Beakman explains that our nervous system is responsible for sensing pain. He uses a schematic to show the nervous system in the body and explains the traveling of signals from the neurons to the brain and back. Lester the rat hurts his tail and there is a slight delay from the event happening and your reaction (about 2 seconds).
The doctor explains to Stephen Hawking that Motor Neuron Disease is affecting his ability to use any muscle activity, such as speaking, walking, breathing, and swallowing. The signals that tell the muscles to move are disrupted, and the result is gradual muscle decay.