GLOSSA RY
GLOSSARY
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.
A
Abducens nucleus (VI) – An aggregate of cranial nerve motor neu-
rons situated beneath the *fourth ventricle in the *pons.
The nucleus receives input from the *vestibular nuclear
complex and is the source of motor fibers of cranial *nerve
VI that innervate the lateral rectus muscle of the eye.Accessory basal nucleus – See Basal accessory nucleus.
Accessory nucleus (XI) – A column of motoneurons that extends
from the region of the *nucleus ambiguus in the medulla to
the 5th-6th segments of the cervical spinal cord. Its axons
form *nerve XI that innervates the sternocleidomastoid and
trapezius muscles.Accumbent neuroepithelium – Putative germinal source of the
neurons and neuroglia of the *nucleus accumbens. After
cessation of neurogenesis, this germinal matrix is trans-
formed into a *glioepithelium.Allocortex – Cortical regions of the telencephalon with a “three-
layered” cytoarchitectonic organization, such as the *hip-
pocampus and *primary olfactory cortex. At most hippo-
campal sites, a central layer of densely packed neurons is
sandwiched between an external and internal fibrous layer
with scattered neurons. The allocortex is a phylogeneti-
cally older telencephalic system than the mammalian “six-
layered” isocortex, or *neocortex.Alvear glioepithelium – Putative source of the glia of the *alveus
(alveolar path) of the *hippocampus.
Alveus – Deep layer of white matter that borders *Ammon’s horn
region of the hippocampus.
Ammon’s horn (hippocampus) – Part of the *hippocampus that
contains a prominent layer of large pyramidal cells that
curves toward the *dentate gyrus. Layers distinguished in
Ammon’s horn are (from deep to superficial) the alveus,
the stratum oriens, the pyramidal layer, the stratum radi-
atum, and the stratum lacunosum moleculare. Ammon’s
horn is subdivided into two large areas: CA1 adjacent to the*presubiculum, and CA3 adjacent to the *dentate gyrus.
The two smaller regions are CA2 and CA4 (in the *hilus of
the dentate gyrus).Amygdala – A large structure in the *uncus of the temporal lobe
that is a basal ganglionic component of the limbic system.
The complex cell groupings in the amygdala have been put
into two major subdivisions: the *corticomedial complex
and the *basolateral complex. This structure is involved
in linking emotions with olfactory, auditory, and visual
sensory information and plays a major role in behavioral
aggression.Amygdalohippocampal area – Also called the cortical amygdaloid
transition area, an ellipitical mass of densely packed cells
in the caudomedial amygdala between the ventral *subicu-
lum and the *cortical nucleus. It is connected to the *bed
nucleus of the stria terminalis and the *ventromedial hypo-
thalamic nucleus.Ansa lenticularis – Fiber tract that originates in the internal (medial)
segment of the *globus pallidus, courses dorsal to the
*zona incerta in *Forel’s fields, and terminates in the *thal-
amus, in particular the *ventral complex and the *centro-
median nucleus.Anterior amygdaloid area – A region of small to medium-sized
cells in the rostral half of the *amygdala that represents a
transition zone between the *substantia innominata and the
amygdaloid complex proper.Anterior commissure – Large fiber bundle that crosses in the
ventral telencephalon and interconnects several forebrain
structures on the right and left sides, including the *olfac-
tory bulb, the *primary olfactory cortex, the *entorhinal
area, the *amygdala, and some components of the *tempo-
ral lobe.Anterior complex (thalamus) – A group of anterior thalamic nuclei
with related connections. Components of the anterior tha-
lamic complex are the anterodorsal nucleus, the anterome-
dial nucleus, and the anteroventral nucleus. The afferents
of the anterior complex come principally from the *hip-pocampal region and the *mammillary body. The ascend-
ing efferents terminate in the *cingulate gyrus while the
descending efferents terminate in the *mammillary body.
The anterior thalamic complex is considered a component
of the “limbic system”.Anterior cortical nucleus (amygdala) – The anterior part of the
large *cortical nucleus that has less dense cellular accumu-
lation in layer II.Anterior corticospinal tract – See Ventral corticospinal tract.
Anterior lobe (cerebellum) – Region of the vermis and hemi-
spheres in front of the primary fissure. It contains the *lin-
gula, *centralis and *culmen in the vermis, and their exten-
sions in the hemispheres.Anterior olfactory nucleus – A nucleus in the olfactory peduncle
that has numerous anatomical connections to the *olfac-
tory bulb and *primary olfactory cortex.Aqueduct – Narrow region of the ventricular system situated
between the *third ventricle rostrally and the *fourth ven-
tricle caudally.Aqueduct (embryonic) – During early development, the *neuro-
epithelium lining the hypertrophied aqueduct of the mes-
encephalon is the source of neurons and neuroglia of the
*superior colliculus, the *inferior colliculus, the *central
gray, the *tegmentum, and several components of the *isth-
mus.Arcuate nucleus (hypothalamus) – A small-celled nucleus that
surrounds the base of the third ventricle posteriorly. It con-
tains releasing hormones and is involved in the central ner-
vous regulation of the anterior pituitary gland.Arcuate nucleus (medulla) –A small group of neurons in the ven-
tral part of the *pyramids caudal to the *pontine gray. The
chief afferents to this nucleus are from motor areas in the
cerebral cortex. This nucleus provides output to the cer-
ebellum. See also Raphe migration.