The Human Brain During the Third Trimester

extends from the region of the *nucleus ambiguus to the

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segments of the cervical *spinal cord.

Nerve XII (hypoglossal) – A somatic motor cranial nerve that origi-

nates in the *hypoglossal nucleus and leaves the *medulla

between the *pyramids and the *inferior olive. The fibers

innervate the intrinsic and extrinsic muscles of the tongue.

Neuroepithelium – The primary source of the neurons and glia

in the central nervous system. The neuroepithelial stem

cells initially form the neural plate, which folds to form

the neural groove. After closure of the neural tube cau-

dally and of the cephalic vesicles rostrally, the *ventricles

form and henceforth the nuclei of neuroepithelial stem

cells shuttle to the ventricular lumen to undergo mitosis.

The neuroepithelium is divisible into mosaic regions or

patches that give rise to neurons and glia of distinct brain

structures or cell types. The identified patches are listed

seperately, e.g. *frontal neuroepithelium. The neuroepi-

thelium is the source of several *secondary germinal matri-

ces that also generate neurons. As neurogenesis ceases, the

pluripotential neuroepithelium is transformed at many sites

into a *glioepithelium, such as the *callosal glioepithelium

or the *fornical glioepithelium, or into the *ependyma that

lines the enduring ventricles.

Nodulus (cerebellum X) – Cerebellar lobule coextensive with the

*inferior lobe of the cerebellar vermis. It is separated from

the *posterior lobe by the *posterolateral fissure. It is con-

tinuous, via the floccular peduncle, with the *flocculus in

the hemisphere. The nodulus gets extensive primary ves-

tibular input from the vestibular ganglion and secondary

input from the *vestibular nuclear complex.

Nucleus accumbens – Ganglionic component of the ventral tel-

encephalon ventromedial to the *striatum. It is distin-

guished from the striatum by its cellular organization, neu-

rochemical composition, and intimate connections with the

*hypothalamus, *amygdala, and other parts of the limbic

system.

Nucleus ambiguus – Aggregate of somatic motor neurons that form

a thin column in the ventrolateral medulla. Its axons inner-

vate the muscles of the larynx and pharynx via *nerve IX.

Nucleus of the lateral lemniscus (dorsal) – Interstitial neurons

in the *lateral lemniscus ventral to the *inferior colliculus

with input from the *cochlear nuclei and the *superior oli-

vary complex. Its axons join the lateral lemniscus to termi-

nate bilaterally in the *inferior colliculus and the *medial

geniculate body.

Nucleus of the lateral lemniscus (ventral) – Interstitial neurons in

the *lateral lemniscus lateral to the *superior olivary com-

plex. Input and output are similar to the dorsal nucleus of

the lateral lemniscus.

Nucleus of the lateral olfactory tract (amygdala) – A small

mass of densely-stained neurons located deep to the lateral

*olfactory tract and superficial to the medial part of the

cortical nucleus. These neurons are reciprocally connected

to the *olfactory bulb.

Nucleus of Roller – A large-celled nucleus ventral to the *hypo-

glossal nucleus. Sometimes classified as a component of

the perihypoglossal nuclear complex.

O

Occipital lobe – The most posterior region of the cerebral cortex,

partially separated from the *parietal lobe by the *parieto-

occipital sulcus. It contains several visual areas with direct

projections from the retina, by way of the *lateral genicu-

late body, or indirect projection by way of the *pulvinar.

The distinctive primary visual projection area, the *striate

cortex, lies along the wall of the *calcarine sulcus.

Occipital neuroepithelium – Putative source of the neurons and

glia of the occipital lobe. It is flanked in the fetal neocor-

tex by the occipital subventricular zone and the distinctive

*stratified transitional field before all its neurons settle in

the visual cortex.

Oculomotor nerve (III) – See Nerve III.

Oculomotor nuclear complex – Situated at the base of the *central

gray beneath the *aqueduct, the cell columns of this com-

plex extend from the anterior pole of the *superior collicu-

lus rostrally to the *trochlear nucleus caudally. Its somatic

motor nuclei innervate the medial rectus, inferior rectus,

superior rectus and inferior oblique muscles of the eye, and

are associated with the fibers of the *medial longitudinal

fasciculus. Most prominent of its autonomic (pregangli-

onic) components is the dorsally located Edinger-Westphal

nucleus.

Olfactory bulb – Laminated brain structure where the first-order

fibers of the olfactory nerve terminate and the second-order

fibers of the lateral and medial *olfactory tracts originate.

It is composed of three classes of neurons: large mitral

cells, the intermediate tufted cells, and the small granule

cells. The late-generated granule cells migrate to the olfac-

tory bulb by way of the *rostral migratory stream.

Olfactory peduncle – A stalk that connects the olfactory bulb to

the brain. It contains the *olfactory tract and the anterior

olfactory nucleus. During the third trimester, the olfac-

tory peduncle contains the *rostral migratory stream in its

core.

Olfactory tubercle – Paleocortical area in the ventral telencepha-

lon deep to the *diagonal band of Broca (vertical limb) and

superficial to the *nucleus accumbens/ventral *striatum. It

contains many dense cellular aggregates, the *islands of

Calleja. Input comes mainly from the lateral *olfactory

tract.

Olfactory tract – Large fiber bundle that originates in the *olfac-

tory bulb and has two parts, the larger lateral olfactory stria

and the smaller medial stria. The fibers of the lateral stria

terminate in the *olfactory tubercle, the *primary olfactory

cortex, and *corticomedial complex of the amygdala.

Optic chiasm – Site of the partial decussation of fibers of the *optic

nerve. Fibers from the nasal half of each retina cross here

to the opposite side while those from temporal half proceed

uncrossed to form the *optic tract.

Optic nerve – The large second cranial nerve containing the axons

of retinal ganglion cells. Beyond the *optic chiasm this

nerve is called the *optic tract.

Optic tract – Large bundle of crossed and uncrossed retinal affer-

ents. In the human brain the majority of the fibers ter-

minate in the *dorsal lateral geniculate nucleus; others

proceed to the *superior colliculus, the *suprachiasmatic

nucleus, and the *pretectum.

Orbital gyrus – Ventromedial region of the *frontal lobe with affer-

ents from the thalamic dorsomedial nucleus and efferents

to the *preoptic area and the *hypothalamus. The region

has been implicated in such visceromotor functions, as the

regulation of blood pressure, respiratory rate, and gastric

motility.

P

Parabigeminal nucleus – This small nucleus, composed of a dorsal

and ventral aggregate of cells in the lateral wall of the

*midbrain tegmentum, receives second-order visual input

from the *superior colliculus. It may be the homologue of

the isthmo-optic nucleus of lower vertebrates.

Parabrachial nucleus – Dorsolateral pontine structure with indis-

tinct boundaries that surrounds the *superior cerebellar

An asterisk in front of a term indicates that it has a separate entry in the Glossary with additional

information. Terms referring to transient developmental structures are underlined.

GLOSSARY