
Indicate that the two types of striatal fi ber bundles are
the Muratoff bundle (aka subcallosal fasciculus) and the
external capsule.
Now, we will draw the cord fi bers, which connect
opposite sides of the nervous system: they subdivide into
commissural and subcortical fi ber bundles and they form
the dense aggregate of deep white matter underneath the
cortical gray matter. Draw a transverse-oriented com-
missural fi ber connection from one side of the cortex to
the other. Th e key commissural fi bers are the anterior
commissure and corpus callosum — the latter contains
roughly 300 million myelinated axons.^1 Next, draw two
vertically oriented representative subcortical projection
fi bers: an ascending one from the thalamus to the cortex,
called the thalamic bundle, and a descending one from
the cortex to the opposite side of the brainstem, called
the pontine bundle. Th e thalamic bundle encompasses
such pathways as the somatosensory aff erents from the
spinal cord and the cerebellar projections of the cortico-
pontocerebellar pathway; these pathways relay in the
thalamus in their ascent to the cerebral cortex. Th e pon-
tine bundle encompasses such pathways as the corti-
cospinal tract, which descends through the internal
capsule and pons, and the corticopontine tract, which
synapses in the pons. Note that although the thalamic
bundle and the corticopontine tract are actually ipsi-
hemispheric projections, they both are steps in larger
pathways that connect opposite sides of the neuroaxis.
Th e cord fi bers form the deep white matter region
between the cortex and the diencephalon, which is ana-
tomically divided into the centrum semiovale and corona
radiata. Th e name corona radiata stands for “radiating
crown”; however, its radiating appearance is appreciable
only in detailed anatomic dissection. Indicate that the
centrum semiovale lies above the level of the lateral ven-
tricles and that the corona radiata lies at their level.^2 – 8