360
UNIT THREE
M
ultiple sclerosis (MS) is a disorder of
the CNS that affects 2.5 million peo-
ple worldwide, and 400,000 in North
America. In addition to overt nervous system
symptoms, aF
ected individuals experience disabil-
ity, mood problems such as depression, and great
fatigue. ±our subtypes of MS are recognized, based
on the pattern of symptomatic periods over time.
In MS, the myelin coating in various sites
through the brain and spinal cord becomes
inflamed due to an immune response and is
eventually destroyed, leaving hard scars, called
scleroses, that block the underlying neurons
from transmitting messages. Muscles that no
longer receive input from motor neurons stop
contracting, and eventually, they atrophy.
Symptoms re²
ect the speci³
c neurons aF
ected.
Shortcircuiting in one part of the brain may aF
ect
fine coordination in one hand; if another brain
part is aF
ected, vision may be altered.
The ³
rst symptoms of MS are often blurred
vision and numb legs or arms, but because in
many cases these are intermittent, diagnosis may
take awhile. Diagnosis is based on symptoms
and repeated magnetic resonance (MR) scans,
which can track development of lesions. A diag-
nostic work-up for MS might also include a lum-
bar puncture to rule out infection and an evoked
potential test to measure electrical signals sent
from the brain. About 70% of aF
ected individu-
als first notice symptoms between the ages of
twenty and forty; the earliest known age of onset
is three years, and the latest, sixty-seven years.
Some affected individuals eventually become
permanently paralyzed. Women are twice as likely
to develop MS as men, and Caucasians are more
often aF
ected than people of other races.
MS may develop when certain infections
in certain individuals stimulate T cells (a type
of white blood cell that takes part in immune
responses) in the periphery, which then cross
the blood-brain barrier. Here, the T cells attack
myelin-producing cells through a ²
ood of in²
am-
matory molecules and by stimulating other cells
to produce antibodies against myelin.
A virus may lie behind the misplaced
immune attack that is MS. Evidence includes
the observations that viral infection can cause
repeated bouts of symptoms, as can MS, and that
MS is much more common in some geographical
regions (the temperate zones of Europe, South
America, and North America) than others, sug-
gesting a pattern of infection.
Various drugs are used to manage MS.
Drugs to decrease bladder spasms can temper
problems of urinary urgency and incontinence.
Antidepressants are sometimes prescribed, and
short-term steroid drugs are used to shorten the
length of acute disabling relapses. Muscle relax-
ants ease stiF
ness and spasms.
Several drugs are used for long-term treat-
ment of MS. Beta interferons are immune system
biochemicals that are widely prescribed, even
after only one attack if MS seems a likely diag-
nosis. This treatment diminishes the intensity
of flare-ups, but effects on the course of illness
over time are not yet known. Beta interferons
may cause flulike adverse effects. They are self-
injected once to several times a week.
Glatiramer is an alternative to beta interfer-
ons. It is prescribed if the course of the disease
is “relapsing remitting,” with periodic flare-ups.
Glatiramer is self-injected daily and dampens
the immune system’s attack on myelin. It con-
sists of part of myelin basic protein, the most
abundant protein of myelin. In response, T cells
decrease inflammation. Glatiramer also stimu-
lates increased production of brain-derived neu-
rotrophic factor, which protects axons.
Mitoxantrone is another drug that halts
the immune response against CNS myelin, but
because it can damage the heart, it is typically used
in severe cases of MS and only up to two years.
Another drug, natalizumab, prevents T cells from
binding blood vessels in the brain, also quelling
the abnormal immune response against myelin. It
too may have rare but serious adverse eF
ects.
10.2
CLINICAL APPLICATION
Multiple Sclerosis
type of neuron is specialized to send a nerve impulse in one
direction.
1.
Multipolar neurons.
Multipolar neurons have many
processes arising from their cell bodies. Only one is an
axon; the rest are dendrites. Most neurons whose cell
bodies lie within the brain or spinal cord are of this
type. The neuron illustrated in F
gure 10.3 is multipolar.
2.
Bipolar neurons.
The cell body of a bipolar neuron
has only two processes, one arising from either end.
Although these processes are similar in structure, one is
an axon and the other is a dendrite. Bipolar neurons are
found in specialized parts of the eyes, nose, and ears.
3.
Unipolar neurons.
Each unipolar neuron has a single
process extending from its cell body. A short distance
from the cell body, this process divides into two
branches, which really function as a single axon: One
branch (peripheral process) is associated with dendrites
near a peripheral body part. The other branch (central
process) enters the brain or spinal cord. The cell bodies
of some unipolar neurons aggregate in specialized
masses of nerve tissue called
ganglia,
located outside
the brain and spinal cord.
Neurons can also be classiF
ed by
functional differences
into the following groups, depending on whether they carry
information into the CNS, completely within the CNS, or out
of the CNS
(f
g. 10.7)
.
1.
Sensory neurons
(afferent neurons) carry nerve
impulses from peripheral body parts into the brain or
spinal cord. At their distal ends, the dendrites of these
neurons or specialized structures associated with
them act as sensory receptors, detecting changes in
the outside world (for example, eyes, ears, or touch
receptors in the skin) or in the body (for example,
temperature or blood pressure receptors). When
sufF
ciently stimulated, sensory receptors trigger
impulses that travel on sensory neuron axons into
the brain or spinal cord. Most sensory neurons are
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