Chapter 12
The Central Nervous System
429
12
In this chapter, we examine the structure of the CNS and the
functions associated with its various regions. We also touch on
complex integrative functions, such as sleep-wake cycles and
memory.
The Brain
Te unimpressive appearance of the human
brain
gives few
hints of its remarkable abilities. It is about two good fistfuls of
quivering pinkish gray tissue, wrinkled like a walnut, with a
consistency somewhat like cold oatmeal. Te average adult hu-
man brain has a mass of about 1500 g (3.3 lb).
Embryonic Development
Describe how space constraints affect brain development.
Name the major regions of the adult brain.
Name and locate the ventricles of the brain.
We begin with an introduction to brain embryology, as the ter-
minology used for the structural divisions of the adult brain is
easier to follow when you understand brain development.
Te brain and spinal cord begin as an embryonic structure
called the
neural tube
(Figure 12.1a)
. As soon as the neural
tube forms, its anterior (rostral) end begins to expand and con-
strictions appear that mark off the three
primary brain vesicles
(Figure 12.1b):
Prosencephalon
(pros
0
en-sef
9
ah-lon), or
forebrain
Mesencephalon
(mes
0
en-sef
9
ah-lon), or
midbrain
Rhombencephalon
(romb
0
en-sef
9
ah-lon), or
hindbrain
(Note that
encephalo
means “brain.”) Te remaining
caudal
(“toward the tail”), or posterior, portion of the neural tube be-
comes the spinal cord, which we will discuss later in the chapter.
Te primary vesicles give rise to the
secondary brain vesi-
cles
(Figure 12.1c). Te forebrain divides into the
telencephalon
(“endbrain”) and
diencephalon
(“interbrain”), and the hindbrain
constricts, forming the
metencephalon
(“a±erbrain”) and
my-
elencephalon
(“spinal brain”). Te midbrain remains undivided.
Each of the five secondary vesicles then develops rapidly to
produce the major structures of the adult brain (Figure 12.1d).
Te greatest change occurs in the telencephalon, which sprouts
two lateral swellings that look like Mickey Mouse’s ears. Tese be-
come the two
cerebral hemispheres
, referred to collectively as the
cerebrum
(ser
9
ĕ-brum). Te diencephalon specializes to form
the
hypothalamus
(hi
0
po-thal
9
ah-mus),
thalamus
,
epithalamus
,
and
retina
of the eye. Less dramatic changes occur in the mesen-
cephalon, metencephalon, and myelencephalon as these regions
transform into the
midbrain
, the
pons
and
cerebellum
, and the
medulla oblongata
, respectively. All these midbrain and hindbrain
structures, except the cerebellum, form the
brain stem
.
Te central cavity of the neural tube remains continuous and
enlarges in four areas to form the fluid-filled
ventricles
(
ventr
5
little belly) of the brain (Figure 12.1e). We will describe the ven-
tricles shortly.
Because the brain grows more rapidly than the membranous
skull that contains it, it folds up to occupy the available space.
Te
midbrain
and
cervical flexures
move the forebrain toward
the brain stem
(Figure 12.2a)
. Te cerebral hemispheres are
forced to take a horseshoe-shaped course and grow posteriorly
and laterally (indicated by black arrows in Figure 12.2b). As a
result, they grow back over and almost completely envelop the
diencephalon and midbrain. By week 26, the continued growth
(e) Adult neural
canal regions
(d) Adult brain structures
(a) Neural tube
(contains
neural
canal)
(c) Secondary brain vesicles
(b) Primary brain vesicles
Anterior
(rostral)
Posterior
(caudal)
Spinal cord
Cerebellum
Brain stem: medulla
oblongata
Brain stem: pons
Brain stem: midbrain
Diencephalon
(thalamus, hypothalamus,
epithalamus), retina
Cerebrum: cerebral
hemispheres (cortex,
white matter, basal nuclei)
Myelencephalon
Metencephalon
Mesencephalon
Diencephalon
Telencephalon
Rhombencephalon
(hindbrain)
Mesencephalon
(midbrain)
Prosencephalon
(forebrain)
Central canal
Fourth ventricle
Cerebral aqueduct
Third ventricle
Lateral ventricles
Figure 12.1
Embryonic development of the human brain.
(a)
Formed by week 4, the
neural tube quickly subdivides into
(b)
the primary brain vesicles, which subsequently form
(c)
the secondary brain vesicles by week 5. These five vesicles differentiate into
(d)
the adult
brain structures.
(e)
The adult structures derived from the neural canal.
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