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The nervous system can be
divided into two main parts: the central nervous system and the
peripheral nervous system.
The
Central Nervous System
The brain and the spinal
cord make up the central nervous system. There are three layers of
tissue that cover the brain:
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the dura matter
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the arachnoid
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the pia matter
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The dura is the fibrous outer
layer. The optic nerve is covered by dura.
Cerebrospinal fluid (CSF) fills the space
between the arachnoid and the pia.
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CSF is produced by the choroid
plexus inside the ventricles (hollow spaces) of the brain. CSF
circulates around the brain and spinal cord, is replaced 3-4 times a
day, and creates intracranial pressure. If the circulation is blocked,
intracranial pressure rises. If pressure is high for an extended period
of time, it will show up funduscopically as papilledema (swelling of the
optic nerve head). |
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Normal optic nerve head. Click on
the pictures for an enlarged view. Use the back button on your
browser to return. |
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Papilledema (2x normal magnification).
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The major divisions of the
brain are the cerebral cortex, the cerebellum, and the brainstem.
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The
brainstem is responsible for
automatic, basic functions of life. Almost all of the cranial nerves,
except I and II, originate here. The brainstem is made up of the
following structures, from front (face) to
back.
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diencephlon
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midbrain
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pons
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medulla
oblongata
The pupillary reflexes
are serviced through the midbrain. |
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The
cerebellum synchronizes muscular actions. It controls the
body's orientation in space and it controls gravity related muscular
activity.
Parkinson’s disease
is a degeneration of the brain stem caused by diminished production
of a neurotransmitter called dopamine. Muscles become rigid causing
jerky movements, such as shuffling feet when walking. Eye movements can
also become jerky. |
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The cerebral
cortex produces the cognitive level of brain function including
speech, critical and abstract thinking, and personality. It is divided
into right and left hemispheres. Each hemisphere is further divided into
frontal, parietal, occipital, and temporal lobes. |
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Frontal lobe —
controls personality, problem solving, reasoning, initiative for movements. The
frontal lobe controls saccades, which allow us to rapidly fixate
from one object to another.
Occipital lobe —
images from the retina are perceived here.
Information from the right
half of each retina goes to the right occipital cortex. Information from
the left half of each retina goes to the left occipital cortex. The
occipital lobe plays a role in the reflexive eye movements called pursuits,
allowing us to follow an object in space.
Parietal lobe —
sensory information received and processed.
Temporal lobe —
Sound and odor interpretation. |
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The Optokinetic Drum or
Tape
Saccades and pursuits are
tested clinically with a drum or tape with alternating black and white
stripes. The drum is rotated slowly in front of the patient as the
patient is
asked to watch the stripes. A normal patient’s gaze will follow a
stripe briefly and then quickly re-fixate on another stripe coming
along, creating a jerky eye movement. The test is useful when testing
babies or uncommunicative patients for eye movement related functioning.
Move your mouse over the
illustration of the drum to see it in action.
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The
spinal cord connects the
brain to the rest of the body. Thirty-one pairs of spinal nerves branch
out to the body parts from the spinal cord.
The spinal cord extends
from the medulla oblongata down the inside of the spinal column. It is divided into three major
sections: cervical (neck), thoracic (chest), and lumbar (lower back). It
is protected by the vertebrae of the back (back bone). |
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| The
Peripheral Nervous System |
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The nerves that originate
from the brain (cranial nerves) and the spinal cord (spinal nerves)
branch out and make up the peripheral nervous system. Peripheral nerves
are of two functional types. Motor nerves control muscular actions. Sensory
nerves transmit information from the sensory organs. Autonomic
nerves supply the other organs, including the primary blood vessels and
muscles.
There are twelve pairs of
cranial nerves, each with specialized
functions.
Take note of cranial
nerves II through VII which affect the eyes. |
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I
Olfactory — connects to the nose and carries information
resulting in the sense of smell.
II
Optic — connects to the retinas and carries
information resulting in the sense of sight.
III
Oculomotor — controls the extaocular muscles, except
for the superior oblique and the lateral rectus. Controls the upper
eyelid (superior levator palpebrae), pupillary dilation, and
accommodation.
IV
Trochlear — controls the superior oblique muscle.
V
Trigeminal — this sensory nerve has three major branches with connections to a
large area of the head including the eyelids, eyeball,
conjunctiva, and lachrymal gland.
VI
Abducens — controls the lateral rectus muscles.
VII
Facial — controls facial muscles and connects to the tongue
and mouth resulting in the sensation of taste. It controls tear
production by the lachrymal gland.
VIII
Vestibulocochlear — controls balance and connects to the
ears resulting in the sense of hearing.
IX
Glossopharyngeal — connects to parts of the tongue, mouth,
and ear.
X
Vagus — connects to the neck and abdomen, including the
heart, the respiratory tract, and the GI tract.
XI
Accessory — controls muscles of the head and neck area.
XII
Hypoglossal — controls muscles of the tongue. |
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Neurotransmission |
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The nerve pathways are
made up of bundles of nerve cells called neurons. Neurons consist of a
cell body, dendrites, and an axon. Nerve impulses travel chemically
across a gap (synapse) from the axon of one neuron to a dendrite of the
next neuron. The impulse then travels from the dendrite to the cell
body, through the axon to the next synapse, and on to the next cell.
Most axons are coated
with an insulating substance called myelin. A degeneration of the myelin
can cause a disruption of nerve impulses, as occurs in multiple
sclerosis (MS).
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| The
Optic Nerve |
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The optic nerve can be
thought of as an extension of the brain. Since it can be directly viewed
using a direct ophthalmoscope, the optic nerve provides a kind of
"window" into the brain. The ophthalmologist can view
pathological changes in the optic nerve which give valuable clues to
associated neurological diseases.
Optic atrophy (whitening
of the normally pink optic nerve head) indicates a decrease in the
number of nerve fibers. Possible causes are glaucoma, trauma,
inflammation, compression (tumor), and ischemia (reduced blood
supply).
Optic neuritis (inflammation)
is commonly caused by MS.
Ischemic optic
neuropathy (ION) is a swelling
of the nerve caused by decreased blood flow to the nerve. It is
associated with temporal arteritis. A common symptom is amaurosis
fugax, which is a temporary monocular vision loss.
Papilledema
is a swelling of the optic nerve head caused by increased intracranial
pressure. It can be caused by pseudotumor cerebri, a brain tumor,
meningitis, or a hemorrhage inside the skull.
Pathology of the optic
nerve is often accompanied by visual acuity changes and is almost always
associated with visual field changes that are characteristic of the
particular disease. This makes sense because all of the nerve fibers
that service the retina pass through the optic nerve head on their way
to and from the brain.
Visual field testing is
an important tool used by the ophthalmologist to diagnose and follow
neurological pathology. |
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Click the thumbnail image to view a
larger image. Use the "back" button on your browser to
return.
Above: Normal optic nerve head.
Above: Optic nerve head atrophy.
Above: Papilledema
Above: Optic neuritis
Above: Glaucomatous cup
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