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Dr. Chris Doumen
Week 7
2401 : Anatomy/Physiology
Central Nervous System
The B rain
TextBook Readings
♦ Pages 431 through 435
and 463 - 467
♦ Make use of the figures
in your textbook ; a
picture is worth a
thousand words !
♦ Work the Problems and
Questions at the end of
the Chapter
The brain starts its development from an
embryonic neural tube. At the end of
the 4th week, the neural tube will have
developed enough to show the beginning
parts of what will become the brain. The
brain starts to develop from 3 regions of
the embryo which are called the primary
brain vesicles.
• Proencephalon
• Mesencephalon
• Rhombencephalon
These develop into secondary brain
vesicles and will form the adult mature
structures.
The adult brain is mushroom shaped and
can be divided into 4 principal parts
• Proencephalon becomes Cerebrum
(Telencephalon) and Diencephalon
(containing Thalamus, Hypothalamus and Epithalamus)
• Mesencephalon becomes the midbrain
portion of The Brain stem
•
The Cerebrum and Diencephalon are also
grouped together under the name
"forebrain"
Terminology:
• One axon : = nerve fiber
• A group of nerve fibers = Nerve
• A group of nerve fibers in the CNS =
tract
• A group of nerve cell bodies outside
the CNS = ganglia
• A group of nerve cell bodies inside the
CNS = nuclei
• Gray matter = unmyelinated cell
bodies and dendrites
• White matter = myelinated axons
.
Collin County
Community
College District
Rhombencephalon becomes
Metencephalon and Myelencephalon
• Metencephalon develops into pons
and Cerebellum
• Myelencephalon becomes the
Medulla oblongata
2401 : Anatomy/Physiology
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The figure above shows the secondary vesicles (top left) as seen from a top view, and the
same structures from a lateral view in growing human embryo.
The Forebra in
The cerebral hemispheres make up the bulk of the brain mass and sit like a mushroom cap on the
brain stem. The surface ( 2 to 4 mm thick) is composed of gray matter called the ce re bral
corte x , containing billions of neuron cell bodies. Beneath the cortex lies the cerebral White
matter, consisting out of myelinated axons and dendrites with islands of gray matter situated
deep in the white matter e.g. th e su bc ortical nuc lei.
During embryonic development, there is a rapid increase in brain size in which the gray matter
enlarges faster than the white matter. The result is that the cortex starts to fold over upon
itself, creating many convolutions
Definitions :
• Gyri ( gyrus) = elevated ridges of brain tissue
• Sulci ( sulcus) = shallow grooves in-between the ridges
• Fissures = deeper grooves, separating large regions of the brain
Most prominent fissure is the longi tu dinal fiss ure, separating the two hemispheres. The
trans ve rs e fiss ure separates the cerebral hemispheres from the cerebellum below.
Each hemisphere is furthermore subdivided in 5 lobes by sulci and most of them named after the
cranial bone that overlie them. : F ront al , p ariet al , occipit al and tem poral lobe . The
5th one is buried under the temporal lobe.
2401 : Anatomy/Physiology
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•
Ce nt ral sulc us separates the frontal lobe from the parietal lobe
• the gyrus anterior of the central sulcus = pre-ce nt ral gyrus
• the gyrus posteriorly of the central sulcus = p os t-c entral g yru s
•
A p ariet o- occi pit al
sulcu s, located on the
posterior medial surface,
separates the parietal lobe
from the occipital lobe
below
•
A deep l at eral sulc us
outlines lower laying
temporal lobe, separating it
from the inferior borders of
the frontal and parietal
lobe
Use this website to learn more about lobes and sulci: http://uta.marymt.edu/~psychol/brain.html
The C erebral Ventricles
During the embryonic development, the neural tube expands as well to form fluid containing
chambers within the cerebellum. These are called the ventricles and the fluid is referred to
as CerebroSpinal Fluid
There are four such ventricles. Two laterals ventricles are located in each hemisphere. They
connect via an inte rve nt ricul ar f orame n with the third ventricle , located in the
diencephalons. This connects via the ce re bral aqu educt to the fourth ventricle, located in
the brainstem, starting between pons and cerebellum and extending inferiorly to connect with
the central canal of the spinal cord.
The C ranial M eninges
The cerebral hemispheres are protected by the bones of the skull and the cranial meninges.
The meninges are fibrous membranes that provide structural and functional support and are
located immediately deep to the skull.
2401 : Anatomy/Physiology
•
Dura m ate r :
It is the Outermost layer. It has
two subblayers called the endosteal
layer( fused to the bone) and the
meningeal layer. A large gap
between these two layers provide
space for fluids such as the large
midsaggital blood sinus.
•
Arach noi d mate r :
Provides the arach noi d villi
( see later) and the sub-arachnoid
space. This latter area is the space
into which Cerebrospinal fluid flows
as well as the location of majopr
blood vessels feeding the brain.
•
Pia m ate r :
The innermost layer that sticks to the brain itself.
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CerebroSpina l Fluid
The CSF is a fluid that circulates in the subarachnoid space ( between arachnoid and pia
mater) around the brain and through cavities within the brain. It nourishes and protects the
brain.
CSF is produced mainly by structures called the ch oroi d ple xu s in the lateral, third and
fourth ventricles. CSF flows from the lateral ventricle to the third ventricle through the
interventricular foramen (also called the foramen of Monro). The third ventricle and fourth
ventricle are connected to each other by the cerebral aqueduct (also called the Aqueduct of
Sylvius). CSF then flows onto the 4th ventricle and into the subarachnoid space through the
foramina of Luschka (there are two of these) and the foramen of Magendie (only one of
these). CSF circulates up the subarachnoid space of the brain and is eventually reabsorbed
into the a blood vascular sinus called the superior sagittal sinus. The absorption occurs
throu gh arach noid villi , extension from the arachnoid meninges
2401 : Anatomy/Physiology
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Choroid Plexus es
The ch oroi d ple xu se s are
networks of capillaries in the
walls of the ventricles. They
are capillaries covered by
epe ndymal cell s (epithelial
cells of the ventricles) that
form CSF by filtration of blood
plasma and then secreting it
into the ventricles. The very
tight junctions between the
ependymal cells prevents fluid
to leak from the blood
between the cells into the
CSF. So, blood plasma has to
pass through the cell. This
provides an efficient cellular
barrier and protects the brain-spinal cord from potential harmful substances in the blood. It
is called the blood-cerebrospinal fluid barrier.
The entire CSF is about 150 ml and contributes to the homeostasis of the brain in 3 main
ways
• provides mechanical protection by serving as a shock absorber
• provide chemical protection by maintains an optimal chemical environment
• contributes in the circulation and exchange of nutrients, waste products between
blood and nervous tissue
CSF circulates continuously and is formed and reabsorbed at a rate of 480 ml/day. Blockage
of CSF movement results in increased pressure on the brain (= hydrocephalus)
The C erebrum
The cerebral cortex is the executive suite of the nervous system. It enables us to perceive,
communicate, remember, understand, appreciate, initiate voluntary movements. Thus all qualities
of the conscious behavior.
Consisting out of gray mater, it is thus a collection of cell bodies, dendrites, unmeyelinated
axons but no fiber tracts. Although correlation of localization and function can sometimes be
pinpointed to certain domains, other function are more difficult to locate and seem to be
complex and overlapping.
We will examine the functional aspects of the cortex with reference to these functional
areas. However, always keep these generalizations about the cerebral cortex in mind.
2401 : Anatomy/Physiology
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1.
No functional area of the cortex acts alone; conscious behavior involves the entire
cortex
2. There are 3 kinds of functional areas:
• Motor areas : control voluntary motor functions
• Sensory areas : provide for conscious awareness and sensation
• Association areas: act to integrate the diverse information
3. Each hemisphere is concerned with the sensory and motor function of the opposite
side
4. The two hemisphere are not entirely equal in function (lateralization, specialization)
White Matter
Different parts of the brain are connected and communicate by means of tracts of white
fibers.
Ass oci ati on fi be rs : connect and transmit impulses between gyri of the same hemisphere;
smaller ones are called arc u at e fibers, longer one are referred to as f asciculi
Commis su ral fi be rs : connect gyri in one hemisphere to corresponding gyri of the opposite
hemisphere (Corpus callosum is the main bridge between right and left side)
Projecti on fibers : They form ascending ( from spinal cord up to the brain, and usually are
sensory ) and descending tracts ( from brain towards and into the spinal cord, usually
motor fibers)