Glossary 285
Lateral corticospinal tract––Contralateral axons in the
dorsal part of the lateral funiculus from upper moto-
neurons in layer V of the primary motor cortex (pyra-
midal cells of Betz in area 4) and other cortical areas.
These axons are somatotopically arranged (cervical
terminating axons medial, lumbosacral terminating
axons lateral). Axons from area 4 terminate primarily
in laminae VI-IX in the ventral horn.
Lateral funiculus––White matter between Lissauer’s tract
and the ventral funiculus. The border between the
lateral and ventral funiculi is indistinct. The lateral
funiculus contains the following tracts: dorsal and
ventral spinocerebellar, lateral corticospinal, rubrospi-
nal, lateral reticulospinal, dorsolateral spinocephalic,
and dorsolateral intraspinal.
Lateral horn motoneurons––Preganglionic (visceral)
motoneurons of the autonomic nervous system. These
neurons are generated in the ventral neuroepithelium
during the early fi rst trimester and migrate dorsolater-
ally during the late fi rst trimester to settle in the lateral
horn. There are no lateral horn motoneurons at cervi-
cal levels. At thoracic and upper lumbar levels, the
axons of these motoneurons leave the spinal cord in
the ventral roots and terminate in the chain of sym-
pathetic ganglia located adjacent to the anterolateral
vertebral column. At sacral levels, the axons of these
motoneurons leave the spinal cord in the ventral roots
and terminate in parasympathetic ganglia located in
the abdominopelvic cavity.
Lateral plate––Dorsal, intermediate, and ventral parts of
the spinal cord neuroepithelium.
Lateral reticulospinal tract––Axons from large neurons
in the medullary reticular formation that terminate pri-
marily in laminae VII at all levels of the spinal cord.
Axons in this tract travel close to the gray matter in the
lateral funiculus and overlap with axons in the intra-
spinal tracts.
Lateral spinothalamic tract––See Spinocephalic tracts.
Lissauer’s tract––Unmyelinated axons of small neurons
in dorsal root ganglia that carry pain input to the dorsal
horn of the spinal cord; also called the dorsolateral
fasciculus. This tract forms the boundary between the
dorsal funiculus and the lateral funiculus.
Lumbar enlargement––The part of the spinal cord that
has a larger diameter and contains the gray matter and
white matter associated with the lumbosacral plexus,
the nerves that innervate the lower limb.
Lumbosacral region––The most caudal part of the spinal
cord that contains the lumbar enlargement. There are
fi ve lumbar spinal nerves, fi ve sacral spinal nerves,
and one coccygeal spinal nerve.
Mantle zone or layer––See Primordial gray matter.
Marginal zone or layer––See Primordial white matter.
Medial longitudinal fasciculus––Axons from neurons in
the medial vestibular nucleus; also called the medial
vestibulospinal tract. This fasciculus is most promi-
nent at cervical levels adjacent to the medial ventral
horn below the ventral white commissure in the ven-
tral funiculus. Axons in this fasciculus overlap with
those in the intraspinal tracts.Medulla––also called medulla oblongata. The part of the
brain that is continuous with the spinal cord and con-
tains the posterior part of the fourth ventricle.Motoneurons––A general term for neurons whose axons
leave the spinal cord in the ventral roots of a spinal
nerve. Axons of these neurons terminate either on
skeletal muscle (somatic motoneurons) or on postgan-
glionic neurons in autonomic ganglia (preganglionic
visceral motoneurons).Neuroepithelium––The primordial nervous system com-
posed of neural stem cells (NEP cells). The ultimate
source of neurons, neuroglia, ependymal cells, tany-
cytes, and other specialized cells in the central nervous
system.Notochord––A solid core of cells that lie in the midline
beneath the neural tube fl oor plate that will form the
spinal cord. These cells provide “induction” signals
that establish “ventralization” of the neural tube so
that motoneurons are only generated by stem cells in
the ventral neuroepithelium.Oval bundle of His––See Dorsal root bifurcation zone.Primordial gray matter––A thin layer of young neurons
between the spinal cord neuroepithelium and the pri-
mordial white matter present only during the early fi rst
trimester. The primordial gray matter differentiates
into specifi c parts of the spinal gray. This area is tra-
ditionally called the mantle zone or layer.Primordial white matter––A thin cell-sparse layer beneath
the pial membrane in the embryonic spinal cord. The
main components of this layer are the end feet of spe-
cialized glial cells and spaces where axons of spinal
cord fi ber tracts will form the dorsal, lateral, and ven-
tral funiculi. This area is traditionally called the mar-
ginal zone or marginal layer.Propriospinal tract––See Intraspinal tracts.Reticulated area––A region in the lateral funiculus adja-
cent to the intermediate gray where white matter and
gray matter intermingle.Roof plate––Non-neural cells that cap the dorsal midline
of the spinal canal during the fi rst trimester. Initially,
it is the most dorsal structure in the embryonic spinal
cord, but it sinks downward at the end of the fi rst tri-
mester as the dorsal neuroepithelium recedes. As that
happens, the dorsal funiculus fi lls with axons sepa-
rated in the midline by aligned non-neural cells that
form the dorsal median septum.