The Human Brain During the Third Trimester

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Secondary fissure (cerebellum) – Fissure in the midline vermis

that separates the *posterior lobe from the *central lobe.

Secondary germinal matrix – Layer or field of proliferative pre-

cursors of neurons and glia outside the primary *neuro-

epithelium. These cells are progeny of the stem cells of

the primary neuroepithelium that persist for varying peri-

ods after the neuroepithelium has disappeared. Examples

of secondary germinal matrices are the *external germinal

layer of the cerebellum, the *subventricular zone of the

neocortex, the *subgranular zone of the dentate gyrus, the

*rostral migratory stream, and the *striatal subventricular

zone. Typically, the secondary germinal matrices are the

source of late-generated small interneurons, or microneu-

rons.

Septum – Midline telencephalic structure beneath the anterior

*corpus callosum that is better developed in lower mam-

mals than in man. It contains *medial and *lateral nuclei.

Its principal connections are with the *hippocampus and

the *hypothalamus by way of the *fornix. The septum is a

focal component of the limbic system.

Sexually dimorphic nucleus – A small, dense cluster of neurons

closely associated with the *medial preoptic nucleus in the

*preoptic area.

Simplex lobule (cerebellum HVI) –Lobule in the cerebellar hemi-

sphere that is continuous with the vermal *declive. It is

delineated anteriorly by the *primary fissure and posteri-

orly by *crus I of the ansiform lobule.

Solitary tract and nucleus – This tract contains primary sensory

fibers that reach the *medulla by way of cranial *nerves

VII, IX, and X, conveying gustatory (VII and IX) and

visceral sensory information (IX and X) to the solitary

nucleus. The *dorsal sensory nucleus of X and the *com-

missural nucleus of X are part of this nuclear complex.

Spinal cord – Caudal tubular component of the central nervous

system that surrounds the *central canal. Its continuous

core of gray matter (the dorsal horn, intermediate gray and

ventral horn) and surrounding white matter (the dorsal,

*lateral and ventral funiculi) are divided into 31 segments

by the discontinuous entry of dorsal root afferents and exit

of ventral root efferents that form the mixed spinal nerves.

The cervical enlargement and the lumbar enlargement of

the spinal cord supply the nerves of the forelimbs and hind

limbs, respectively.

Spinal nucleus (V) – See Trigeminal, spinal nucleus.

Spinal tract (V) – See Trigeminal, spinal tract.

Spinocephalic tract – Large ascending fiber tract in the *lateral

funiculus of the spinal cord whose fibers or collaterals are

widely distributed throughout the *medulla, *pons, *mid-

brain, *thalamus, and the limbic forebrain. The spinoce-

phalic tract, which includes the spinoreticular, spinomes-

encephalic and spinothalamic tracts, conveys nociceptive

and other affective stimuli to the brain. It is distinguished

from the phylogenetically more recent ascending *cuneate

and *gracile funiculi that convey primarily cognitive som-

esthetic information to the neocortex.

Spinocerebellar tracts – Several ascending pathways, including

the dorsal and ventral spinocerebellar tracts in the *lateral

funiculus that convey proprioceptive information from

muscles and joints to the *cerebellum.

Stratified transitional field – Transient intermediate field in the

fetal *cerebral cortex between the *neuroepithelium (or

*subventricular zone) and the *cortical plate. It is a

crisply laminated site with alternating fiber-rich and cell-

rich layers that vary in their configuration in different lobes

of the cerebral cortex; e.g., frontal stratified transitional

field; occipital stratified transitional field. The different

fibrous layers are continuous with the incoming thalamo-

cortical afferents, the outgoing corticofugal efferents, and

the decussating fibers of the *corpus callosum. The cellu-

lar layers are composed of sojourning and migrating neu-

rons all of which eventually settle in the *cortical plate.

Stria medullaris (thalamus) – Mediodorsal fiber bundle in the

diencephalon coursing in an anteroposterior direction and

terminating in the *habenular nuclei.

Striatal neuroepithelium – Primary germinal source of neurons

of the *caudate nucleus, *putamen, and *globus pallidus.

It has a large anterolateral and anteromedial division, also

known as the lateral and medial eminences, and a small

posterior division that generates the neurons of the tail of

the caudate nucleus.

Striatal subventricular zone – A large *secondary germinal matrix

flanking the striatal neuroepithelium that generates the bulk

of the neurons of *caudate nucleus, *putamen and *globus

pallidus.

Striate cortex – Principal projection area of the *visual radiation,

situated along the *calcarine sulcus. This distinctive thin

cortex is rich in small granule cells and contains a white

band (of Gennari) within layer IV.

Stria terminalis – Arched fiber bundle that originates in the cor-

ticomedial and basolateral *amygdala, courses along the

medial surface of the *caudate nucleus, and terminates in

the *bed nucleus of the stria terminalis, the anterior *hypo-

thalamus, and the *preoptic area.

Striatum – Term used for two components of the *basal ganglia: the

*caudate nucleus and the *putamen.

Strionuclear glioepithelium – Putative source of the glia of the

stria terminalis, stria medullaris and possibly other nearby

fiber tracts.

Strionuclear neuroepithelium – Putative germinal source of the

neurons and glia of the *bed nucleus of the stria termina-

lis. It is situated beneath the *striatal neuroepithelium in a

notch of the *lateral ventricle near the *foramen of Monro.

Throughout the third trimester, this germinal site is most

likely the source of glia, and is labeled as the *strionuclear

glioepithelium.

Subcommissural organ – A highly vascularized circumventricular

neuroendocrine organ located beneath the *posterior com-

missure in the roof of the posterior *third ventricle and

anterior *aqueduct. It is devoid of neurons and may disap-

pear from the human brain after birth.

Subfornical organ – A highly vascularized circumventricular neu-

roendocrine organ situated between the two columns of

the fornix at the confluence of the *third ventricle and the

*foramen of Monro. Among its afferents are fibers from

the *preoptic area and the anterior *hypothalamus. Its

efferents target the hypothalamic *paraventricular nucleus

and the *supraoptic nucleus.

Subgranular zone (hippocampus) – A long-persisting secondary

germinal matrix beneath the *granular layer of the dentate

gyrus. It is a division of the *hippocampal neuroepithe-

lium and is the source of late-generated dentate granule

cells.

Subicular complex – Collective term for the *parasubiculum, the

*presubiculum and the *subiculum of the *parahippocam-

pal gyrus.

Subiculum – *Allocortical component of the *hippocampus,

between CA1 and the *presubiculum.

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