286 Glossary
Rubrospinal tract––Somatotopically arranged axons of
the contralateral red nucleus in the lateral funiculus
that intermingle with ventral fi bers of the lateral corti-
cospinal tract and terminate in laminae V-VIII of the
spinal gray. Medial axons terminate at cervical levels,
lateral axons terminate at lumbosacral levels.
Sojourn zone––The peripheral parts of the dorsal and
intermediate neuroepithelia that contain dense accu-
mulations of premigratory young neurons that stay
(“sojourn”) in the neuroepithelium prior to entering
the gray matter. These zones have ruffl ed edges that
are diffi cult to distinguish from the immediately adja-
cent gray matter. The ventral neuroepithelium may
also contain a sojourn zone, but its ruffl ed outer edges
are less obvious.
Somite––Paired oval clusters of cells outside the embry-
onic spinal cord that contain a dermatome (precursor
of the dermis), myotome (precursor of axial skeletal
musculature), and sclerotome (precursor of the axial
skeleton).
Spinal canal––The enlarged fl uid-fi lled core of the spinal
cord present during the embryonic and early fetal
period. It is surrounded laterally by the dorsal, inter-
mediate, and ventral parts of the neuroepithelium. It
is sealed dorsally by the roof plate and ventrally by the
fl oor plate.
Spinal cord––That part of the central nervous system that
is continuous with the medulla of the brain and occu-
pies the vertebral canal in the spine.
Spinal nerve––A mixed nerve that separates into dorsal
and ventral roots at its points of entry (dorsal root) or
exit (ventral root) from the spinal cord. The incom-
ing axons are those of the dorsal root ganglion. The
outgoing axons are those of the ventral horn somatic
motoneurons and lateral horn visceral motoneurons.
A total of 31 spinal nerves are connected to the spinal
cord.
Spinocephalic tracts––A complex of several tracts occu-
pying a crescent-shaped area in the peripheral ventral
and lateral funiculi. The traditional anatomists named
this region the ventral and lateral spinothalamic tracts.
Modern research indicates that the majority of these
axons terminate in the brainstem reticular formation
(spinoreticular tract), the inferior olive nucleus (spino–
olivary tract), the deep layers of the superior collicu-
lus (spinotectal tract), the central gray in the midbrain,
and the hypothalamus. Less than 50% of these axons
terminate in the thalamus in the ventroposterolateral
nuclear complex and in the intralaminar nuclei.
Spinocerebellar tracts––Combined dorsal and ventral spi-
nocerebellar tracts in the peripheral lateral funiculus.
Spino-olivary tract––Axons in the spinocephalic tracts
that terminate in the accessory nuclei associated with
the inferior olive complex in the lower medulla.
Spinoreticular tract––Axons in the spinocephalic tracts
that terminate in the reticular formation of the medulla
and pons.
Spinotectal tract––Axons in the spinocephalic tracts that
terminate in the deep layers of the superior colliculus.
Subgelatinosal plexus––Intermingled axon terminal
branches (lightly myelinated) and dendrites at the base
of the substantia gelatinosa.
Substantia gelatinosa––Component of the dorsal horn
that contains interneurons in laminae II and III. These
neurons include the central cells, tufted cells, stalked
cells, and islet cells that receive sensory input from
dorsal root ganglion cells.
Sulcus limitans––A shallow depression in the spinal neu-
roepithelium traditionally a landmark that separates
the alar and basal plates. This sulcus marks the area
of the spinal canal that will be retained as the central
canal in the maturing and adult spinal cord.
Tectospinal tract––Contralateral axons from neurons in
the deep layers of the superior colliculus that travel
below and intermingle with the medial longitudinal
fasciculus in the ventral funiculus. All axons in this
tract terminate at cervical levels in laminae VI–VIII
in the intermediate gray. This tract overlaps with the
intraspinal tracts.
Thoracic region––Part of the spinal cord that is the target
and source of 12 pairs of spinal nerves associated with
the 12 thoracic vertebrae. The ventral horn gray matter
is narrow here because there are only axial muscles to
innervate.
Ventral canal––A transient part of the spinal canal that
fi rst expands then shrinks in relation to the changing
size of the ventral neuroepithelium during the fi rst tri-
mester.
Ventral corticospinal tract––Ipsilateral axons in the
medial part of the ventral funiculus from upper moto-
neurons in layer V of the primary motor cortex (pyra-
midal cells of Betz in area 4). These axons are somato-
topically arranged (cervical terminating axons dorsal,
lumbosacral terminating axons ventral). Axons cross
the midline in the ventral white commissure before
terminating in laminae VI-IX in the ventral horn.
Ventral funiculus––White matter between the lateral
funiculus and the ventral white commissure. The
border between the lateral and ventral funiculi is
indistinct. The ventral funiculus contains the follow-
ing tracts: medial longitudinal fasciculus, tectospinal,
ventral corticospinal, ventral reticulospinal, vestibulo-
spinal, ventromedial spinocephalic, and ventromedial
intraspinal.
Ventral gray commissure––A bridge of small neurons in
the midline beneath the central canal.
