The Central Nervous System 2098.2 Cerebrum
The cerebrum, consisting of five paired lobes within two
convoluted hemispheres, contains gray matter in its cortex
and in deeper cerebral nuclei. Most of what are considered
to be the higher functions of the brain are performed by the
cerebrum.the brain and deeper within the brain in aggregations known
as nuclei. White matter consists of axon tracts (the myelin
sheaths produce the white color) that underlie the cortex and
surround the nuclei. The adult brain contains an estimated
100 billion (10^11 ) neurons, weighs approximately 1.5 kg (3 to
3.5 lb), and receives about 15% of the total blood flow to the
body per minute. This high rate of blood flow is a consequence
of the high metabolic requirements of the brain; it is not, as
Aristotle believed, because the brain’s function is to cool the
blood. (This fanciful notion—completely incorrect—is a strik-
ing example of prescientific thought, having no basis in experi-
mental evidence.)
Scientists have demonstrated that the brains of adult mam-
mals, including humans, contain neural stem cells that can
develop into neurons and glial cells. Neurogenesis —the for-
mation of new neurons from neural stem cells—has been dem-
onstrated in two locations. One location is the subventricular
zone, a thin layer adjacent to the ependyma that lines the lateral
ventricles. In mammals other than humans, new neurons gen-
erated in this area migrate to the olfactory bulbs, which are
involved in the sense of smell. In humans, the new interneurons
generated in the subventricular zone migrate instead to an area
called the striatum, involved in the regulation of motor control
and cognitive functions. Robust neurogenesis also occurs in
a second brain area—the subgranular zone of the hippocam-
pus (see fig. 8.15 ). Neurogenesis here results in newly formed
interneurons that function within the hippocampus, perhaps to
aid the learning and memory that depend on the hippocampus
(discussed in section 8.2).
Figure 8.4 The ventricles of the brain. ( a ) An anterior view and ( b ) a lateral view.
Interventricular
foramenMesencephalic
aqueduct
Fourth ventricle(a)To central canal
of spinal cordThird ventricleLateral ventricleInterventricular
foramenThird ventricle(b)To central canal
of spinal cordFourth ventricleMesencephalic
aqueductLateral ventricle| CHECKPOINT
- Identify the three brain regions formed by the middle
of the fourth week of gestation and the five brain
regions formed during the fifth week. - Describe the embryonic origin of the brain ventricles.
Where are they located and what do they contain?
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the organization of the sensory and motor
areas of the cerebral cortex, and the nature of the
basal nuclei (basal ganglia). - Distinguish between the functions of the right
and left cerebral hemispheres, and describe the
significance of the limbic system. - Identify the areas of cerebral cortex involved in
speech and language. - Describe the brain regions involved in memory and
some synaptic events associated with learning and
memory.
The cerebrum ( fig. 8.5 ), which is the only structure of the
telencephalon, is the largest portion of the brain (accounting
for about 80% of its mass) and is the brain region primarily
responsible for higher mental functions. The cerebrum consists
of right and left hemispheres, which are connected internally
by a large fiber tract called the corpus callosum (see fig. 8.1 ).
The corpus callosum is the major tract of axons that function-
ally interconnects the right and left cerebral hemispheres.Cerebral Cortex
The cerebrum consists of an outer cerebral cortex, composed
of 2 to 4 mm of gray matter and underlying white matter. The
cerebral cortex is characterized by numerous folds and grooves