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The CNS consists of the brain and spinal cord, which are located
in the dorsal body cavity. 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.
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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 though they were malignant to treat
symptoms that may develop when a tumor applies pressure to the brain.
Brain
The brain is divided into the cerebrum, diencephalons, brain stem,
and cerebellum.
Cerebrum
The largest and most obvious portion of the brain is the cerebrum,
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 that provides a communication pathway between
the two halves.
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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 occipital lobe, and
the temporal lobe. A fifth lobe, the insula or Island of Reil,
lies deep within the lateral sulcus.
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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
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the sense of smell, going to the cerebral cortex.
The hypothalamus is a small region below the thalamus, which plays
a key role in maintaining homeostasis because it regulates many visceral
activities. The 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
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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
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portion of the brain stem. This region primarily consists of nerve
fibers that form conduction tracts between the higher brain centers
and spinal cord. 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).
Spinal Cord
The spinal cord 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. 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 spinal cord has two main functions:
- 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.
- 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.
Brain Tumor
Glioma refers to tumors that arise from the support cells of the
brain. These cells are called glial cells. These tumors include
the astrocytomas, ependymomas and oligodendrogliomas. These tumors
are the most common primary brain tumors.
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