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Welcome to the website of the
Southwest Virginia MS Support Group
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Central Nervous System
The
Central Nervous System (CNS) is made up of the
brain and the
spinal cord which are
located in the dorsal body cavity. The CNS contains billions of
specialized cells known as
neurons. Neurons do
all the processing and communication, and trillions
of support cells, called
glial cells, which assist
its functioning, repair damage and isolate it from
the rest of the body.
Neurons have specific projections called
Dendrite and
Axons that contribute to their unique function
of transmitting signals throughout the body.
Dendrites carry electrical signals to the Neuron,
while Axons carry them away from the Neuron.
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It contrasts with the Peripheral
Nervous System
which passes nerve signals between the CNS and rest
of the body.
There are two different
types of region in the CNS called
grey matter and
white matter.
(Click for a picture of where the grey and white matter
are.) Grey matter is where all the processing is done and white matter is responsible for communicating
between regions of grey matter and between the grey matter and the reset of the body.
The
brain is surrounded by the cranium, and the spinal
cord is protected by the vertebrae. The brain is
continuous with the spinal cord at the foramen
magnum. In addition to bone, the CNS is surrounded
by connective tissue membranes, called meninges, and
by cerebrospinal fluid.
Meninges
There are three layers of meninges around the
brain and spinal cord. The outer layer, the dura
mater, is tough white fibrous connective tissue. The
middle layer of meninges is arachnoid, which
resembles a cobweb in appearance, is a thin layer
with numerous threadlike strands that attach it to
the innermost layer. The space under the arachnoid,
the subarachnoid
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space, is filled with cerebrospinal fluid and
contains blood vessels. The pia mater is the
innermost layer of meninges. This thin,
delicate membrane is tightly bound to the
surface of the brain and spinal cord and
cannot be dissected away without damaging the
surface. Meningiomas are tumors of the nerve tissue
covering the brain and spinal cord. Although
meningiomas are usually not likely to spread,
physicians often treat them as
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though they were malignant to treat symptoms that may develop when a
tumor applies pressure to the brain.
Brain
The brain is divided into four parts.
Cerebrum
The largest and most obvious portion of the
brain is the
cerebellum, which is divided by a deep
longitudinal fissure into two cerebral hemispheres.
The two hemispheres are two separate
entities but are connected by an arching band of white fibers,
called the corpus
callosum, a great band of
commissural fibers uniting the cerebral
hemispheres of higher mammals including
humans,
that provides a communication pathway between the
two halves. Each cerebral hemisphere is divided into
five lobes, four of which have the same name
as the bone over them: the
fontal lobe, the
parietal lobe, the
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occipital lobe,
and the temporal lobe. A fifth lobe, the insula or
Island of Reil, lies deep within the lateral sulcus.
Diencephalon
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The diencephalons is centrally located and is
nearly surrounded by the cerebral hemispheres.
It includes the
thalamus,
hypothalamus, and
epithalamus. The thalamus, about 80 percent of
the diencephalons, consists of two oval masses
of gray matter that serve as relay stations
for sensory impulses, except for the sense of smell, going to the cerebral cortex.
The cortex is the outer or superficial part of
an organ or bodily structure (as the kidney,
adrenal gland, or a hair); especially : the
outer layer of gray matter of the cerebrum and
cerebellum. The
hypothalamus is a small region below the
thalamus, which plays a key role in
maintaining homeostasis because it regulates
many visceral activities. The
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epithalamus is the most dorsal portion of the
diencephalons. This small gland is involved with the
onset of puberty and rhythmic cycles in the body. It
is like a biological clock.
Brain Stem
The brain stem is the region between the
diencephalons and the spinal cord. It consists
of three parts:
midbrain, pons, and
medulla oblongata. The midbrain is the most superior
portion of the brain stem. The pons is the
bulging middle portion of the brain stem. This
region primarily consists of nerve fibers that
form conduction tracts between the higher
brain centers and spinal cord.
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The medulla oblongata, or simply medulla, extends
inferiorly from the pons. It is continuous with the
spinal cord at the foramen magnum. All the ascending
(sensory) and descending (motor) nerve fibers
connecting the brain and spinal cord pass through
the medulla.
Cerebellum
The cerebellum, the second largest portion of the
brain, is located below the occipital lobes of the
cerebrum. Three paired bundles of myelinated nerve
fibers, called cerebellar peduncles, form
communication pathways between the cerebellum and
other parts of the central nervous system.
Ventricles and
Cerebrospinal Fluid
A series of interconnected, fluid-filled cavities
are found within the brain. These cavities are the
ventricles of the brain, and the fluid is
cerebrospinal fluid (CSF).
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The spinal cord is major column of nerve tissue that is
connected to the
brain
and lies within the vertebral canal and from which
the spinal nerves emerge. It extends from the foramen magnum at
the base of the skull to the level of the first
lumbar vertebra. The cord is continuous with the
medulla oblongata at the foramen magnum which is the
great band of commissural fibers uniting the
cerebral hemispheres of higher mammals including
humans.
Like the brain, the spinal cord is surrounded by
bone, meninges, and cerebrospinal fluid.
The spinal cord is divided into 31 segments with
each segment giving rise to a pair of spinal nerves.
At the distal end of the cord, many spinal nerves
extend beyond the conus medullaris to form a
collection that resembles a horse's tail. This is
the cauda equina. In cross section, the spinal cord
appears oval in shape.
The thirty-one pairs of spinal nerves originate in the
spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and
1 coccygeal. The spinal cord and the brain constitute the
central nervous system (CNS). The spinal cord consists of
nerve fibers that transmit
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impulses to and from the brain. Like the brain, the spinal
cord is covered by three connective-tissue envelopes called
the meninges. The space between the outer and middle envelopes
is filled with cerebrospinal fluid (CSF), a clear colorless
fluid that cushions the spinal cord against jarring shock.
Also known simply as the cord.
The spinal cord has two main functions:
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Serving as a conduction pathway for impulses
going to and from the brain. Sensory impulses
travel to the brain on ascending tracts in the
cord. Motor impulses travel on descending tracts.
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Serving as a reflex center. The reflex arc is the functional
unit of the nervous system.
Reflexes are responses to stimuli that do not
require conscious thought and consequently, they
occur more quickly than reactions that require
thought processes. For example, with the
withdrawal reflex, the reflex action withdraws the
affected part before you are aware of the pain.
Many reflexes are mediated in the spinal cord
without going to the higher brain centers.
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The writer of part of this, Reynolds Price (1933- ), called the spinal cord
"the literally irreplaceable cable between my brain and the
rest of my body." (A Whole New Life. An Illness and a Healing.
1994, p. 58)
The Peripheral Nervous System (PNS) consists of
the rest of the Neurons in the body outside of the
Central Nervous System (CNS) . It passes nerve
signals between the CNS and rest of the body. These include the
Sensory Neurons, which detect any sensory stimuli
and alert the CNS of their presence, and Motor
Neurons, which connect the CNS to the muscles and
carry out instructions from the CNS for movement.
In multiple sclerosis
(MS), neurons get stripped of their insulating material (myelin)
in patches of damage called lesions or plaques.
These lesions predominantly affect the white matter (about 95%) but a few
lesions are found in
grey matter (about 5%).
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