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Once established
in the intestines, poliovirus can enter the blood stream and invade the
central nervous system - spreading along nerve fibers.A
sequence of events occur.
1.Begin
to infect the motor neurons of the anterior horn through the same mechanisms
it uses to infect epithelial cells of the gut.
2.Host
cell protein synthesis and RNA synthesis are then inhibited by the virus
as replication of viral components begin.
3.Neurons
noticeably swell due to increased chromoplasmic content from the viral
replication.
4.Motor
neuron then undergoes cellular lysis for viral replicants to exit the host
cell.
5.Because
the motor neurons rooted in the anterior horn travel outward through the
peripheral nerves to innervate muscle fiber, the death of a motor neuron
will result in dennervation of its motorgroup and hence lead to weakness
of the muscle.
Taken from Bears, Connors, and Paradiso, Fundamentals of Neuroscience
Progression
of Paralysis: How Can Polio Kill You?
The muscles
of the legs are affected more often than the arm muscles. The limb becomes
floppy and lifeless; a condition known as acute flaccid paralysis.
More extensive paralysis, involving the trunk and muscles of the thorax
and abdomen, can result in quadriplegia. In the most severe cases,
mainly bulbar polio, the motor neurons of the brain stem are killed resulting
in decreased breathing capability and causing difficulty in swallowing
and speaking. Without respiratory support, bulbar polio can result in death.
Paralytic
vs. Non-paralytic Poliomyelitis
A.Non
Paralytic
The CNS manifestation of poliomyelitis
can either be paralytic or non-paralytic depending on the severity of the
damage caused by the poliovirus infection. If only a small population
of motor neurons are killed in the anterior horn of the spinal grey, then
the dennervated muscle fibers originally belonging to the motor units of
the damaged axon can be reinnervated by another surviving motor neuron
and hence we observe some sort of recovery of function. However,
the muscle will become more hypertrophic since the innervation ratio of
motor neurons to muscle fibers will be lesser.
Current Status of Poliovirus Infections,
Melnick, J. L. Clinical Microbiology Reviews, July 1996 p.
293-300
B.Paralytic
If large populations of motor neurons
die in the anterior horn of the spinal grey, the deennervated muscles will
not be reinnervated since CNS neurons lack the ability to regenerate and
thus the muscle is subject to atrophy. Paralytic poliomyelitis has all
the symptoms of non-paralytic plus weakness in one or more muscle groups.
It is generally separated into three sub-categories.
1.
Spinal paralytic poliomyelitis:
This is the most frequent type of paralytic poliomyelitis and is associated
with paralysis or paresis of trunk muscles or muscles responsible for movement
of the limbs. The effected limbs tend to be the lower extremities.
The paresis/paralysis does not fit any particular pattern.
2. Bulbar paralytic poliomyelitis:
This type of poliomyelitis leads to weakness of muscles innervated by the
cranial nerves. The symptoms generally include difficulty in swallowing,
loss of voice quality, and sometimes tongue and facial paralysis.
3. Bulbospinal paralytic poliomyelitis:
This type of paralytic poliomyelitis generally leads to severe respiratory
impairment.
Ten to twenty-five percent of paralytic cases seen during the polio
epidemics were of bulbar or bulbospinal type.
Commonalties
Both paralytic and non-paralytic poliomyelitis
are usually accompanied by nausea, headache, sore throat, back and neck
pain, and stiffness. There are also changes in reflexes and elevated
spinal fluid cell count.
Taken from Bears, Connors, and Paradiso,
Fundamentals of Neuroscience