Polio is cause by Poliovirus. They enter into human body by oral route (most common). They replicate in body then spread to other organs.
Spread to CNS causes the above mentioned symptons. Virus may enter spinal cord and replicate within motor neurons. Their replication causes inflammatory reaction and destruction of nerve fibers which leads to paralysis of muscles supplied by the nerve effected. Muscles fiber functions by nerve stimulation. When impulse arrives at muscles (neuromuscular junction) then it causes muscles to contract. The intensity of stimulation can be modulated by higher centers of brain. Stimulation of nerve is essential for muscle function.
Atrophy and weakness of muscles - as nerves are destroyed so they are no longer able to stimulate muscles. Nerves also transport growth factors to muscles. Continous stimulus of muscles are important for their survival so over a period of time the muscles show symptoms of weakness which may progress to complete atrophy of muscles.
Respiratory paralysis - Diaphragm is important muscle fror respiration. It is supplied by phrenic nerve. If poliovirus invade and destroy this nerve then it will lead to paralysis of diagram. This will lead to death.
When nerves within brain are invaded and destroyed - the symptoms will depend on the area of brain involved.
Important - neurological symptoms are due to invasion (by poliovirus) induced inflammation and destruction of nerve.
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