The Central Nervous System
Structural Organization of the Brain
I. During embryonic development, five regions of the brain are formed: the telencephalon,
diencephalon, mesencephalon, metencephalon, and myelencephalon.
A. The telencephalon and diencephalon constitute the forebrain; the mesencephalon is
the midbrain, and the hindbrain is composed of the metencephalon and the
myelencephalon
B. The CNS begins as a hollow tube, and thus the brain and spinal cord are hollow. the
cavities of the brain are known as ventricles.
Cerebrum
I. The cerebrum consists of two hemispheres connected by a large fiber tract called the corpus
callosum.
A. The outer part of the cerebrum, the cerebral cortex, consists of gray matter.
B. Under the gray matter is white matter, but nuclei of gray matter, known as the basal
nuclei, lie deep within the white matter of the cerebrum.
C. Synaptic potentials within the cerebral cortex produce the electrical activity seen in
an electroencephalogram (EEG).
II. The two cerebral hemispheres exhibit some degree of specialization of function, a
phenomenon called cerebral lateralization.
A. In most people, the left hemisphere is dominant in language and analytical ability,
whereas the right hemisphere is more important in pattern recognition, musical
creation, singing, and the recognition of faces.
B. The two hemispheres cooperate in their functions; this is aided by communication
between the two via the corpus callosum.
III. Particular regions of the left cerebral cortex appear to be important in language ability; when
these areas are damaged, characteristic types of aphasias result.
A. Wernicke 抯 area is involved in speech comprehension, whereas Broca 抯 area is
required for the mechanical performance of speech.
B. Wernicke 抯 area is believed to control Broca 抯 area by means of the arcuate
fasciculus.
C. The angular gyrus is believed to integrate different sources of sensory information
and project to Wernicke 抯 area.
IV. The limbic system and hypothalamus are regions of the brain that have been implicated as
centers for various emotions.
V. Memory can be divided into short-term and long-term categories.
A. The medial temporal lobes, in particular the hippocampus and perhaps the
amygdaloid nucleus, appear to be required for the consolidation of short-term memory
into long-term memory.
B. Particular aspects of a memory may be stored in numerous brain regions.
C. Long-term potentiation is a phenomenon that may be involved in some aspects of
memory.
Diencephalon
I. The diencephalon is the region of the forebrain which includes the thalamus, epithalamus,
hypothalamus, and pituitary gland.
A. The thalamus serves an important relay center for sensory information, among its
other functions.
B. The epithalamus contains a choroid plexus, where cerebrospinal fluid is formed.
The pineal gland, which secretes the hormone melatonin, is also part of the
epithalamus.
C. The hypothalamus forms the floor of the third ventricle, and the pituitary gland is
located immediately inferior to the hypothalamus.
II. The hypothalamus is the main control center for of visceral activities.
A. The hypothalamus contains centers for the control of thirst, hunger, body
temperature, and (together with the limbic system) various emotions.
B. The hypothalamus regulates the secretions of the pituitary gland. It controls the
posterior pituitary by means of a fiber tract, and it controls the anterior pituitary by
means of hormones.
Midbrain and Hindbrain
I. The midbrain contains the superior and inferior colliculi, which are involved in visual and
auditory reflexes, respectively and nuclei that contain dopaminergic neurons that project to the
corpus striatum and limbic system of the forebrain.
II. The hindbrain consists of two regions: the metencephalon and the myelencephalon.
A. The metencephalon contains the pons and cerebellum. The pons contains nuclei for
four pairs of cranial nerves, and the cerebellum plays an important role in the control of
skeletal movements.
B. The myelencephalon consists of only one region, the medulla oblongata. The
medulla contains centers for the regulation of such vital functions as breathing and the
control of the cardiovascular system.
Spinal Cord Tracts
I. Ascending tracts carry sensory information from sensory organs up to the spinal cord to the
brain.
II. Descending tracts are motor tracts and are divided into two groups: the pyramidal and the
extrapyramidal systems.
A. Pyramidal tracts are the corticospinal tracts. They begin in the precentral gyrus and
descend, without synapsing, into the spinal cord.
B. Most of the corticospinal fibers decussate in the pyramids of the medulla oblongata.
C. Regions of the cerebral cortex, the basal nuclei, and the cerebellum, control
movements indirectly by synapsing with other regions that give rise to descending
extrapyramidal fiber tracts.
D. The major extrapyramidal motor tract is the reticulospinal tract, which has its origin
in the reticular formation of the midbrain.
Cranial and Spinal Nerves
I. There are twelve pairs of cranial nerves. Most of these are mixed, but some are exclusively
sensory in function.
II. There are thirty-one pairs of spinal nerves. Each of these contains both sensory and motor
fibers.
A. The dorsal root of a spinal nerve contains sensory fibers, and the cell bodies of
these neurons are contained in the dorsal root ganglion.
B. The ventral root of a spinal nerve contains motor fibers.
III. A reflex arc is the pathway that involves a sensory neuron and a motor neuron; one or more
association neurons also may be involved in some reflexes.
(From: http://www.mhhe.com/biosci/ap/foxhumphys/student/olc/chap08summary.html)