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[ Pobierz całość w formacie PDF ]14
The Brain and Cranial Nerves
Did you know...?
A newborn baby’s brain will triple in size by its
first birthday.
Learning Outcomes
After completing this chapter, you should be able to do the following:
14-1
Name the major brain regions, vesicles, and ventricles, and
describe the locations and functions of each.
14-2
Explain how the brain is protected and supported, and discuss
the formation, circulation, and function of cerebrospinal fluid.
14-3
Describe the anatomical differences between the medulla
oblongata and the spinal cord, and identify the medulla
oblongata’s main components and functions.
14-4
List the main components of the pons, and specify the
functions of each.
14-5
List the main components of the cerebellum, and specify the
functions of each.
14-6
List the main components of the mesencephalon, and specify
the functions of each.
14-7
List the main components of the diencephalon, and specify the
functions of each.
14-8
Identify the main components of the limbic system, and
specify the locations and functions of each.
14-9
Identify the major anatomical subdivisions and functions of
the cerebrum, and discuss the origin and significance of the
major types of brain waves seen in an electroencephalogram.
14-10
Describe representative examples of cranial reflexes that
produce somatic responses or visceral responses to specific
stimuli.
Clinical Notes
Epidural and Subdural Hemorrhages p. 466
Aphasia and Dyslexia p. 488
Disconnection Syndrome p. 488
461
Chapter 14
The Brain and Cranial Nerves
An Introduction to the Brain and
Cranial Nerves
14-1
The brain has several
principal structures, each with
specific functions
This chapter introduces the functional organization of the
brain and cranial nerves, and describes simple cranial re-
flexes. The adult human brain contains almost 97 percent of
the body’s neural tissue. A “typical” brain weighs 1.4 kg (3
lb) and has a volume of 1200 cc (71 in.
3
). Brain size varies
considerably among individuals. The brains of males are, on
average, about 10 percent larger than those of females, ow-
ing to differences in average body size. No correlation exists
between brain size and intelligence. Individuals with the
smallest brains (750 cc) and the largest brains (2100 cc) are
functionally normal.
In this section we introduce the anatomical organization of the
brain. We begin with an overview of the brain’s major regions
and landmarks; then we discuss the brain’s embryological
origins and some prominent internal cavities: the ventricles of
the brain.
Major Brain Regions and Landmarks
The adult brain is dominated in size by the cerebrum
(
Figure
14–1
). Viewed from the anterior and superior
Left cerebral
hemisphere
Gyri
Sulci
CEREBRUM
Conscious thought processes,
intellectual functions
Memory storage and processing
Conscious and subconscious regulation
of skeletal muscle contractions
Fissures
DIENCEPHALON
THALAMUS
Relay and processing
centers for sensory
information
HYPOTHALAMUS
Centers controlling
emotions, autonomic
functions, and hormone
production
CEREBELLUM
Coordinates complex
somatic motor
patterns
Adjusts output of
other somatic motor
centers in brain and
spinal cord
Spinal
cord
MESENCEPHALON
Processing of visual
and auditory data
Generation of reflexive
somatic motor
responses
Maintenance of
consciousness
Brain
stem
PONS
Relays sensory
information to
cerebellum and
thalamus
Subconscious
somatic and visceral
motor centers
MEDULLA OBLONGATA
Relays sensory information to thalamus and
to other portions of the brain stem
Autonomic centers for regulation of visceral
function (cardiovascular, respiratory, and
digestive system activities)
Figure 14–1
An Introduction to Brain Structures and
Functions.
462
Unit 3
Control and Regulation
surfaces, the
cerebrum
(se-RE-brum or SER-e-brum) of the
adult brain can be divided into large, paired
cerebral hemi-
spheres
. The surfaces of the cerebral hemispheres are highly
folded and covered by
neural cortex
(
cortex
, rind or bark), a
superficial layer of gray matter. This
cerebral cortex
forms a
series of elevated ridges, or
gyri
(JI-ri singular,
gyrus
) that
serve to increase its surface area. The gyri are separated by
shallow depressions called
sulci
(SUL-si) or by deeper grooves
called
fissures
. The cerebrum is the seat of most higher men-
tal functions. Conscious thoughts, sensations, intellect, mem-
ory, and complex movements all originate in the cerebrum.
The
cerebellum
(ser-e-BEL-um) is partially hidden by
the cerebral hemispheres, but it is the second-largest part of
the brain. Like the cerebrum, the cerebellum has hemispheres
that are covered by a layer of gray matter, the
cerebellar cortex
.
The cerebellum adjusts ongoing movements by comparing
arriving sensations with previously experienced sensations,
allowing you to perform the same movements over and over.
The other major anatomical regions of the brain can best
be examined after the cerebral and cerebellar hemispheres have
been removed (
Figure 14–1
). The walls of the
diencephalon
(di-en-SEF-a-lon;
dia
, through
tracts and relay centers, the pons also contains nuclei
involved with somatic and visceral motor control.
•
The spinal cord connects to the brain at the
medulla
oblongata
. Near the pons, the posterior wall of the
medulla oblongata is thin and membranous. The inferior
portion of the medulla oblongata resembles the spinal
cord in that it has a narrow central canal. The medulla
oblongata relays sensory information to the thalamus and
to centers in other portions of the brain stem. The
medulla oblongata also contains major centers that
regulate autonomic function, such as heart rate, blood
pressure, and digestion.
The boundaries and general functions of the dien-
cephalon and brain stem are indicated in
Figure 14–1
. In con-
sidering the individual components of the brain, we will
begin at the inferior portion of the medulla oblongata. This
region has the simplest organization found anywhere in the
brain, and in many respects it resembles the spinal cord. We
will then ascend to regions of increasing structural and func-
tional complexity until we reach the cerebral cortex, whose
functions and capabilities are as yet poorly understood.
encephalos
, brain) are com-
posed of the
left thalamus
and
right thalamus
(THAL-a-mus;
plural,
thalami
). Each thalamus contains relay and processing
centers for sensory information. The
hypothalamus
(
hypo
-,
below), or floor of the diencephalon, contains centers involved
with emotions, autonomic function, and hormone produc-
tion. The
infundibulum
, a narrow stalk, connects the hypo-
thalamus to the
pituitary gland
, a component of the
endocrine system. The hypothalamus and the pituitary gland
are responsible for the integration of the nervous and en-
docrine systems.
The diencephalon is a structural and functional link be-
tween the cerebral hemispheres and the components of the
brain stem. The
brain stem
contains a variety of important
processing centers and nuclei that relay information headed
to or from the cerebrum or cerebellum. The brain stem in-
cludes the
mesencephalon
,
pons
, and
medulla oblongata
.
1
Embryology of the Brain
To understand the internal organization of the adult brain, we
must consider its embryological origins. The central nervous
system (CNS) begins as a hollow cylinder known as the
neural
tube
. This tube has a fluid-filled internal cavity, the
neurocoel
.
In the cephalic portion of the neural tube, three areas enlarge
rapidly through expansion of the neurocoel. This enlargement
creates three prominent divisions called
primary brain vesi-
cles
. The primary brain vesicles are named for their relative po-
sitions: the
prosencephalon
(pr¯z-en-SEF-a-lon;
proso
, forward
enkephalos
, brain), or “forebrain”; the
mesencephalon
, or
“midbrain”; and the
rhombencephalon
(rom-ben-SEF-a-lon), or
“hindbrain.”
The fates of the three primary divisions of the brain are
summarized in Table 14–1. The prosencephalon and
rhombencephalon are subdivided further, forming
secondary
brain vesicles
. The prosencephalon forms the
telencephalon
(tel-en-SEF-a-lon;
telos
, end) and the diencephalon. The tel-
encephalon will ultimately form the cerebrum of the adult
brain. The walls of the mesencephalon thicken, and the neu-
rocoel becomes a relatively narrow passageway, comparable to
the central canal of the spinal cord. The portion of the
rhombencephalon adjacent to the mesencephalon forms the
metencephalon
(met-en-SEF-a-lon;
meta
, after). The dorsal
portion of the metencephalon will become the cerebellum,
and the ventral portion will develop into the pons. The por-
tion of the rhombencephalon closer to the spinal cord forms
the
myelencephalon
•
The
mesencephalon
(
mesos
, middle), or midbrain,
contains nuclei that process visual and auditory
information and control reflexes triggered by these
stimuli. For example, your immediate, reflexive
responses to a loud, unexpected noise (eye movements
and head turning) are directed by nuclei in the midbrain.
This region also contains centers that help maintain
consciousness.
•
The
pons
of the brain connects the cerebellum to the
brain stem (
pons
is Latin for “bridge”). In addition to
1
Some sources consider the brain stem to include the diencephalon. We will use
the more restrictive definition here.
(mi-el-en-SEF-a-lon;
myelon
, spinal
463
Chapter 14
The Brain and Cranial Nerves
cord), which will become the medulla oblongata.
ATLAS: Embry-
ology Summary 12: The Development of the Brain and Cranial Nerves
gata expands to form chambers called
ventricles
(VEN-tri-kls).
The ventricles are lined by cells of the
ependyma
.
l
p. 392
Each cerebral hemisphere contains a large
lateral ventri-
cle
(
Figure 14–2
). The
septum pellucidum
, a thin medial par-
tition, separates the two lateral ventricles. Because there are
two
lateral ventricles, the ventricle in the diencephalon is
called the
third ventricle
. Although the two lateral ventricles
Ventricles of the Brain
During development, the neurocoel within the cerebral hemi-
spheres, diencephalon, metencephalon, and medulla oblon-
TABLE 14–1
Development of the Brain
Primary Brain Vesicles (3 weeks)
Secondary Brain Vesicles (6 weeks)
Brain Regions at Birth
Telencephalon
Cerebrum
Prosencephalon
Diencephalon
Diencephalon
Mesencephalon
Mesencephalon
Mesencephalon
Cerebellum
and
Pons
Metencephalon
Rhombencephalon
Medulla
oblongata
Myelencephalon
Cerebral hemispheres
Lateral ventricles
Interventricular
foramen
Third ventricle
Aqueduct of
midbrain
Fourth ventricle
Pons
Cerebellum
Medulla oblongata
Central canal
Spinal cord
(a)
(b)
Figure 14–2
Ventricles of the Brain.
The orientation and extent of the ventricles as they would appear if the brain were transparent.
(a)
A lateral view.
(b)
An anterior view.
ATLAS: Plates 10; 12a–c; 13a–e
464
Unit 3
Control and Regulation
are not directly connected, each communicates with the third
ventricle of the diencephalon through an
interventricular
foramen
(
foramen of Monro
).
The mesencephalon has a slender canal known as the
aqueduct of midbrain
(or
the mesencephalic aqueduct
,
aqueduct of Sylvius
,or
cerebral aqueduct
). This passageway
connects the third ventricle with the
fourth ventricle
. The su-
perior portion of the fourth ventricle lies between the posterior
surface of the pons and the anterior surface of the cerebellum.
The fourth ventricle extends into the superior portion of the
medulla oblongata. This ventricle then narrows and becomes
continuous with the central canal of the spinal cord.
The ventricles are filled with cerebrospinal fluid (CSF).
The CSF continuously circulates from the ventricles and cen-
tral canal into the
subarachnoid space
of the surrounding cra-
nial meninges. The CSF passes between the interior and
exterior of the CNS through three foramina in the roof of the
fourth ventricle; these foramina will be described in a later
section.
The Cranial Meninges
The layers that make up the cranial meninges—the cranial
dura mater, arachnoid mater, and pia mater—are continuous
with those of the spinal meninges.
l
p. 433
However, the
cranial meninges have distinctive anatomical and functional
characteristics (
Figure 14–3a
):
•
The cranial
dura mater
consists of outer and inner fibrous
layers. The outer layer is fused to the periosteum of the
cranial bones. As a result, there is no epidural space
superficial to the dura mater, as occurs along the spinal
cord. The outer, or
endosteal
, and inner, or
meningeal
,
layers of the cranial dura mater are typically separated by
a slender gap that contains tissue fluids and blood
vessels, including several large venous sinuses. The veins
of the brain open into these sinuses, which deliver the
venous blood to the
internal jugular veins
of the neck.
•
The cranial
arachnoid mater
consists of the arachnoid
membrane (an epithelial layer) and the cells and fibers of
the arachnoid trabeculae that cross the subarachnoid
space to the pia mater. The arachnoid membrane covers
the brain, providing a smooth surface that does not
follow the brain’s underlying folds. This membrane is in
contact with the inner epithelial layer of the dura mater.
The subarachnoid space extends between the arachnoid
membrane and the pia mater.
•
The
pia mater
sticks to the surface of the brain, anchored
by the processes of astrocytes. It extends into every fold,
and accompanies the branches of cerebral blood vessels
as they penetrate the surface of the brain to reach internal
structures.
The
A
&
P Top 100
# 48
The brain is a large, delicate mass of neural tissue
containing internal passageways and chambers filled with
cerebrospinal fluid. Each of the six major regions of the brain
has specific functions. As you ascend from the medulla
oblongata (which connects to the spinal cord) to the
cerebrum, those functions become more complex and
variable. Conscious thought and intelligence are provided by
the neural cortex of the cerebral hemispheres.
CHECKPOINT
1. Name the six major regions of the brain.
2. What brain regions make up the brain stem?
3. Which primary brain vesicle is destined to form the
cerebellum, pons, and medulla oblongata?
See the blue Answers tab at the end of the book.
Dural Folds
In several locations, the inner layer of the dura mater extends
into the cranial cavity, forming a sheet that dips inward and
then returns. These
dural folds
provide additional stabiliza-
tion and support to the brain.
Dural sinuses
are large collect-
ing veins located within the dural folds. The three largest
dural folds are called the falx cerebri, the tentorium cerebelli,
and the falx cerebelli (
Figure 14–3b
):
14-2
The brain is protected and
supported by the cranial
meninges, cerebrospinal fluid, and
the blood–brain barrier
1.
The
falx cerebri
(FALKS SER-e-bri;
falx
, curving or sickle-
shaped) is a fold of dura mater that projects between the
cerebral hemispheres in the longitudinal fissure. Its infe-
rior portions attach anteriorly to the crista galli and poster-
iorly to the
internal occipital crest
, a ridge along the inner
surface of the occipital bone. The
superior sagittal sinus
and the
inferior sagittal sinus
, two large venous sinuses,
lie within this dural fold. The posterior margin of the falx
cerebri intersects the tentorium cerebelli.
2.
The
tentorium cerebelli
(ten-TO-ree-e-um ser-e-BEL-e;
tentorium
, a covering) protects the cerebellar hemi-
The delicate tissues of the brain are protected from mechani-
cal forces by the bones of the cranium, the
cranial meninges
,
and cerebrospinal fluid. In addition, the neural tissue of the
brain is biochemically isolated from the general circulation by
the
blood–brain barrier
. Refer to
Figures 7–3
and
7–4
(pp. 215–217) for a review of the bones of the cranium. We
will discuss the other protective factors here.
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