Glossary 287
Ventral horn––Ventral wings of the butterfl y-shaped gray
matter that contain motoneurons (lamina IX) and ven-
tral horn interneurons (lamina VIII). This is the ear-
liest differentiating region of the spinal cord gray
matter.
Ventral horn interneurons––Neurons in the ventral gray
(lamina VIII) whose axons do not extend to the brain
or enter the ventral roots. These neurons are 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.
Ventral horn motoneurons––Neurons in lamina IX whose
axons leave the spinal cord in the ventral roots of the
spinal nerves and innervate skeletal muscle. These
neurons are generated in the ventral neuroepithelium
during the early fi rst trimester. During the late fi rst
trimester and second trimester, the motoneurons seg-
regate into smaller and smaller clusters (in cross-sec-
tions) that are aligned longitudinally to form columns.
Each column supplies specifi c groups of muscles. The
most medial columns supply axial muscles. Ventro-
lateral columns supply proximal limb muscles in the
cervical and lumbar enlargements. Dorsolateral col-
umns supply distal limb muscles in the cervical and
lumbar enlargements.
Ventral median fi ssure––A deep cleft in the ventral mid-
line that separates the medial walls of the ventral
funiculus on the right and left sides of the spinal cord.
The dorsal border of this fi ssure is formed by the ven-
tral white commissure.
Ventral neuroepithelium––Neuroepithelium surrounding
the ventral canal that produces ventral horn motoneu-
rons, autonomic motoneurons, and ventral horn inter-
neurons. This is the fi rst neuroepithelium to produce
neurons, and the fi rst to recede in the middle fi rst
trimester. A portion of this neuroepithelium may be
transformed to produce glia.Ventral root––Part of the spinal nerve that exits the cen-
tral nervous system near the junction of the ventral and
lateral funiculi. It is composed of the axons of ventral
horn motoneurons that project to skeletal muscle and
lateral horn motoneurons that project to autonomic
ganglia.Ventral rootlets––Fine bundles of axons visible in the
white matter of the ventral funiculus prior to coalesc-
ing and exiting the spinal cord in the ventral root.Ventral spinocerebellar tract––Axons of proprioceptive
sensory processing neurons in the intermediate zone
and spinal border cells in the intermediate gray that
project to the cerebellum.Ventral spinothalamic tract––See Spinocephalic tract.Ventral reticulospinal tract––Axons from large neurons
in the pontine reticular nucleus that terminate primar-
ily in laminae VIII at all levels of the spinal cord.
Axons in this tract travel close to the gray matter in the
ventral funiculus and overlap with axons in the intra-
spinal tracts.Ventral white commissure––Axons that cross the mid-
line beneath the ventral gray commissure. This is one
of the earliest identifi able components of the white
matter that contains early and late myelinating axons.Vestibulospinal tract––Axons of the lateral vestibular
nucleus (Deiter’s) in the medial part of the peripheral
ventral funiculus. These axons terminate in laminae
VII and VIII throughout the entire length of the spinal
cord. These axons overlap with medial axons in the
spinocephalic tracts.