Why can paralysis caused by destroyed motor neurons be permanent?

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Polio vaccine — polio A health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.

Paralysis caused by polio becomes permanent because the poliovirus destroys motor neurons in the spinal cord and brainstem. These specialized cells are responsible for transmitting the signals from the central nervous system that trigger muscle contraction.1

While nerve cells that are only mildly injured may recover their function over time, cells that are completely destroyed cannot be replaced because nerve cells are unable to regenerate. Consequently, when these neurons are lost, the communication pathway to the muscles is permanently severed, leading to lasting paralysis and the progressive atrophy of the affected muscles.1