Complex sentence: The upper motor neuron lesion in the brain in turn impairs the ability of some nerve receptors in the spine to properly receive gamma amino butyric acid, leading to hypertonia in the muscles signaled by those damaged nerves.
Split into simpler sentences: The upper motor neuron lesion in the brain impairs the ability of some nerve receptors in the spine to properly receive gamma amino butyric acid. That leads to hypertonia in the muscles signaled by those damaged nerves.