The Human Brain During the Third Trimester

In the third trimester, this germinal matrix has already been

transformed into a glioepithelium/ependyma.

Mesencephalic nucleus (V) – Large neurons scattered along the lat-

eral border of the *central gray of the midbrain and pons.

They are primary sensory neurons that enter the brain from

the periphery early in development relaying proprioceptive

information from the muscles of mastication. They may

function in the reflex control of bite strength.

Meyer’s loop – Part of the visual radiation that curves into the tem-

poral lobe on its way to the *occipital lobe.

Midbrain – The most anterior part of the brainstem divided into the

*tectum (“roof”) and the *tegmentum (“floor”).

Midbrain tectum – See Tectum.

Midbrain tegmentum – See Tegmentum.

Middle cerebellar peduncle – Massive tract of *pontocerebellar

fibers that originate in the *pontine gray and enter the cer-

ebellum posterolateral to the *inferior cerebellar peduncle.

Molecular layer (cerebellar cortex) – Superficial layer that con-

tains the dendrites of Purkinje cells, the parallel fibers

(axons) of granule cells, the climbing fibers of the *inferior

olive, and the small basket and stellate cells and their pro-

cesses.

Molecular layer (dentate gyrus) – Superficial fibrous layer of the

hippocampal *dentate gyrus. It contains the dendrites of

granule cells, some scattered neurons, and axon terminals

from multiple sources.

Motor nucleus, V – See Trigeminal, motor nucleus.

Motor nucleus, VII See Facial, motor nucleus.

Myelination gliosis – Sometimes called premyelination gliosis.

Transient increase in the concentration of proliferating glia

in fiber tracts before the onset of myelination. It is evi-

dent during late fetal development in the *cuneate fascicu-

lus and deep parts of the *gracile fasciculus.

N

Neocortex – The “six-layered” cortex of the cerebral hemispheres,

also known as the isocortex. Its principal divisions are the

*frontal lobe, the *paracentral lobule, the *parietal lobe,

the *occipital lobe, the *temporal lobe, and the *insula.

The neocortex is considered to be a phylogenetically more

recent laminated telencephalic structure than the “three-

layered” *allocortex.

Neocortical neuroepithelium – Extensive primary germinal matrix

that lines the dorsal and lateral walls of the hypertrophied

embryonic *lateral ventricles. The nuclei of the dividing

cells shuttle to the lumen of the ventricle to undergo mitotic

division there. It is the ultimate source of neurons and

glia in the neocortex and some other structures, such as the

*primary olfactory cortex and the *basolateral complex of

the amygdala.

Neocortical stratified transitional field – See Stratified transi-

tional field (cortical).

Neocortical subventricular zone – A *secondary germinal matrix

situated either above the *neocortical neuroepithelium or,

after dissolution of the *neuroepithelium, adjacent to the

*lateral ventricle *ependyma. It is derived from the pri-

mary cortical neuroepithelium but is distinguished from it

in that the nuclei of its proliferating cells do not shuttle to

the lumen of the ventricle during mitosis.

Nerve III (oculomotor) – Cranial motor nerve originating in the

*oculomotor nuclear complex. It innervates all the extra-

ocular muscles – except the lateral rectus and superior

oblique – and the skeletal muscles of the eyelid, the smooth

sphincter muscles of the iris, and the ciliary muscles of the

lens.

Nerve IV (trochlear) – Cranial motor nerve composed of axons of

the *trochlear nucleus that innervates the superior oblique

muscle of the eye. This nerve is unique because it exits

from the dorsal surface of the *midbrain behind the *infe-

rior colliculus.

Nerve V (trigeminal) – A mixed sensory and motor cranial nerve

that has three peripheral branches, the ophthalmic, the

maxillary, and the mandibular. All three branches contain

peripheral sensory fibers from the trigeminal ganglion that

terminate in the *trigeminal principal sensory nucleus, the

*trigeminal spinal nucleus, and the substantia gelatinosa

in upper cervical segments of the spinal cord. A bundle

of fibers in the mandibular branch, originating in the *tri-

geminal motor nucleus, innervates the muscles of mastica-

tion.

Nerve VI (abducens) – A motor cranial nerve that originates in

the *abducens nucleus and emerges near the midline at the

caudal border of the *pons. The fibers innervate the lateral

rectus muscle of the eye.

Nerve VII (facial) – A mixed sensory and motor nerve, the facial

nerve has three components. Primary sensory gustatory

fibers from the geniculate ganglion enter the solitary tract

and terminate in the *solitary nucleus. Somatic motor

fibers from the *facial motor nucleus innervate the mus-

cles of facial expression. Visceral motor (parasympathetic)

fibers from preganglionic neurons in the dorsolateral pon-

tine *reticular formation (the indistinct salivatory nucleus)

target the pterygopalatine and submandibular ganglia.

Nerve VIII (cochlear, vestibular) – A sensory cranial nerve that

contains primary auditory afferents from the spiral gan-

glion in the cochlea and primary vestibular afferents from

the vestibular (Scarpa’s) ganglion. The auditory afferents

terminate in the dorsal and ventral *cochlear nuclei. The

vestibular afferents terminate in the nuclei of the *vestibu-

lar nuclear complex and some reach the cerebellar *nodu-

lus and *flocculus.

Nerve IX (glossopharyngeal) – A mixed sensory and motor cranial

nerve that exits the *medulla between the *inferior olive

and the *inferior cerebellar peduncle. The sensory part

of nerve IX relays gustatory input from the posterior third

of the tongue and visceral sensory input from the tonsils,

the Eustachian tube, and the carotid sinus via primary sen-

sory neurons in the superior and inferior (nodose) ganglia.

These fibers enter the solitary tract and terminate in the

*solitary nucleus. The somatic motor part of nerve IX

originates in the *nucleus ambiguus and innervates the

pharyngeal and laryngeal muscles. The visceral motor

fibers from parasympathetic preganglionic neurons in the

salivatory nucleus terminate in the otic ganglion.

Nerve X (vagus) – A mixed sensory and motor nerve, with some

somatic and many visceral afferents and efferents (para-

sympathetic) that are widely distributed throughout the

body, including the pharynx, larynx, trachea, esophagus,

and all the thoracic and abdominal viscera. Vagal affer-

ents terminate in the *solitary nucleus and other medullary

sites. Most of its preganglionic motor neurons are located

in the *dorsal motor nucleus (X).

Nerve XI (accessory) – This nerve has a cranial and a spinal com-

ponent. The cranial motor fibers innervate the intrinsic

muscles of the larynx and the spinal motor fibers inner-

vate the sternocleidomastoid and upper trapezius muscles.

The spinal fibers originate in a column of motoneurons that

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