284 Glossary
Dorsal median septum––Aligned non-neural cells attached
to the pia that extend downward in the dorsal midline.
This septum separates the two gracile fasciculi on
either side of the midline and may be formed by the
receding cellular portion of the roof plate during the
late fi rst trimester and early second trimester.
Dorsal neuroepithelium––Neuroepithelium surrounding
the dorsal canal that produces dorsal horn neurons.
This neuroepithelium still produces neurons in the late
fi rst trimester.
Dorsal root––Part of a spinal nerve that is formed by
axons of dorsal root ganglion cells that bring sensory
input to the dorsal funiculus.
Dorsal root bifurcation zone––Region in the dorsal funi-
culus where incoming axons from the dorsal root
divide into ascending and descending branches.
Dorsal root boundary cap––Specialized glial cells that
accumulate at the entry points of the dorsal roots of the
spinal nerve. These cells may act as targets that direct
the growth of dorsal root axons into the spinal cord.
Dorsal root collateralization zone––Region of the dorsal
funiculus that contains the local collateral branches of
dorsal root ganglion cell axons. These axons curve
around the dorsomedial edge of the dorsal horn before
penetrating the spinal gray matter. This region also
contains axons of the intraspinal tracts.
Dorsal root ganglion––Primary sensory neurons embed-
ded in the dorsal root of a spinal nerve that bring
exteroceptive (touch, pressure), proprioceptive (stretch
receptors in muscles and tendons), visceroceptive
(receptors in the vital internal organs), and nociceptive
(pain) input to the spinal cord.
Dorsal spinocerebellar tract––Axons of neurons in
Clarke’s column and the central cervical nucleus that
extend ipsilaterally to the cerebellum in the peripheral
dorsal lateral funiculus.
Dorsolateral fasciculus––See Lissauer’s tract.
Ependyma––Cells lining the central canal in the mature
spinal cord. During the late fi rst trimester and early
second trimester, these cells gradually appear as the
spinal neuroepithelium recedes.
Fasciculus cuneatus––Central fi ber tract in the dorsal
funiculus above the 6th thoracic level (lateral to the
fasciculus gracilis and medial to the dorsal root col-
lateralization zone). Its main component is ipsilateral
axons from large neurons in the dorsal root ganglia
above the 6th thoracic level that carry exteroceptive
information from the upper part of the body to the
nucleus cuneatus in the lower medulla. A smaller
component is axons of sensory relay neurons in lam-
inae III–V that terminate in the nucleus cuneatus as
part of the postsynaptic–dorsal column system.
Fasciculus gracilis––Medial fi ber tract in the dorsal funic-
ulus. Its main component is ipsilateral axons from
large neurons in the dorsal root ganglia below the
6th thoracic level that carry exteroceptive information
from the lower part of the body to the nucleus gracilis
in the lower medulla. A smaller component is axons
of sensory relay neurons in laminae III–V that termi-
nate in the nucleus gracilis as part of the postsynaptic–
dorsal column system.
Fibrous roof plate––The end feet of roof plate cells that
extend into the spinal canal in the dorsal midline.
Floor plate––Specialized cells that bridge the ventral mid-
line of the spinal canal during the fi rst trimester. Ini-
tially, it is the most ventral structure in the embryonic
spinal cord, lying directly above the notochord. The
fl oor plate moves upward at the end of the fi rst tri-
mester, with the receding ventral neuroepithelium. It
may be involved in ventral motoneuron induction and
may provide directional cues for growing commissural
axons.
Intermediate gray––Spinal gray matter that forms a
bridge between the dorsal and ventral horns (laminae
VI–VII).
Intermediate interneurons––Neurons in the intermediate
gray whose axons do not extend to the brain or enter
the ventral roots. These neurons may be part of the
intrinsic locomotive pattern generator system in the
spinal cord. Many of their axons travel up and down
the spinal cord in the intraspinal tracts.
Intermediate neuroepithelium––Neuroepithelium sur-
rounding the sulcus limitans that produces intermedi-
ate gray neurons.
Intraspinal tracts––Axons of spinal interneurons that
travel in the white matter adjacent to the gray matter
of the dorsal horn, intermediate gray, and ventral horn;
also called the propriospinal tract or the spinospinal
tract. These tracts overlap with the medial longitudi-
nal fasciculus, the tectospinal tract, the ventral and lat-
eral reticulospinal tracts, and the dorsal root collater-
alization zone.
Laminae VI–VII––Gray matter of the intermediate gray.
Lamina VIII––Gray matter of the ventral horn interneu-
rons.
Lamina IX––Gray matter of the ventral horn motoneu-
rons.
Lateral cervical nucleus––Large neurons scattered adja-
cent to and within the reticulated area at cervical
levels. These neurons get input from lamina IV cells
at all levels of the spinal cord. Their axons cross the
midline in the ventral white commissure and travel to
the thalamus in the medial lemniscus.
