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The nervous system
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Dr.Abdul-Aziz Ahmed
Organization of the Nervous System
The nervous system, in coordination with the endocrine system, provides
the means by which cell and tissue functions are integrated. The nervous
system mediates a tremendous range of functions, from the unconscious
control of visceral functions to sensory perceptions, voluntary movement,
behavior, emotions, dreams, intelligence, anticipation, judgment.
Cells of the Nervous System
Nervous tissue consists of two principal types of cells
 neurons
 supporting cells.
I.Neurons
The neurons are the functional cells of the nervous system. They exhibit
membrane excitability and conductivity and secrete neurotransmitters and
hormones, such as epinephrine and antidiuretic hormone.The major parts
of a neuron
1. A soma is the cell body of a neuron that contains the cell nucleus and
the rough endoplasmic reticulum.
2. The axon is the cellular process that carries action potentials away from
the soma. Axons are often long and may have multiple branches.
3. Dendrites have a structure similar to axons but receive impulses from
other neurons. Many neurons have an extensive set of dendrites,
referred to as the dendritic tree.
Figure: The neurons
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The nervous system
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Dr.Abdul-Aziz Ahmed
II.The supporting cells
The cells supporting in the CNS include:
1. Schwann cells are the supporting cells in the PNS. They produce the
myelin sheath that isolates axons of neurons in the PNS.
2. Astrocytes are highly branched cells that envelope neurons and brain
capillaries , it also provide neurons with lactate , maintain a stable [K+] in
the brain extracellular fluid and synthesize neurotransmitter precursors
for neurons (e.g., glutamine synthesis for glutaminergic neurons).
3. Microglia are highly reactive cells that are activated by injury or
infection, which causes them to proliferate and become phagocytic
4. Oligodendrocytes produce and maintain the myelin sheaths around
neurons in the CNS by wrapping around the axons many times.
5.ependymal cell , lining the cerebral ventricles.
Divisions of the nervous system
The nervous system has two major divisions:
I.The central nervous system, consists of the brain and spinal cord.
II.The peripheral nervous system, the peripheral nervous system is further
divided into :
1. sensory fiber, that carry sensory information to the central nervous
system,
2.motor fiber that carry instructions from the central nervous system to
the skeletal muscles(somatic) and to cardiac muscle, smooth muscle and
gland(autonomic).
Figure . The organizationof the human nervous system
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Meninges
Inside the skull and vertebral column, the brain and spinal cord are loosely
suspended and protected by several connective tissue sheaths called the
meninges:
1.pia mater, the surfaces of the spinal cord, brain, are covered with a
delicate connective tissue layer called the pia mater .
2. Arachnoid, it is very delicate, nonvascular, and waterproof layer,
encloses the entire CNS. The arachnoid layer is named for its spider-web
appearance. The CSF is contained in the subarachnoid space.
3. Dura mater , lie immediately outside the arachnoid it is a continuous
sheath of strong connective tissue, , which provides the major protection
for the brain and spinal cord. The dura forms two major folds. The first, a
longitudinal fold called the falx cerebri, separates the cerebral
hemispheres and tentorium cerebella which separates the cerebral
hemispheres, from the cerebellum.
Figure: Cranial dura mater. The skull is open to show the falx cerebri and
the right and left portions of the tentorium cerebelli, as well as some of
the cranial venous sinuses
Meningitis and encephalitis
Infection (i.e., viral, bacterial, or fungal) of the meninges results in
meningitis, whereas infection of the brain parenchyma results in
encephalitis. The classic clinical triad of meningitis is fever, headache, and
nuchal rigidity (stiff neck).
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Ventricular System
The ventricular system is a series of interconnected chambers inside the
CNS that are filled with CSF.
1.The lateral ventricles of the cerebrum, each lateral ventricle forms a
large C shape as the ventricle curves around from the frontal lobe into the
temporal lobe.
2.The slit-like third ventricle,located in the midline and is closely
associated with the thalamus. The lumen of the third ventricle is
continuous with each lateral ventricle through an interventricular foramen
(the foramen of Monro).
3.The fourth ventricle located in the hind brain and connected to the third
ventricle by
cerebral aqueduct. The fourth ventricle is continuous
inferiorly with the central canal of the spinal cord.
Figure : The ventricular system of the brain.
Cerebrospinal Fluid
Cerebrospinal fluid is a clear, colorless ultrafiltrate of blood plasma,
composed of 99%. Humans secrete approximately 500 mL of CSF each day.
However, only approximately 150 ml is in the ventricular system at any
one time, meaning that the CSF is continuously being absorbed. The CSF is
produced by tiny reddish masses of specialized capillaries from the pia
mater, called the choroid plexus, that project into the ventricles. Once
produced, the CSF flows freely through the ventricles and pass to the
subarachnoid space through two foramina of Luschka and foramen
Magendie. Reabsorption of CSF into the vascular system occurs along the
sides of the superior sagittal sinus in the anterior and middle fossa by the
arachnoid villi, these villi function as one-way valves, permitting CSF
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outflow into the blood but not allowing blood to pass into the arachnoid
spaces. The CSF provides a supporting and protective fluid in which the
brain and spinal cord float, and it helps to maintain a constant ionic
environment that serves as a medium for diffusion of nutrients,
electrolytes, and metabolic end products into the extracellular fluid
surrounding CNS neurons and glia. Any physical force delivered to either
the skull or spine is to some extent diffused and cushioned by, the CSF.
The Cerebral Circulation
The blood flow to the brain is supplied by the two internal carotid arteries
anteriorly and the vertebral arteries posteriorly.
I.internal carotid artery, is distributed through the anterior and middle
cerebral arteries.
 The anterior cerebral arteries supply the medial surface of the
frontal and parietal lobes and the anterior half of the thalamus, the
corpus striatum, part of the corpus callosum.
 The middle cerebral artery passes laterally, supplying the lateral
basal ganglia ,the motor and premotor frontal, language cortices
(frontal and superior temporal), the primary and association
auditory cortex (superior temporal gyrus), and the primary and
association somesthetic cortex for the face and hand (postcentral
gyrus, parietal).
The consequences of ischemia of these areas may be the most
devastating, resulting in damage to the fine manipulative skills of the face
and upper limbs and to receptive and expressive communication functions
(e.g., aphasia).
II.The two vertebral arteries arise from the subclavian and enter the skull
through the foramen magnum and unite to form the basilar artery, which
then diverges to terminate in the posterior cerebral arteries. Branches of
the basilar and vertebral arteries supply the medulla, pons, cerebellum,
midbrain,. The posterior cerebral arteries supply the remaining occipital
and inferior regions of the temporal lobes and the thalamus. The distal
branches of the internal carotid and vertebral arteries communicate at the
base of the brain through the circle of Willis.
The cerebral circulation is drained by two sets of veins that empty into the
dural venous sinuses. This system of sinuses returns blood to the heart
primarily through the internal jugular veins.
Hydrocephalus
Hydrocephalus can be defined in general terms as an increase in CSF
volume or pressure as a result of an imbalance in CSF production, flow, or
absorption.
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Blood-Brain Barriers
Maintenance of a chemically stable environment is essential to the
function of the brain. Only water, carbon dioxide, oxygen and other lipidsoluble molecules(such as Alcohol, nicotine, and heroin) enter the brain
with relative ease; the transport of other substances between the brain
and the blood is slower and more controlled. Glucose enters the brain by
facilitated diffusion via GLUT1 carriers present in capillary endothelial cell
membranes.The blood-brain barrier depends on
 the tight junctions between endothelial cells of brain capillaries
 brain capillaries are completely surrounded by basement membrane
 the processes of supporting astrocyte cells of the brain
Acute cerebral lesions, such as trauma and infection, increase the
permeability of the blood-brain barrier.
The cerebral capillaries are much more permeable at birth than in
adulthood. In severely jaundiced infants, bilirubin can cross the immature
bloodbrain barrier, producing kernicterus and brain damage.In adults, the
mature blood-brain barrier prevents bilirubin from entering the brain, and
the nervous system is not affected.
(Note: A few small areas of the brain, known as circumventricular organs,
lack a blood-brain barrier; that is, the area postrema, posterior pituitary,
subfornical organ, median eminence, pineal gland, and organum
vasculosum laminae terminalis (OVLT). This arrangement allows certain
neurons to be directly exposed to solutes in the blood, which facilitates
some neuroendocrine control processes.
Figure: The three components of the blood-brain barrier.
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Dr.Abdul-Aziz Ahmed