The Central Nervous System
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
Te unimpressive appearance of the human
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).
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
. 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
means “brain.”) Te remaining
(“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-
(Figure 12.1c). Te forebrain divides into the
(“interbrain”), and the hindbrain
constricts, forming the
(“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
, referred to collectively as the
ĕ-brum). Te diencephalon specializes to form
of the eye. Less dramatic changes occur in the mesen-
cephalon, metencephalon, and myelencephalon as these regions
transform into the
, and the
, respectively. All these midbrain and hindbrain
structures, except the cerebellum, form the
Te central cavity of the neural tube remains continuous and
enlarges in four areas to form the ﬂuid-ﬁlled
little belly) of the brain (Figure 12.1e). We will describe the ven-
Because the brain grows more rapidly than the membranous
skull that contains it, it folds up to occupy the available space.
move the forebrain toward
the brain stem
. 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
(d) Adult brain structures
(a) Neural tube
(c) Secondary brain vesicles
(b) Primary brain vesicles
Brain stem: medulla
Brain stem: pons
Brain stem: midbrain
white matter, basal nuclei)
Embryonic development of the human brain.
Formed by week 4, the
neural tube quickly subdivides into
the primary brain vesicles, which subsequently form
the secondary brain vesicles by week 5. These ﬁve vesicles differentiate into
The adult structures derived from the neural canal.