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Brain
51
Interventricular foramen
Corpus callosum (cut)
Cut edge of tela choroidea of 3rd ventricle
3rd ventricle
Choroid plexus
Internal cerebral vein
Superior thalamostriate vein
(vena terminalis)
Head of caudate nucleus
Septum pellucidum
Columns of fornix
Anterior tubercle
Stria terminalis
Interthalamic adhesion
Pes hippocampi
Inferior horn of lateral ventricle
Dentate gyrus
Collateral eminence
Hippocampus
Fimbria of hippocampus
Lamina affixa
Stria medullaris
Habenular trigone
Pulvinar (lifted)
Lateral geniculate body
Medial geniculate body
Brachium of superior colliculus
Posterior commissure
Brachium of inferior colliculus
Habenular commissure
Pineal gland
Superior colliculus
Inferior colliculus
Collateral trigone
Cerebellum
Calcar avis
Posterior horn of lateral ventricle
Calcarine sulcus (fissure)
3.18 THALAMIC ANATOMY
The thalamus is viewed from above. The entire right side of the
brain, just lateral to the thalamus, has been removed, the head
of the caudate nucleus has been sectioned, the corpus callosum
and all tissue dorsal to the thalamus have been removed, and
the third ventricle has been opened from its dorsal surface.
The pineal gland is present in the midline, just caudal to the
third ventricle; it produces melatonin, a hormone that helps
regulate circadian rhythms, sleep, and immune � responses.
The superior and inferior colliculi are shown, depicting the
dorsal surface of the midbrain. On the left, the temporal horn
of the lateral ventricle, with the hippocampal formation, has
been exposed to show the relationship of these structures to
the thalamus. The terminal vein and choroid plexus accompany the stria terminalis along the lateral margin of the thalamus. The stria medullaris runs along the medial surface of the
dorsal thalamus.
52
Overview of the Nervous System
3rd ventricle
Interthalamic adhesion
Internal medullary lamina
MD
LP
Mi
rnal
Inte
M
n
dli
Intralaminar
nuclei
)
l
dia
Me
llary
edu
ina
ian
ed
m
e(
l am
Pulvinar
a
lamin
m
VA
LD
LP
VPL
VL
3rd ventricle
CM
VI
VP
M
VP
M
VP
Reticular nucleus
of thalamus
L
VP
External medullary lamina
Pulvinar
Lateral geniculate body
Thalamic nuclei
Midline (median) nuclei
Schematic section through thalamus
(at level of broken line shown in
figure at right)
CM
LD
LP
M
MD
VA
VI
VL
VP
VPL
VPM
Centromedian
Lateral dorsal
Lateral posterior
Medial group
Medial dorsal
Ventral anterior
Ventral intermedial
Ventral lateral
Ventral posterior (ventrodorsal)
Ventral posterolateral
Ventral posteromedial
3.19 THALAMIC NUCLEI
The thalamus is subdivided into nuclear groups (medial,
lateral, and anterior) that are separated by medullary (white
matter) lamina. Many of these thalamic nuclei are “specific”
thalamic nuclei that are reciprocally connected with discrete
regions of the cerebral cortex. Some nuclei, such as those embedded within the internal medullary lamina (intralaminar
nuclei such as the centromedian and parafascicular nuclei)
and the outer, lateral shell nucleus (reticular nucleus of the
thalamus), have very diffuse, nonspecific associations with the
cerebral cortex.
Medial geniculate body
Schematic representation of thalamus
(external medullary lamina and reticular nuclei removed)
Lateral cell mass
Medial cell mass
Anterior cell mass
CLINICAL POINT
Thalamic syndrome (posterolateral thalamic syndrome, or Dejerine-Roussy syndrome) results from obstruction of the thalamo�
geniculate arterial supply to the region of the thalamus where the
�ventroposterolateral nucleus is located. Initially, all sensation is lost
in the contralateral body, epicritic more completely than protopathic.
Commonly, severe spontaneous pain occurs contralaterally, described
as stabbing, burning, or tearing pain; it is diffuse and persistent. Even
light stimulation can evoke such pain (hyperpathia), and other �sensory
stimuli or emotionally charged situations can result in these painful
sensations. Even when the threshold for pain and temperature sensation (protopathic sensations) is elevated, the thalamic pain may be
present; it is called analgesic dolorosa. If the vascular lesion includes
the subthalamic nucleus or associated basal ganglia circuitry, the patient may also experience hemiballismus (or choreoform or athetoid)
movements in addition to the sensory deficits.
4
BRAIN STEM AND
CEREBELLUM
4.1
Brain Stem Surface Anatomy: Posterolateral View
4.2
Brain Stem Surface Anatomy: Anterior View
4.3
Cerebellar Anatomy: External Features
4.4
Cerebellar Anatomy: Internal Features
53
54
Overview of the Nervous System
Posterolateral View
Thalamus
Pulvinar
Optic tract
Pineal gland
Lateral geniculate body
Medial geniculate body
Brachia of superior and inferior colliculi
Cerebral peduncle
Trochlear (IV) nerve
Superior colliculi
Pons
Inferior colliculi
Trigeminal (V) nerve
Superior cerebellar peduncle
Superior medullary velum
Middle cerebellar peduncle
Vestibulocochlear (VIII) nerve
Medial eminence
Facial (VII) nerve
Inferior cerebellar peduncle
Rhomboid fossa of 4th ventricle
Vestibular area
Facial colliculus
Cuneate tubercle
Gracile tubercle
Dorsal roots of 1st spinal nerve (C1)
Fasciculus cuneatus
Olive
Hypoglossal (XII) nerve
Glossopharyngeal (IX) and vagus (X) nerves
Hypoglossal trigone
Vagal trigone
Accessory (XI) nerve
Fasciculus gracilis
4.1 BRAIN STEM SURFACE ANATOMY:
�POSTEROLATERAL VIEW
The entire telencephalon, most of the diencephalon, and the
cerebellum are removed to reveal the dorsal surface of the brain
stem. The three cerebellar peduncles (superior, middle, and inferior) are sectioned and the cerebellum removed. The dorsal
roots provide input into the spinal cord, and the cranial nerves
provide input into and receive output from the brain stem. The
fourth nerve (trochlear) is the only cranial nerve to exit �dorsally
from the brain stem. The tubercles and trigones on the floor of
the fourth ventricle are named for nuclei just beneath them.
The superior and inferior colliculi form the dorsal surface of
the midbrain, and the medial and lateral geniculate nuclei (associated with auditory and visual processing, respectively) are
shown at the caudalmost region of the diencephalon.
CLINICAL POINT
The facial colliculus is an elevation on the floor of the fourth ventricle in the pons under which is located the abducens nucleus (cranial
nerve VI) and the axons of the facial nerve nucleus (VII), which arc
around the abducens nucleus. A tumor or other lesion on one side
of the floor of the fourth ventricle may induce symptoms related to
cranial nerves VI and VII, including ipsilateral paralysis of lateral gaze
(lateral rectus) and medial gaze (resulting from damage to nerve VI’s
interneurons, which ascend to the nucleus of CN III via the medial
longitudinal fasciculus) as well as ipsilateral facial palsy.
The cerebellar peduncles convey the cerebellar afferent and efferent fibers. The superior peduncle conveys the major efferents to
the red nucleus and thalamus (especially the ventrolateral nucleus),
whereas the inferior peduncle conveys the major efferents to the vestibular and reticular nuclei. The middle peduncle conveys the corticoponto-�cerebellar fibers. Afferents enter the cerebellum especially
through the inferior peduncle but also through the superior peduncle.
Damage to the lateral hemisphere of the cerebellum or its associated
peduncles can result in ipsilateral symptoms, including limb ataxia,
mild hypotonia, dysmetria (misjudgment of distance), decomposition of movement (especially movement involving several joints),
intention tremor (with movement), dysdiadochokinesia (inability
to � perform rapid alternating movements), and inability to dampen
movements appropriately (rebound phenomena).
Brain Stem and Cerebellum
55
Insula
Olfactory tract
Anterior View
Anterior perforated substance
Optic chiasm
Infundibular stalk
Tuber cinereum
Optic tract
Cerebral peduncle
Mammillary body
Temporal lobe
Oculomotor (III) nerve
Trochlear (IV) nerve
Pons
Posterior perforated substance
in interpeduncular fossa
Trigeminal (V) nerve
Lateral geniculate body
Facial (VII) nerve
Abducens (VI) nerve
Vestibulocochlear (VIII) nerve
Basilar groove
Flocculus
Middle cerebellar peduncle
Olive
Pyramid
Ventral roots of 1st spinal nerve (C1)
Pyramidal decussation
4.2 BRAIN STEM SURFACE ANATOMY:
�ANTERIOR VIEW
The left temporal lobe is dissected to show the anterior ―ventral)
surface of the brain stem. The cerebral peduncles, direct caudal
extensions of the posterior limb of the internal capsules, carry
corticospinal and corticobulbar fibers from the internal capsule to the spinal cord and brain stem, respectively. The decussation of the pyramids marks the boundary between the caudal
medulla and the cervical spinal cord. Cranial nerve XI (accessory) is associated with the lateral margin of the upper cervical
spinal cord. Cranial nerves XII (hypoglossal), X (vagus), and
IX (glossopharyngeal) emerge from the ventrolateral margin
of the medulla. Cranial nerves VI (abducens), VII (facial),
and VIII (vestibulocochlear) emerge from the boundary between the medulla and the pons. Cranial nerve V (trigeminal)
emerges from the lateral margin of the upper pons. Cranial
nerve III (oculomotor) emerges from the interpeduncular
fossa in the medial portion of the caudal midbrain. The optic
nerve, chiasm, and tract (cranial nerve II) and the olfactory
tract (cranial nerve I) are not peripheral nerves; they are central nervous system tracts that were mistakenly called cranial
nerves by anatomists in centuries past.
CLINICAL POINT
The oculomotor nerve (III) emerges from the ventral surface of the
brain stem in the interpeduncular fossa, at the medial edge of the
Choroid plexus of 4th ventricle
at foramen of Luschka
Glossopharyngeal (IX) nerve
Vagus (X) nerve
Hypoglossal (XII) nerve
Accessory (XI) nerve
cerebral peduncle. In conditions of increased intracranial pressure in
the anterior and middle cranial fossa, such as that caused by a tumor,
edema from injury, or other space-occupying lesions, the brain stem
can herniate through the tentorium cerebelli, a rigid wing of dura. The
resultant transtentorial herniation can compress the third nerve on
one side (ipsilateral fixed and dilatated pupil resulting from parasympathetic disruption and paralysis of medial gaze resulting from motor
fiber disruption) and compress the cerebral peduncle on that same
side, resulting in contralateral hemiparesis.
The medullary pyramids contain the descending corticospinal tract
fibers from the ipsilateral cerebral cortex, particularly from the motor
and premotor cortex. The major crossing of the corticospinal tract
takes place in the decussation of the pyramids (80%), producing the
crossed, descending, lateral corticospinal tract in the spinal cord. An
infarct in the upper reaches of the anterior spinal artery or the paramedian branches of the vertebral artery can result in damage to the ipsilateral pyramid (contralateral hemiparesis); to the ipsilateral medial
lemniscus (contralateral loss of epicritic somatosensory sensations
such as fine, discriminative touch, vibratory sensation, and joint position sense); and the ipsilateral hypoglossal nerve (cranial nerve XII;
paralysis of the ipsilateral tongue, which deviates toward the weak side
when protruded). This condition is called Dejerine’s syndrome. The
hemiparesis is not spastic and is characterized by mild loss of tone, loss
of fine hand movements, and a plantar extensor response (Babinski’s
sign). It appears that isolated damage to the pyramids does not result
in spasticity. Damage to other descending systems, from either the
�motor-related cortices or the upper motor neurons, must accompany
pyramidal tract damage to produce spasticity. Thus, the term pyramidal tract syndrome, when used to describe spastic hemiplegia, is a misnomer and is anatomically incorrect.