The Human Brain During the Third Trimester

Lateral ventricles (embryonic) – During embryonic development,

the *neuroepithelium lining the hypertrophied lateral ven-

tricles and the adjacent subventricular zone are the source

of neurons and neuroglia in the telencephalon. In the

mature brain the enduring shrunken regions of the lateral

ventricles are lined by cells of the *ependyma.

Lateral vestibular nucleus – Also called Deiter’s nucleus. A col-

lection of large neurons lying dorsolaterally along the wall

of the *fourth ventricle. It receives primary sensory input

from the vestibular ganglion via cranial *nerve VIII and

its large neurons are the source of the vestibulospinal tract.

Smaller neurons send axons to vestibular areas of the cer-

ebellum and to the *medial longitudinal fasciculus.

Layer I (cerebral cortex) – The cell-sparse layer beneath the pia

in all parts of the *cerebral cortex. This is the first cortical

layer to develop and contains the earliest generated cortical

neurons, the Cajal-Retzius cells.

Lingula (cerebellum I-II) – The ventral lobule of the anterior lobe

of the *cerebellar vermis with a tongue-like projection

over the surface of the *superior cerebellar peduncle. This

lobule does not extend to the hemispheres.

Lingula (occipital lobe) – Tongue-shaped gyrus beneath the

cuneus, ventral to the calcarine fissure.

Locus coeruleus – Aggregate of large pigmented cells in the *pons.

It is the major source of ascending and descending norad-

renergic fibers that are widely distributed throughout the

central nervous system.

M

Mammillary body – Distinctive region in the posteroventral hypo-

thalamus, composed of the medial and lateral mammillary

nuclei. Its principal afferents are from the *septum and

*subiculum that course in the *fornix; its efferents form the

*mammillothalamic and *mammillotegmental tracts.

Mammillotegmental tract – Descending fiber bundle containing

*mammillary body efferents to the brain stem, including

the *dorsal tegmental nucleus.

Mammillothalamic tract – Ascending fiber bundle containing

efferents of the *mammillary body that terminate in the

thalamic *anterior complex.

Massa intermedia (thalamus) – Interthalamic bridge across the

*third ventricle that contains several midline structures,

including the *reuniens nucleus and the thalamic *periven-

tricular complex.

Medial accessory olive – A small nucleus in the *inferior olive

complex that contains densely packed neurons along the

lateral border of the medial lemniscus. It receives proprio-

ceptive input from the spinal cord and its efferents reach

the contralateral cerebellum (mainly the vermis) by way of

the *inferior cerebellar peduncle.

Medial forebrain bundle – A diffuse fiber tract that extends from

the *olfactory tubercle, through the *lateral hypothalamic

area, to the *substantia nigra in the midbrain *tegmentum.

It is more conspicuous in the brains of lower animals than

in the human brain.

Medial geniculate body – Principal thalamic relay station in the

auditory pathway to the *cerebral cortex. Its afferents orig-

inate in the *trapezoid body, the *superior olivary com-

plex, the *lateral lemniscus nuclei, and the *inferior col-

liculus. Its efferents form the *auditory radiation that ter-

minates in the *temporal lobe. The connections with the

cerebral cortex are reciprocal.

Medial lemniscus – Large fiber bundle conveying tactile and other

somatosensory input to the thalamus. It originates in the

*gracile and *cuneate nuclei in the *medulla, crosses to the

opposite side (sensory decussation), ascends through the

*pons and *midbrain, and terminates in the *ventral pos-

terolateral and *ventral posteromedial nuclei of the thala-

mus.

Medial lemniscus (decussation) – Also known as the sensory

decussation, a medial area in the lower *medulla, above the

*pyramids, where axons of the medial lemniscus cross to

the opposite side.

Medial longitudinal fasciculus – A dorsomedial tract in the

*midbrain, *pons and *medulla that contains ascending

and descending fibers coursing beneath the *oculomotor

nuclear complex, *trochlear nucleus, *abducens nucleus,

and *hypoglossal nucleus. It turns ventrally in the pos-

terior medulla and extends into the ventral funiculus of

the cervical spinal cord. Its ascending axons originate

throughout the *vestibular nuclear complex. Its descend-

ing axons originate in the *medial vestibular nucleus and

the *reticular formation.

Medial nucleus (amygdala) – Part of the *corticomedial complex

that contains more loosely packed small-to-medium sized

cells than the adjacent *cortical nucleus. It gets input from

layer III of the *primary olfactory cortex, thalamic *peri-

ventricular complex, hypothalamic *ventromedial nucleus,

and *lateral hypothalamic area. It projects to the *orbital

cortex, thalamic *periventricular complex, and *ventrome-

dial hypothalamic nucleus.

Medial preoptic nucleus – A rounded mass of neurons in the

*medial preoptic area that plays a role in sexual functions.

Medial septal nucleus – An indistinct nucleus in the midline septum

that is continuous with the vertical limb of the *diagonal

band of Broca.

Medial vestibular nucleus – Component of the *vestibular nuclear

complex situated underneath the *fourth ventricle medial

to the other vestibular nuclei. Its neurons receive primary

sensory input from the vestibular ganglion via cranial

*nerve VIII and project to the *fastigial nucleus, the *floc-

culus *nodulus, and *uvula. Medial vestibular axons also

extend through cervical levels of the spinal cord in the

*medial longitudinal fasciculus.

Median preoptic nucleus – A small nucleus in the *preoptic area

that forms a narrow cap around the *anterior commissure

in the midline.

Medulla – Region of the neuraxis surrounding the posterior *fourth

ventricle, also known as the medulla oblongata, bounded

by the *pons rostrally and the *spinal cord caudally.

An extremely heterogeneous region containing sensory,

somatic motor, and visceral motor nuclei as well as large

ascending, descending, and decussating fiber tracts.

Medullary neuroepithelium – Primary germinal matrix that lines

the core and recesses of the posterior embryonic *fourth

ventricle. Its several subdivisions are the source of neu-

rons and glia of the different sensory, relay and motor

nuclei of the medulla. During the third trimester, the

medullary neuroepithelium has already been transformed

into a glioepithelium/ependyma. See also Precerebellar

neuroepithelium.

Medullary layer (cerebellar cortex) – The deep white matter in

the *cerebellar cortex. It contains ascending climbing and

mossy fibers and descending Purkinje cell axons.

Mesencephalic neuroepithelium – Primary germinal matrix that

lines the embryonic *aqueduct. Its different subdivisions

produce neurons and glia for the *superior colliculus, the

*inferior colliculus, the *central gray, and the *tegmentum.

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