Simplify the following text for a Grade 11 native English-speaking person.
Original: In spastic diplegia in humans , GABA absorption becomes impaired by nerves damaged from the condition 's upper motor neuron lesion , which leads to hypertonia of the muscles signaled by those nerves that can no longer absorb GABA .
Simplified: In spastic diplegia in humans , GABA absorption by some nerves becomes impaired , which leads to hypertonia of the muscles signaled by those nerves .