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.
Derek is tasked with performing a risky surgical procedure in order to remove a spinal tumor from a patient. He and Meredith discuss the problem and potential solutions. During the discussion, Derek draws a diagram of the spine that illustrates the issue the patient is facing. Derek faces a dilemma: if he cuts the cord, he will kill the tumor, but the patient will lose function in both of his legs. Derek tries to find a way to cure the tumor without cutting the cord.