Motor neurons exit from the anterior horn cells at each segmental level of the cord to innervate the fibers in specific muscles. A single motor unit may contain up to 1,000 muscle fibers, depending on the size of the muscle and how specialized it is. Poliomyelitis damages or destroys the motor neuron cell bodies in the anterior horn cells. As a result, entire muscles or parts of muscles lose their innervation, becoming weak or atrophied and incapable of movement. Unlike the muscle paralysis seen with spinal cord injury, sensation in people with polio remains intact because the sensory nerves and the posterior section of the cord are unaffected.
Acute poliomyelitis is an enteroviral infection that occurs in several phases.
The first phase consists of a fever and gastrointestinal tract irritation lasting a day or two. At this point, the infection is highly contagious and the virus is spread through throat secretions and the GI tract. In the second phase the polio virus enters the vascular system, causing muscle aches, a stiff neck, increasing fever, and a headache. These symptoms are very similar to meningitis and can lead to an incorrect diagnosis. After the virus enters the vascular system, it can invade the anterior horn cells, affecting 90 to 95 percent of them.