Human Physiology, 14th edition (2016)

The Central Nervous System 207

8.1 Structural Organization of the Brain

The brain is composed of an enormous number of asso-

ciation neurons and accompanying neuroglia, arranged in

regions and subdivisions. These neurons receive sensory

information, direct the activity of motor neurons, and per-

form such higher brain functions as learning and memory.

consciousness, are creations of the brain. Whimsical though it

seems, the study of brain physiology is the process of the brain

studying itself.

The study of the structure and function of the central ner-

vous system requires a knowledge of its basic “plan,” which is

established during the course of embryonic development. The

early embryo contains on its surface an embryonic tissue layer

known as ectoderm; this will eventually form the epidermis of

the skin, among other structures. As development progresses,

a groove appears in this ectoderm along the dorsal midline of

the embryo’s body. This groove deepens, and by the twentieth

day after conception it has fused to form a neural tube. The

part of the ectoderm where the fusion occurs becomes a sepa-

rate structure called the neural crest, which is located between

the neural tube and the surface ectoderm ( fig. 8.2 ). Eventually

the neural tube will become the central nervous system, and the

neural crest will become the ganglia of the peripheral nervous

system, among other structures.

By the middle of the fourth week after conception, three

distinct swellings are evident on the anterior end of the neural

tube, which is going to form the brain: the forebrain (pros-

encephalon), midbrain (mesencephalon), and hindbrain

(rhombencephalon). During the fifth week, these areas become

modified to form five regions. The forebrain divides into the

telencephalon and diencephalon; the mesencephalon remains

unchanged; and the hindbrain divides into the metencephalon

and myelencephalon ( fig.  8.3 ). These regions subsequently

become greatly modified, but the terms used here are also used

to indicate general regions of the adult brain.

The basic structural plan of the CNS can now be understood.

The telencephalon (refer to fig. 8.3 ) grows disproportionately in

Kevin sustained head injuries in an automobile accident.

After a hospital stay, he returned home but needed to take

Ambien to help him sleep, and his right arm remained

partially paralyzed. His speech was slow and difficult,

although his meaning was clear. When his wife saw him

sway and stagger when he walked, she accused him of

having too much to drink, which he denied. She took him

to be examined, and the physician told Kevin to point to

his wife. When he attempted this, his arm made oscilla-

tory movements back and forth. The physician stroked

the sole of Kevin’s bare right foot, and this made his large

toe extended upward.

Some of the new terms and concepts you will

encounter include:

• Cerebral lateralization, Broca’s and Wernicke’s
  areas, and aphasias


• Reticular activating system, cerebellum functions,
  and Babinski’s reflex

Clinical Investigation

Figure 8.1 The CNS consists of the brain and

the spinal cord. Both of these structures are covered with

meninges and bathed in cerebrospinal fluid.

Central canal

Gyrus Sulcus

Cerebrum

Meninges

Corpus

callosum

Tentorium

cerebelli

Spinal cord Cerebellum

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe the embryonic origin of the CNS.


2. Identify the five brain regions and the major
   structures they contain, including the ventricles.

The central nervous system (CNS), consisting of the brain
and spinal cord ( fig. 8.1 ), receives input from sensory neurons
and directs the activity of motor neurons that innervate mus-
cles and glands. The association neurons within the brain and
spinal cord are in a position, as their name implies, to associate
appropriate motor responses with sensory stimuli, and thus to
maintain homeostasis in the internal environment and the con-
tinued existence of the organism in a changing external envi-
ronment. Further, the central nervous systems of all vertebrates
(and most invertebrates) are capable of at least rudimentary
forms of learning and memory. This capability—most highly
developed in the human brain—permits behavior to be modi-
fied by experience. Perceptions, learning, memory, emotions,
and perhaps even the self-awareness that forms the basis of