As a so-called nerve-type virus (neuro-tropic), it attacks motor nerve cells of the brain and spinal cord (alpha motor nerve cells) and is excreted orally or in the stool.
It destroys some of the motor nerve cells (alpha motor neurons) and the corresponding nerve strings (axons). The corresponding muscle fibres lose contact and are no longer innervated. This results in a flaccid paralysis.
Many motor nerve cells are now destroyed or irreparably damaged. During the recovery phase, surviving motor nerve cells serving the same muscle sprout new fibres to innervate some of the orphaned striated muscle fibres that were previously innervated by the dead nerve cells.
While this promotes a certain recovery of muscles, it places additional stress on the nerve cell body to nourish the additional fibres. The survived motor cell nerves enlarge, now having to support more muscle fibres. These are called giant motor units. This is a repairing mechanism that occurs after the acute poliomyelitis phase to support paralysed muscle groups.
Poliomyelitis causes brain damage.
According to current findings, the poliomyelitis infection not only affects the peripheral nervous system but the brain as well. Therefore, it is not only a peripheral muscular nerve illness. Of more clinical importance is often possible damage to the brain – the encephalic portion (e.g. respiration -> whereby the patient has to be placed in an ‘iron lung’, etc.).