Download ortant Facts

Six
The Human Nervous Syslem: An Analomical ViewoinL
Sixlh Edilion, Mufiay L. Barr and John A. Kieman J,B,
Lippincott Company, Philadelphia, O I993
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ortant Facts
hoceeding laterally from the ventral midline, the anatomical landmarks at
different levels are as follows, Each represents a functionally important
nucleus or tract within the brain stem. The student must also know the sites
of emergence of cranial nerues III to XII in relation to these landmarks.
Medulla: furamid, olive, inferior cerebellar peduncle; cuneate and gracile
tubercles (below obe><); floor of fourth ventricle (above obe><).
Pons: Basal part of pons, middle cerebellar peduncle, superior cerebellar
peduncle, floor of fourth ventricle.
Midbrain: Interpeduncular frossa; basis pedunculi, inferior or superior colliculus.
In the floor of the fourth ventricle motor nuclei of cranial nerves are typically
medial and sensory nuclei lateral to the sulcus limitans. There are named
areas for hypoglossal, vagal and vestibular nuclei. The,Tacial colliculus
overlies the abducens nucleus.
The superior and inferior medullaryvela form the roof of the fourthventricle,
which narrows into the central canal caudallv and the cerebral aoueduct
rostrally.
Cerebrospinal fluid enters the fourth ventricle from the aqueduct and leaves
by way of the median and lateral apertures.
brain stem consists of the medulla oblongata, pons, and midbrain. Although each of
The
the three regions has special features, they
have certain fiber tracts in common. and each
region includes nuclei of cranial nerves. The
fourth ventricle is partly in the medulla and
partly in the pons. It is advantageous therefore
88
to describe the medulla, pons, and midbrain
together. This chapter is concerned with the
sutface landmarks of the brain stem. For more
details ofthe internal features ofthe brain stem
(such as certain nuclei and tracts) that are
mentioned in this chapter, see Chapter 7 or
refer to the Index. The central connections and
4
5
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I
Chapter 6: Brain Stem: External
B
functions of the cranial nerve s are explained in
Chapter 8.
Medulla Oblongata
The medulla oblongata (ormedulla) is about 3
cm long and widens gradually in a rostral di-
It rests on the basilar portion of the
occipital bone and is concealed from above by
rection.
Anatomy g9
the cerebellum. The junction of the spinal cord
and medulla is at the upper rootlet of the first
cervical nerve,
num. The rostr
marked on the
sulcus (Fig. 6-l and 6-2). On the dorsal surface, the junction between the pons and
medulla is an imaginary transverse line that
passes between the caudal margins of the mid-
Optic tract
Mamillary body
Interpeduncular
fossa
Oculomotor
n.
Basis pedunculi
of cerebral peduncle
Trigeminal n.
Basilar sulcus
Middle cerebellar
peduncle
Abducens
Facial
Basal portion
of pons
n,
n.
Vestibulocochlear n.
Clossopharyngeal n.
Ove
Vagus n.
Pyramid
Hypoglossal n
Ventrolateral sulcus
Ventral
median fissure
Cranial root of
accessory n.
Spinal root of
accessory n.
Ventral root of
f irst cervical n.
Figure 6-1. Ventral aspect of the brain stem.
90
Regional Anatomy of the Central Neryous System
Lateral geniculate body
Medial geniculate body
Optic tract
Superior colliculus
lnferior brachium
Inferior colliculus
Superior brachium
Trochlear n.
Cerebral peduncle
Position of lateral lemniscus
Superior cerebel lar peduncle
Basal portion
of pons
Trigeminal n.
Middle cerebellar peduncle
I
nferior cerebellar peduncle
Vestibulocochlear n.
Abducens n.
Facial
Pyramid
n,
Clossopharyngeal n.
Cuneate tubercle
Vagus n.
Cracile tubercle
Hypoglossal n,
Dorsolateral sulcus
Olive
Fasciculus cuneatus
Cranial root of accessory n
Tuberculum cinereum
Spinal rootof accessory
n,
Fasciculus gracilis
Ventrolateral sulcus
First cervtcal n.
Figure 5-2. Lateral aspect of the brain stem.
SCTT
dle cerebellar peduncles (Fig. 6-3). The dorsal
surface, therefore, contains the caudal half of
the fourth ventricle; this rostral end of the
medulla is known as the open portion, That
part of the medulla between the obex (see
below) and the first cervical segment of the
spinal cord is called the closed portion; ir
contains a continuation of the central canal of
the spinal cord,
The ventricle results from a flexure of the
embryonic brain with a dorsal concavity (the
pontine flexure) and subsequent development
of the large cerebellum with its thick peduncles. These events caused divergence of the
dorsal halves of the maturing brain stem, so
that its lumen widened out to form the fourth
ventricle,
The longitudinal grooves previously de-
me(
rup
dles
nal
me(
ther
The
late
suri
Chapter 6: Brain Stem: External
Anatomy
Pineal body
Superior colliculus
Medial geniculate body
Superior brachium
Lateral geniculate body
Inferior brachium
lnferior colliculus
Trochlear n
Superior medullary velum
Position
Medial em
of lateral lemniscus
Superior cerebellar peduncle
Sulcus limltans
I nferior cerebellar
peduncle
Striae medullares
Middle cerebellar peduncle
Area vestibuli
Hypoglossal triangle
Dorsal cochlear nucleus
Vestibulocochlear
Clossopharyngeal
Obex
n
n
Vagal triangle
Cuneate tubercle
Area postrema
Cracile tuberc
Vagus n
Dorsolateral sulcus
Dorsal median sulcus
Fasciculus cuneatus
Cranial root of accessory
Tuberculum cinereum
Spinal root of accessory
Fascicgrlus gracilis
n.
n.
Dorsal root
of first cervical
n.
Figure 5-3. Dorsal aspect of the brain stem.
scribed for the spinal cord continue on the
medulla. The ventral median fissure is interrupted at the spinomedullary junction by bundles ofdecussating fibers, These are corticospi-
nal fibers that cross from the pyramid of the
medulla to the opposite side of the cord, where
they constitute the lateral corticospinal tract.
The dorsal median, dorsolateral, and ventrolateral sulci also extend from the cord onto the
surface of the medulla.
On each side, several bulges or eminences
are outlined by the sulci. Ventrally, the pyramid (see Fig. 6-l) consists of corticospinal
fibers. This is the origin of the term ,,pyramidal
ftact" as a synonym for corticospinal tract.
Laterally (Fig. 6-2), the olive is a prominent
oval swelling that marks the position of the
inferior olivary nucleus. Rootlets of the glossopharyngeal, vagus, and accessory nerves are
attached to the medulla just dorsal to the olive,
9l
92
Regional Anatomy of the Central Nervous System
The tuberculum cinereum, immediately dorsal to these nerve rootlets, is a rather incon-
nerves as well as those of the cranial division
of the accessory nerve are attached to the
pa
spicuotis ridge that marks the position of the
dorsal spinocerebellar tract and the more
deeply situated spinal tract of the trigeminal
nerve and its associated nucleus, The latter are
comparable to the dorsolateral tract (of Lis-
medulla along a line between the olive and the
tuberculum cinereum, The accessory nerve is
tre
motor, whereas the glossopharyngeal and vagus
nerves are mixed, having sensory and motor
components. The cranial root of the accessory
nerve is joined by the spinal root, and the
glossophanTngeal, vagus, and accessory nerves
leave the posterior cranial fossa through the
tic
jugular foramen, Roots of the hypoglossal
nerve, a motor nerve, emerge along the ventrolateral sulcus between the pyramid and the
m,
olive, and the nerve leaves the posterior fossa
rh
sauer) and the outer laminae of the dorsal
horn in the spinal cord. The gracile and cuneate fasciculi continue from the spinal cord
into the dorsal area of the medulla (see Fig. 6-2
and 6-3), The gracile and cuneate tubercles are slight elevations at the rostral ends of
the corresponding fasciculi of the spinal cord.
They contain the gracile and cuneate nuclei, in
which the fibers of the fasciculi end. The apex
of the V-shaped boundary of the inferior portion of the fourth ventricle, which is folded
caudally over the most rostral I to 2 mm of the
central canal, is known as the obex.
Seven cranial nerves are attached to the
medulla or to the junction of the medulla and
pons (see Figs. 6-1, 6-2, and 6-3), The abducens nerve emerges near the midline be-
tween the pons and the pyramid of the
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medulla, The nerve passes forward in the subarachnoid space beneath the pons, traverses
the cavernous venous sinus (described in Clls.
25 and 26), and enters the orbit through the
superior orbital fissure. The facial and vestibulocochlear nerves are attached to the
brain stem at the caudal border of the pons
well out laterally. The facial nerve, which is
the more medial, has two roots, motor and
sensory, The sensory root, which includes
some efferent parasympathetic fibers, lies be-
tween the larger motor root and the vestibulocochlear nerve; it is therefore known as
the nervus intermedius (of Wrisberg). The
cochlear division of the vestibulocochlear nerve
ends in the dorsal and ventral cochlear nuclei,
which are situated on the base of the inferior
cerebellar peduncle, whereas the vestibular division penetrates the brain stem deep to the
root of the inferior cerebellar peduncle. The
facial and vestibulocochlear nerves enter the
internal acoustic meatus in the petrous temporal bone,
Roots of the glossopharyngeal and vagus
cr(
rh
te)
sit
co
m'
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hr
th
through the hypoglossal canal.
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m
ns
de
This part of the brain stem, which is about 2,5
cm long, owes its name to the appearance
presented on its ventral surface (see Fig. 6- l),
which is that of a bridge connecting the right
and left cerebellar hemispheres. The appearance is deceptive as far as the constituent nerve
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fibers are concerned, as noted below.
The pons consists of quite different basal
(ventral) and dorsal portions (see Figs. 7 -9 and
fe.
7- r0),
The basal
portion
is distinctive of this
flc
is
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SO
TI
ni
th
part
of the brain stem. A shallow groove, the basilar srilcus, runs along its ventral surface in the
midline. The pons merges laterally into the
middle cerebellar peduncles, with the attachment of the trigeminal nerve marking the
transition between the pons and the peduncle
(see Figs. 6- I and 6-2). The motor root of the
F,
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b(
in
ol
flt
ffigeminal nerve is rostromedial to the larger
sensory root. The trigeminal nerve enters the
middle cranial fossa at the medial end of the
petrous temporal bone, where the trigeminal
m
di
ganglion is located. The three divisions of the
nerve diverge from the ganglion, embedded in
the dura mater, The ophthalmic division passes
through the superior orbital {issure to reach
the orbit. The maxillary division traverses the
foramen rotundum, and the mandibular division traverses the foramen ovale.
Fibers from the cerebral cortex terminate
ipsilaterally on nerve cells that compose the
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Chapter 6: Bfain Stem: F.xternal
pontine nuclei, and axons of the latter cells
cross the midline and then constitute the contralateral middle cerebellar peduncle. In effect,
the basal pons is a large synaptic or relay station, providing a connection between the cortex of each cerebral hemisphere and the opposite cerebellar hemisphere as part of a circuit
contributing to efficient voluntary movements. The cerebral cortex, basal pons, and
cerebellum all increased in size during mammalian evolution and are best developed in the
human brain. The corticospinal tracts traverse
the basal portion of the pons before they enter
the pyramids (see Fig. 7-9).
The dorsal portion or tegmentum of the
pons is similar to much of the medulla and
midbrain, in that it contains,ascending and
descending tracts and nuclei ofcranial nerves.
The dorsal surface of the pons is formed by the
flooi of the fourth ventricle .
The rostral part of the pons is known as the
isthmus of the brain stem. A slight bandlike elevation runs obliquely across the dorsolateral surface of the isthmus toward the inferior colliculus of the midbrain (see Fig. 6-2).
This elevation is produced by the lateral lemniscus, which carries auditory fibers through
the pons.
Fourth ntricle
The
floor of the fourth ventricle (rhom-
boid fossa) is broad in its midportion, narrowing toward the obex caudally and the aqueduct
of the midbrain rostrally (see Fig. 6-3). The
floor is divided into symmetrical halves by a
median sulcus; the sulcus limitans further
divides each half into medial and lateral areas.
The vestibular nuclear complex lies beneath
the floor of most of the lateral area. This area is
therefore known as the vestibular area of the
rhomboid fossa. Motor nuclei are located beneath the floor of the medial area, The caudal
part of the rhomboid fossa is marked by two
triangles or trigones, The rostral end of the
dorsal nucleus of the vagus nerve lies beneath the vagal triangle (or ala cinerea), and
the rostral end of the hypoglossal nucleus
Iies beneath the hypoglossal triangle. The area
Anatomy 93
postrema is a narrow strip between the vagal
triangle and the most caudal part of the margin
of the ventricle. Because the appeararrce of this
part of the rhomboid fossa suggested the tip of
a pen to early anatomists, the term calamus
scriptorius was applied to it,
The facial colliculus, a slight swelling at
the lower end of the medial eminence (see
Fig. 6-3), is formed by fibers from the moror
nucleus of the facial nerve looping over the
abducens nucleus. There is a pigmented area,
the locus coeruleus, at the rostral end of the
sulcus limitans, indicating the site of a cluster
of noradrenergic nerve cells that contain
melanin pigment, In the middle of the floor of
the fourth ventricle, delicate strands of nervc
fibers emerge from the median sulcus, run laterally as the striae medullares, and enter the
inferior cerebellar peduncle. The connections
of these fibers are explained in Chapter 7.
The tent-shaped roof of the fourth ventricie
protrudes toward the cerebellum. The rostral
part of the roof is formed on each side by the
superior cerebellar peduncles, which consist mainly of fibers proceeding from cerebellar
nuclei into the midbrain, The V-shaped interval between the converging peduncles is
bridged by the superior medullary velum,
a sheet of tissue that consists of a layer of pia
mater and one of ependyma with nerve fibers
in between. The remainder of the roof consists
of a thinner pial-ependymal membrane, the
inferior medullary velurn, which often adheres to the undersurface of the cerebellum. A
deficiency of variable size in the inferior
medullary velum constitutes the median aperture of the fourth ventricle, alternatively
known as the foramen of Magendie. This
hole provides the principal communication
between the ventricular system and the subarachnoid space (Fig. 6-4).
The lateral walls of the fourth ventricle in-
clude the inferior cerebellar peduncles,
which curve from the medulla into the cerebellum on the medial aspects of the middle
peduncles (see Fig. 6-3). Lateral recesses of
the ventricle extend around the sides of the
medulla and open ventrally as the lateral apertures of the fourth ventricle (the fbrarnina
94
Regional Anatomy of the Central Nervous System
Figure 6-4. Median aperture of
the fourth ventricle (foramen of
Magendie), opening from the
fourth ventricle into the cerebellomedullary cistern of the
subarachnoid space.
of Luschka), which
are two other channels
through which cerebrospinal fluid enters the
subarachnoid space (Fig. 6- 5 ). These foramina
are at the junction of the medulla, pons, and
cei:ebellum (the cerebellopontine angles) near
the attachment to the brain stem of the vestibulocochlear and glossopharyngeal nerves.
The choroid plexus of the fourth ventricle is
suspended from the inferior medullary velurrr,
the plexus extends into the lateral recesses,
and a small tuft usually protrudes through
each foramen of Luschka.
Midbrain
The midbrain is about I.5 cm long. Its ventral
surface extends from the pons to the mamillarybodies of the diencephalon (see Fig. 6-I).
The robust column of white matter on each
side is the basis pedunculi (crus cerebri),
which consists of fibers of the pyramidal motor
system and corticopontine fibers. The deep depression between these two columns is the
interpeduncular fossa. Many small blood
vessels penetrate the midbrain in the floor of
the interpeduncular fossa; this region is there-
fore known as the posterior perforated
sqbstance. The oculomotor nerve emerqes
(x 2.5)
from the side of the interpeduncular fossa and
passes forward through the cavernous venous
sinus and then through the superior orbital
fissure into the orbit.
The lateral surface of the midbrain (see Fig.
6-2) is formed mainly by the cerebral peduncle, which constitutes the major portion of
tair
tex
otl
this region of the brain stem on each side. The
cerebral peduncle comprises the basis pedunculi and some internal structures, the substantia nigra and the tegmentum, which are d.escribed in Chapter 7.
The dorsal surface of the midbrain bears
four rounded elevations. the paired inferior
and superior colliculi (also called the corpora quadrigemina). These colliculi (see Figs.
6-2 and 6-3) make up the tectum and indicate the extent of the midbrain on the dorsal
surface. The inferior colliculus is a relay nucleus on the auditory pathway. Fibers that con -
nal
nect the inferior colliculus with the specific
thalamic nucleus for hearing (medial genicuIate nucleus) form an elevation known as the
thr
inferior brachium (see Figs. 6-2 and 6-3).
The superior colliculus is involved in the control of ocular movements and related movements of the head in response to visual and
other stimuli. The superior brachium con-
IOS
are
pul
em
dal
mi,
the
Ldt
tw
Fig
thr
Chapter 6: Bfain Stem: External
Anatouy 95
C lossopharyngeal,
vagus, and
accessory nerves
Figure 6-5. Lateral apertures of the fourth ventricle (foramina of
Luschka). Tlrfts of choroid plorus (arcows) occupy the foramina, into
which marker sticks (black) have been inserted. (x 1.5)
tains fibers proceeding from the cerebral cortex and the retina to the superior colliculus.
Other fibers in the superior brachium terminate in the pretectal area ventral and just
rostral to the superior colliculi; these fibers
dial and lateral geniculate bodies, and a prominent part of the thalamus known as the pulvinar (see Figs. 7-14 and 7-15).
are pafi of a pathway from the retina for the
pupillary light reflex, The trochlear nerve
SUGGESTED READING
emerges from the brain stem immediately cau-
Bertram EGM, Moore I(Ll An Atlas of the Human
Brain and Spinal Cord, Baltimore, Williams &
dal to the inferior colliculus, curves around the
midbrain, and enters the orbit after traversing
the cavernous venous sinus.
The posterior part of the thalamus projects
caudally beyond the plane of transition between the diencephalon and the midbrain (see
Fig. 6-3). Consequently transverse sections at
the level of the superior colliculi
include
thalamic nuclei, in particular those of the me-
Wilkins, 1982
Montemurro DG, Bruni JE: The Human Brain in
Dissection, 2nd ed. New York, Oxford University Press, 1988
Noback CR, Strominger NL, Demarest RJ: The Human Nervous System: Introduction and Review, 4th ed. Philadelphia, Lea 6 Febiger, l99t
Smith CG: Serial Dissections of the Human Brain.
Baltimore, Urban & Schwarzenberg, lgBI