110 Neuroanatomy: Draw It to Know It
Here, we will draw the spinal cord in axial cross-section.
Show that the spinal cord is ovoid and has a thin fi ssure
on its anterior surface. Next, draw the internal gray
matter, which resembles a butterfl y. Th en, draw the cen-
tral canal in the center of the gray matter; it is mostly
obliterated by the second decade of life. Th e white matter
of the spinal cord is segmented into posterior, lateral,
and anterior funiculi (aka columns). Label the posterior
white matter as the posterior funiculus, the middle white
matter as the lateral funiculus, and the anterior white
matter as the anterior funiculus. Lastly, show that inter-
spinal rostro-caudal white matter projections travel via
the proprius fasciculus, which surrounds the gray matter
horns. Next, introduce the posterior median septum,
which divides the posterior white matter into two halves,
and label the anterior median fi ssure in parallel along the
anterior surface of the spinal cord.
Th e gray matter of the spinal cord divides into three
diff erent regions, which are further classifi ed as Rexed
laminae. First, label the regions from posterior to ante-
rior as the posterior horn, intermediate zone, and ante-
rior horn. Th en, label the Rexed laminae, which are
numbered from I to X. In the posterior horn, label lami-
nae I–VI: they are the sensory laminae; then, in the
intermediate zone, label lamina VII, which is the spinoc-
erebellar and autonomic lamina; next, in the anterior
horn, label laminae VIII and IX, which are the motor
laminae; and fi nally, label lamina X around the central
canal.
Now, let’s address a few neuroanatomic highlights of
the Rexed laminae. Lamina I is the marginal nucleus (aka
posteromarginal nucleus); lamina II is the substantia
gelatinosa — so named because its lack of myelinated
fi bers gives it a gelatinous appearance on myelin staining ;
and laminae III and IV comprise nucleus proprius (the
proper sensory nucleus). Laminae I through V receive
the central processes of sensory fi bers in a complicated
way; generally, laminae I, II, and V receive small, poorly
myelinated or unmyelinated fi bers, which carry pain and
temperature sensation, and laminae III and IV receive
large cutaneous sensory fi bers — note, however, that the
majority of large fi bers do not synapse within the Rexed
laminae at all but instead directly ascend the posterior
columns.^1
Laminae V and VI receive descending motor fi bers
and assist in sensorimotor integration. Lamina VII con-
tains the dorsal nucleus of Clarke, a key spinocerebellar
nuclear column, and the intermediolateral column, a key
autonomic nuclear column. Laminae VIII and IX con-
tain motor neurons. Lamina X surrounds the central
canal.^2 , 3
Now, let’s illustrate the relative size of the white and
gray matter regions of the spinal cord at diff erent ana-
tomic heights. Show that in the lumbosacral cord, the
amount of white matter is small, because the ascending
fi bers have yet to coalesce and the descending motor
fi bers have already terminated on their anterior horn
cells. Indicate that the amount of gray matter is large
because of the numerous neurons needed to innervate
the lower limbs. Th en, show that in the thoracic cord,
the amount of white matter is moderately large because
of the presence of the lumbosacral aff erents and eff er-
ents, and then show that the amount of gray matter is
small, because thoracic innervation to the trunk requires
far fewer neurons than lumbosacral or cervical innerva-
tion to the limbs. Finally, in the cervical spinal cord,
indicate that both the gray and white matter regions are
large: the white matter bundles are dense with ascending
and descending fi bers from throughout the spinal cord
and the gray matter horns are large because of the large
populations of neurons required to innervate the upper
limbs.^2 , 4 – 12Spinal Cord Overview