Download lymphatic system

Anatomy & Physiology
Lesson 6
THE LYMPHATIC SYSTEM

The lymphatic system is the body system that
is responsible for carrying out immune
responses.
 It consists of lymph (a fluid) which flows within
lymphatic vessels (lymphatics), the lymph
nodes, spleen, thymus gland, and red bone
marrow (where stem cells which develop into
lymphocytes are found).
 Lymph and interstitial fluid are essentially the
same—after the fluid flows out of the interstitial
spaces into the lymphatics it is called lymph.
 Lymphatic tissue is a form of specialized
reticular CT that contains large numbers of
lymphocytes.
FUNCTIONS OF THE LYMPHATIC
SYSTEM
 Draining
interstitial fluid—Lymphatic
vessels drain tissue spaces of excess
interstitial fluid.
 Transporting dietary lipids—Lymphatic
vessels carry lipids and lipid-soluble
vitamins (A, D, E, and K) absorbed by the
GI tract to the blood.
 Protecting against invasion—Lymphatic
tissue carries out immune responses.
LYMPHATIC VESSELS AND
CIRCULATION
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Lymphatic vessels start as lymphatic capillaries, which
are small, close-ended vessels found in intercellular
spaces throughout the body (except in avascular tissues,
the CNS, splenic pulp, and bone marrow).
Lymphatic capillaries converge to form larger lymphatic
vessels.
The walls of lymphatic capillaries are specially designed
to allow the flow of fluid into them, but not out—the
endothelial cells that compose the capillary walls overlap
slightly, creating one-way valves.
Anchoring filaments attach to capillary walls. When
edema is present, they open the spaces between cells
even further, allow more fluid to flow into the capillaries.
LYMPHATIC VESSELS AND
CIRCULATION

Lymphatic vessels are similar to veins, but have more
valves and thinner walls.
 Lymphatic vessels periodically flow through lymphatic
tissue structures called lymph nodes.
 In the skin, lymphatic vessels lie in subcutaneous tissue
and usually follow veins.
 In the viscera, lymphatic vessels tend to follow arteries,
forming plexuses around them.
 Fluid from blood is constantly seeping out of veins and
into interstitial spaces. This fluid enters lymphatic
capillaries to form lymph. The lymph then flows into
lymphatic vessels and through a series of lymph nodes.
After passing the most proximal lymph node in a chain,
lymphatic vessels unite to form lymph trunks, which
connect to either the right lymphatic duct or the
thoracic duct (left lymphatic duct). The lymphatic ducts
drain into the jugular veins, returning the fluid back into
the circulatory system.
LYMPHATIC VESSELS AND
CIRCULATION

Lymph flow is maintained primarily by pressure
from surrounding skeletal muscle contractions.
 Breathing action also helps with lymphatic flow,
creating a pressure gradient between the
abdominal region (high pressure) and the
thoracic region (low pressure). Also, every time
a lymphatic vessel distends, the smooth muscle
in its walls contracts, help to move lymph from
one segment of the vessel to the next.
 Lymphatic vessels contain many one-way
valves, which prevent the lymph from flowing
backward during muscular relaxation or between
breaths.
LYMPHATIC VESSELS AND
CIRCULATION

Lymphatic Capillary
LYMPHATIC TISSUES

The primary lymphatic organs are called such
because they produce B and T cells, the
lymphocytes that carry out immune responses.
These are:
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The major secondary lymphatic organs are:
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Red bone marrow—B cells and pre-T cells.
Thymus gland—T cells.
Lymph nodes
Spleen
Lymphatic nodules are also included among
the secondary lymphatic organs, although they
are not technically organs, since they are not
encapsulated.
THYMUS GLAND
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The bilobed thymus gland is located in the mediastinum,
posterior to the sternum.
Each lobe is divided into lobules. The lobules consist of
an outer cortex (composed of tightly packed
lymphocytes, epithelial cells, and macrophages) and the
inner medulla (mostly epithelial cells and scattered
lymphocytes).
Pre-T cells migrate from red bone marrow to thymus,
where they proliferate and mature into T cells.
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T-helper cells assist B cells in producing antibodies.
T-suppressor cells prevent B cells from producing antibodies.
The epithelial cells produce thymic hormones, which
apparently help with T cell maturation.
 The thymus gland is large in infants, reaches its
maximum size at about 10-12 years of age, then begins
to atrophy after puberty.
 Most T cells arise before puberty, but some continue to
mature throughout life.
LYMPH NODES

Lymph nodes are bean-shaped structures located along
the length of lymphatic vessels throughout the body
(often concentrated in specific areas).
 They range from 1 to 25 mm (1 inch) in length.
 Lymphocytes are produced and stored within the
nodes.
 Lymph passes through a lymph node in one direction,
entering through afferent lymphatic vessels and
leaving through efferent lymphatic vessels.
 Lymph nodes filter foreign substances from lymph as it
moves back toward the bloodstream. These substances
are trapped by reticular fibers within the nodes, where
macrophages destroy some substances through
phagocytosis and lymphocytes destroy others by
immune responses.
SPLEEN
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Measuring 12 cm in length, the spleen is the
largest mass of lymphatic tissue in the body.
It is located in the left hypochondriac region,
between the stomach and diaphragm, lateral to
the liver.
Lymph is not filtered in the spleen, since it does
not have any afferent lymphatic vessels.
The spleen does have an artery, vein, and
efferent lymphatic vessel, however.
It is encapsulated by dense connective tissue
and the stroma (framework) is composed of
trabeculae, reticular fibers, and fibroblasts.
SPLEEN
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The parenchyma (functional part) of the spleen is
composed of two kinds of tissue:
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White pulp
Red pulp
White pulp is lymphatic tissue—mostly lymphocytes (B
cells) arranged around central arteries.
White pulp functions in immunity, proliferating B cells into
antibody-producing plasma cells.
Red pulp is composed of blood-filled venous sinuses
interspersed with thin plates of tissue called splenic
(Billroth’s) cords. Splenic cords contain red blood
cells, macrophages, lymphocytes, plasma cells, and
granulocytes.
Red pulp conducts the main functions of the spleen:
phagocytosis of bacteria and worn-out or damaged red
blood cells and platelets.
The fetal spleen also paticipates in blood cell formation.
LYMPHATIC NODULES

Lymphatic nodules are unencapsulated concentrations
of lymphatic tissue.
 They are usually small, solitary, and scattered
throughout the lamina propria of the mucous membranes
lining the GI tract, respiratory airways, urinary tract, and
reproductive tract. This type of lymphatic tissue is
referred to as mucosa-associated lymphoid tissue
(MALT).
 Some lymphatic nodules occur in multiple, large
aggregations in certain parts of the body. These include
the five tonsils, Peyer’s patches in the ileum of the
small intestine, and in the appendix.
 The tonsils are strategically located to participate in
immune responses against foreign substances that are
inhaled or ingested. T cells destroy intruders directly,
while B cells develop into antibody-secreting plasma
cells that destroy foreign substances.
IMMUNITY

Resistance is the ability to ward off disease
through our defenses. Lack of resistance is
called susceptibility.
 Resistance can be broadly classified into two
groups:
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Nonspecific resistance—mechanisms that provide
general protection against a broad range of
pathogens (skin and mucous membrane barriers,
antimicrobial chemicals, phagocytosis, inflammation,
and fever).
Immunity—involves activation of specific
lymphocytes that combat a particular pathogen or
foreign substance. The lymphatic system is
responsible for immunity.
IMMUNITY

Immunity is the ability of the body to defend
itself against specific invading agents, such as
bacteria, toxins, viruses, and foreign tissues.
 Antigens are substances that are recognized as
foreign by the immune system and provoke
immune responses.
 Two properties distinguish immunity from
nonspecific defenses.
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Specificity for particular antigens, which includes the
ability to distinguish self from non-self molecules.
Memory for most previously encountered antigens so
that a second encounter stimulates and even more
rapid and vigorous response.
IMMUNITY
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Immunity consists of two kind of immune responses,
both triggered by antigens:
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Cell-mediated (cellular) immune (CMI) responses
Antibody-mediated (humoral) immune (AMI) responses
Antibodies (Abs) or immunoglobulins are proteins that
bind to and inactivate a particular antigen.
 CMI responses are particularly good at dealing with
intracellar pathogens (fungi, parasites, viruses), some
cancer cells, and foreign tissue transplants.
 AMI responses are more effective against antigens
dissolved in body fluids and extracellular pathogens
(primarily bacteria that are found in body fluids but
usually do not enter body cells).
 Many pathogens provoke both types of immune
responses.
CELL-MEDIATED IMMUNE
RESPONSES
 Begins
with activation of a few T cells
(lymphocytes) by a particular antigen.
 An activated T cell undergoes proliferation
and differentiation into a clone of effector
cells, a group of identical cells capable of
recognizing the same antigen and carrying
out some aspect of the immune attack.
 Ultimately, the immune response results in
the elimination of the intruding antigen.
ANTIBODY-MEDIATED IMMUNE
RESPONSES
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The body not only contains millions of different T
cells, but also millions of different B cells, each
capable of responding to a specific antigen.
 While T cells leave the lymphatic tissue to seek
out and destroy an antigen, B cells remain in the
lymph nodes, spleen, or other lymphatic tissue.
 In the presence of a foreign antigen, B cells
become activated, differentiating into plasma
cells that secrete specific antibodies.
 The antibodies then circulate in the lymph and
blood until they reach the site of invasion, where
they proceed to destroy the antigen.
ANTIBODIES
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An antibody binds to the antigen that triggered
its production like a key fits into a specific lock.
Antibodies belong to a group of glycoproteins
called globulins, thus the name
immunoglobulins.
Most antibodies are composed of four
polypeptide chains.
Most antibodies have two antigen binding sites,
which allows an antibody to bind to two different
sites on an antigen.
Five types of antibodies are: IgG, IgA, IgM, IgD,
and IgE.
ANTIBODIES
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IgG—Most abundant. About 75% of antibodies in blood. Found in
blood, lymph, and the intestines. Act by enhancing phagocytosis,
neutralizing toxins, and triggering the complement system. Only
class of antibodies that cross the placental barrier, providing immune
protection for newborns.
IgA—About 15% of blood antibodies. Found mostly in sweat, tears,
saliva, mucous, milk, and GI secretions. Some found in blood and
lymph. Levels decrease during stress, lowering resistance. Provide
localized protection on mucous membranes against bacteria and
viruses.
IgM—5-10% of antibodies in blood. First antibodies excreted by
plasma cells after an initial exposure to an antigen. Found in blood
and lymph. Activate complement and cause agglutination and lysis
of microbes.
IgD—<1% of blood antibodies. Found in blood, lymph, and on the
surfaces of B cells as antigen receptors. Involved in B cell
activation.
IgE—<0.1% of antibodies in blood. Located on mast cells and
basophils. Involved in allergic and hypersensitivity reactions.
Provide protection against parasitic worms.
THE NERVOUS SYSTEM
The nervous system is the body’s control
center (closely seconded by the endocrine
system) and communication network.
 The nervous system serves three basic
functions:
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Sensory function—senses changes within the body
and external environment.
Integrative function—analyzes sensory information,
stores some aspects, and makes decisions regarding
apropriate responses.
Motor function—may respond to stimuli by initiating
muscular contractions or glandular secretions.
THE NERVOUS SYSTEM
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The two principal divisions of the nervous
system are:
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The central nervous system (CNS)—consists of the
brain and spinal cord. All neurological impulses are
either sent or received by the CNS.
The peripheral nervous system (PNS)—consists of
cranial nerves and spinal nerves and is connected
to sensory receptors, muscles, glands in peripheral
parts of the body. Conveys impulses into our out of
the CNS.
Sensory or afferent neurons provide input to
the CNS from various parts of the body.
 Motor or efferent neurons transmit output from
the CNS to muscles and glands.
THE NERVOUS SYSTEM
 The
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PNS can further be divided into:
The somatic nervous system (SNS)—
conveys sensory information from cutaneous
and special sense receptors in the head, body
walls, and limbs to the CNS. Conducts motor
impulses to our skeletal muscles. This portion
of the SNS is voluntary.
The autonomic nervous system (ANS)—
conveys sensory information from the viscera
to the CNS. Conducts impulses from the CNS
to smooth muscle, cardiac muscle, and
glands. The ANS is involuntary since it is not
normally under our conscious control.
THE NERVOUS SYSTEM
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The ANS can be subclassified, once again, into
the sympathetic division and the
parasympathetic division.
 Both divisions provide directions to the viscera,
but they are constructed differently and use
different neurotransmitters when conveying
impulses to their target organs.
 These two subdivisions generally have opposing
actions. For example, sympathetic neurons
speed the heartbeat, while parasympathetic
neurons slow it down.
 Sympathetic processes often involve energy
expenditure, while parasympathetic processes
restore and conserve body energy.
THE NERVOUS SYSTEM
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The nervous system is composed of two basic
cell types: Neurons and neuroglia or glial
cells.
Neurons are responsible for conducting energy
(impulses) from one part of the body to another.
Neurons have a cell body, dendrites (which
carry information toward the cell) and an axon
(which conveys information away from the cell
body).
Neuroglia outnumber neurons 5 to 10 times.
Neuroglia perform functions of support and
protection.
There are several types of neuroglia cells.
NEUROGLIA (CNS)
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Astrocytes—Star-shaped cells with many
processes found in the CNS. Participate in
metabolizing neurotransmitters and maintain the
proper balance of K+ for nerve impulse
generation. Participate in brain development by
assisting migration of neurons. Help to form the
blood-brain barrier. Provide a link between
blood vessels and neurons.
 Oligodendrocytes—Smaller than astrocyts with
fewer processes. Round or oval cell body.
Form a supporting network around neurons in
the CNS. Produce myelin sheath around axons
of CNS neurons. Each oligodendrocyte wraps
myelin around several axons.
NEUROGLIA (CNS)
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Microglia—Small cells with few processes.
Derived from monocytes. Phagocytic cells that
engulf and destroy microbes and cellular debris
in the CNS. May migrate to injured nervous
tissue.
 Ependymal cells—Epithelial cells that are
arranged in a single layer and may range from
cuboidal to columnar in shape. Many are
ciliated. Line the ventricles of the brain (spaces
filled with cerebrospinal fluid) and the central
canal of the spinal cord. Form cerebrospinal
fluid (CSF) and assist in the circulation of CSF.
NEUROGLIA (PNS)
 Neurolemmocytes
(Schwann cells)—
Flattened cells arranged around axons in
the PNS. Each cell produces part of the
myelin sheath around a single axon of a
PNS neuron.
 Satellite cells—Flattened cells arranged
around the cell bodies of neurons in
ganglia (collections of neuronal cell bodies
in the PNS). Support neurons in PNS
ganglia.
MYELINATION
 The
axons of most neurons are
surrounded by a multilayered lipid and
protein covering called a myelin sheath.
The sheath insulates the axon, increasing
the rate of impulse conduction.
 Axons with a myelin sheath are said to be
myelinated, while those without are
unmyelinated.
 Oligodendrocytes produce myelin sheaths
in the CNS and Neurolemmocytes do the
same in the PNS.
MYELINATED AND
UNMYELINATED AXONS
MYELINATED NEURON ANATOMY
NERVES
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A nerve is a bundle of motor and sensory
neurons, together with CT and blood vessels.
There are 43 major nerves. They arise in the
CNS.
Nerve impulses are conducted from one neuron
to another across synapses—tiny gaps
between the axon of one neuron and the
dendrites of an adjacent neuron.
Chemicals called nerve transmitter
substances or neurotransmitters transmit the
information between nerves.
Cold slows these impulse transmissions while
heat speeds them up.
THE SPINAL CORD

The spinal cord is a roughly cylindrical column
of nervous tissue that is 16 to 18 inches in length
and ¾ inch in diameter.
 It is enclosed and protected by the spinal
vertebrae and extends from the base of the
brain to the second lumbar vertebra, in adults.
 It has two obvious enlargements:
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The cervical enlargement extends from C4 to T1
and is the location from which nerves serving the
upper limbs arise.
The lumbar enlargement extend from T9 to T12 and
is the location from which the nerves serving the
lower limbs arise.
Two grooves divide the spinal cord into right and
left halves: The anterior median fissure and
the posterior median sulcus.
THE SPINAL CORD
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31 pairs of spinal nerves emerge from the
spinal cord.
 The spinal nerves are the communication paths
between the spinal cord and most of the body.
 Each spinal nerve is connected to the spinal
cord by two roots:
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The posterior or dorsal (sensory) root contains
sensory nerve fibers that convey impulses from the
periphery to the spinal cord. Each posterior root also
has a swelling called the posterior or dorsal
(sensory) root ganglion, which contains the sensory
nerve cell bodies.
The anterior or ventral (motor) root contains motor
neuron axons and conducts impulses from the spinal
cord to the periphery.
THE SPINAL CORD
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The spinal cord is composed of collections of
neurons and bundles of nerve fibers.
 When viewed in cross-section, the spinal cord is
composed of gray matter (shaped like a butterfly
or a sideways “H”) surrounded by three columns
of white matter.
 Gray matter consists primarily of neuronal cell
bodies, neuroglia, unmyelinated axons, and
dendrites of association and motor neurons.
 White matter is composed of bundles of
myelinated axons and motor and sensory
neurons.
THE SPINAL CORD
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The spinal cord performs to principal functions:
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1) The white matter tracts propagate nerve impulses.
Along these “highways,” sensory impulses travels
from the periphery to the brain and motor impulses
flow from the brain to the periphery.
2) The gray matter of the spinal cord receives and
integrates incoming and outgoing information. As one
aspect of this function, it can sometimes bypass the
usual impulse transmissions to the brain, allowing for
and controlling simple reflexes itself.
Both of these functions are essential to the
maintenance of homeostasis.
THE SPINAL CORD
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Meninges are protective CT coverings that
surround the spinal cord (and brain).
 There are three layers of meninges:
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The dura mater is the outermost layer.
The arachnoid forms the middle layer. Between the
arachnoid and the dura mater is a thin subdural
space with contains interstitial fluid.
The deepest layer is the pia mater. Between the pia
mater and the arachnoid is the subarachnoid space,
which is filled with cerebrospinal fluid.
The spinal cord is also protected by a cushion of
fat and CT located in the epidural space
between the dura mater and the vertebral canal.
THE SPINAL CORD
CRANIAL NERVES
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12 pairs of nerves emerge from the underside of
the brain.
 These nerves supply the muscles and sensory
organs of the head. In addition, the vagus
nerve (cranial nerve X) supplies the digestive
organs, heart, and air passages in the lungs.
 Some of these nerves supply sensory
information only, some are primarily responsible
for motor impulses (with some sensory aspects,
as well), and some mix both sensory and motor
functions.
SPINAL NERVES
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The 31 pairs of spinal nerves are named and
numbered for the region and level of the spinal
cord from which they emerge.
The first cervical pair emerges from between the
occipital bone and the atlas.
All other spinal nerves leave the spinal cord
through channels between adjacent vertebrae.
There are 8 pairs of cervical nerves, 12 pairs of
thoracic nerves, 5 pairs of lumbar nerves, 5 pairs
of sacral nerves, and 1 pair of coccygeal nerves.
Each pair of nerves is responsible for the control
of certain parts of the body.
SPINAL NERVES
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A simplified summary of the areas of influence of
the various spinal nerves is as follows:
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C1-C5—skin and muscles of the head, neck, and
upper part of the shoulders.
C5-C8 & T1—upper extremities and shoulder regions.
T2—under the arms and the back of the arms.
T3 & T6—intercostal muscles and skin of the anterior
and lateral chest.
T7-T11—intercostal muscles, abdominal muscles and
overlying skin, deep back muscles, and skin of the
dorsal thorax.
L1-L4—anterolateral abdominal wall, external
genitals, and part of the upper legs.
L4-L5 & S1-S4—buttocks, perineum, and lower
extremeties, including the legs and feet.
THE BRAIN
 The
brain is one of the largest organs in
the body, averaging about 3 pounds in
weight.
 It possesses about 100 billion neurons and
1000 billion neuroglia.
 The brain can be divided into four principal
parts: the brain stem, cerebellum,
diencephalon, and cerebrum.
THE BRAIN

The brain stem is continuous with the spinal
cord and consists of the medulla oblongata,
pons, and midbrain.
 Posterior to the brain stem is the cerebellum.
 Superior to the brain stem is the diencephalon,
which consists primarily of the thalamus and
hypothalamus.
 The cerebrum covers the diencephalon and fills
most of the cranium. It is divided into right and
left halves called hemispheres..
THE BRAIN
THE BRAIN

The brain is protected by the cranial bones and
cranial meninges.
 The cranial meninges surround the brain, are
continuous with the spinal meninges, have the
same basic structure and bear the same names.
 The outer layer is called the dura mater, the
middle layer the arachnoid, and the inner layer
the pia mater.
 Extensions of the dura mater separate the two
hemispheres of the cerebrum, the two
hemispheres of the cerebellum, and the
cerebrum from the cerebellum.
CEREBROSPINAL FLUID (CSF)

The brain and spinal cord are nourished and
protected from chemical or physical injury by
cerebrospinal fluid (CSF).
 CSF circulates continuously through the
subarachnoid space around the brain and spinal
cord, and through cavities within the brain.
 The four CSF-filled cavities in the brain are
called ventricles.
 The two larger lateral ventricles are located in
the two hemispheres of the cerebrum, the
narrow third ventricle is situated at the midline,
superior to the hypothalamus and between the
right and left halves of the thalamus, and the
fourth ventricle lies between the brain stem
and the cerebellum.
CEREBROSPINAL FLUID (CSF)

The CNS contains a total of about 3-5 ounces of
CSF.
 CSF is a clear, colorless liquid that contains
glucose, proteins, lactic acid, urea, cations,
anions, and some white blood cells. It is
constantly being produced from blood by
specialized brain cells.
 CSF contributes to homeostasis in three main
ways:
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Mechanical protection—serves as a shock absorber
and allows the brain to “float” in the cranial cavity.
Chemical protection—provides and optimal
chemical environment for accurate neuronal
signaling.
Circulation—is a medium for exchange of nutrients
and waste products between the blood and nervous
tissue.
FUNCTIONS OF PARTS OF THE
BRAIN
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Brain stem:
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Medulla oblongata—Relays motor and sensory
impulses between other parts of the brain and the
spinal cord. Functions in consciousness and arousal.
Helps to regulate heartbeat, breathing, and blood
vessel diameter. Coordinates swallowing, vomiting,
coughing, sneezing, and hiccupping. Contains nuclei
of origin for cranial nerves VII, IX, X, XI, and XII.
Pons—Relays impulses from one side of the
cerebellum to the other and between the medulla and
midbrain. Helps to control breathing. Contains nuclei
of origin for cranial nerves V, VI, VII, and VIII.
FUNCTIONS OF PARTS OF THE
BRAIN
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Brain stem (cont):
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Midbrain—Relays motor impulses from the cerebral
cortex to the pons and sensory impulses from the
spinal cord to the thalamus. Coordinates movements
of the eyeballs in response to visual and other stimuli
and the head and trunk in response to auditory
stimuli. Contributes to movement control. Contains
nuclei of origin for cranial nerves III and IV.
Cerebellum
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Compares intended movements with actual
movements to smooth and coordinate complex,
skilled movements. Regulates posture and balance.
FUNCTIONS OF PARTS OF THE
BRAIN
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Diencephalon
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Epithalamus—Contains pineal gland, which secretes
melatonin, habenular nuclei, and the choroid plexus
of the third ventricle.
Thalamus—Relays all sensory input to the cerebral
cortex. Provides crude appreciation of touch,
pressure, pain, and temperature. Involved in motor
actions, arousal, emotions, memory, cognition, and
awareness.
Subthalamus—Helps to control muscle movement.
Hypothalamus—Controls and integrates activities of
the ANS and pituitary gland. Regulates emotional
and behavioral patterns and diurnal rhythms.
Controls body temperature, eating, and drinking
behavior. Helps maintain waking and establishes
sleep patterns.
FUNCTIONS OF PARTS OF THE
BRAIN
 Cerebrum
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Sensory areas of the cerebral cortex
interpret sensory impulses, motor areas
control muscular movements, and
associational areas function in emotional and
intellectual processes. Basal ganglia
coordinate gross, automatic muscle
movements and regulate muscle tone.
Limbic system functions in emotional
aspects of behavior related to survival.
The cerebral cortex is the superficial layer of
the cerebrum, composed of gray matter. It is
2-4 mm thick and contains billions of neurons.
BRAIN LATERALIZATION

Although the brain may appear bilaterally
symmetrical, it actual has structural and
functional differences between the hemispheres.
 The left hemisphere recieves sensory input from
and controls the right side of the body, while the
right hemisphere receives sensory input from
and controls the left side of the body.
 Other important functional differences (in most
people) are as follows:
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Left hemisphere—Control of: muscles on right side
of body, spoken and written language, numerical and
scientific skills, reasoning.
Right hemisphere—Control of: muscles on left side
of body, musical and artistic awareness, space and
pattern perception, insight, imagination, generating
mental images to compare spacial relationships.
STRUCTURES OF THE EYE