Human Body: Pushing the Limits

Sight is the king of the senses. More than 80 percent of what we know of the world comes through our eyes -- without our sight we're lost. In this episode, we reveal the inner workings of our visual system as they've never been seen before, vividly confirming that ancient human adaptations prove no less crucial in modern life.

Less than one-twentieth of an inch below the surface of the skin are the "antennae" that allow us to sense the world around us. This vital layer is the gateway to the original information superhighway -- the nervous system. Millions of nerves carry sensations across the body and up to the brain at hundreds of miles an hour. But our nervous system also has abilities almost beyond imagining.

The driving force behind every one of us is the most powerful organ in the natural world: the human brain. Our central processing unit generates as many electrical impulses in a single day as all the telephones in the world combined. With new state-of-the-art imagery of this complex, mysterious machine, we reveal how our brain accelerates when faced with intense stress or danger and taps into its deepest layers to unlock prehistoric survival instincts.