The man with the broken neck can not feel his foot, replies that he feels no pain when he is asked, but still withdraws automatically when his foot is pinched. What does this imply about the human nervous system? We know that there are two aspects of the central nervous system: the brain and the spinal cord. Many of the body's actions involve an extended process of signals, passing from the sensory modalities up to the brain and then back through the motor pathways to the organ that performs the action. But some processes seem to take place without any input from the brain. In instinctual situations, you are up and running the moment you see a bear, even before you process the bear's present actions-whether it is preparing for attack or just lumbering through without any notice of your presence. These actions occur with less processing of the input than actions such as watering a plant or kicking a soccer ball. The type of responses that occur in these instinctual situations must be processed in a different, more efficient way. After all, if the bear is about to attack you, you can't pause to think about its other options. The bear will not wait for you. Perhaps these types of responses don't involve as much (if any) processing by the brain, and are dealt with only at the level of the spinal cord, making them have shorter duration. This would explain the response of the man whose neck is broken. Connections between the brain and the spinal cord may have been severed or lessened, but the spinal cord is still able to have small amounts of communication with the parts of the body connected to it directly. So even though the brain and spinal cord are typically thought of as dual point processing centers, they each also have processes independent of the other, allowing for faster processing in situations that can not be delayed.

Interesting extension. Need to be careful, though, not to presume an equivlence between "instinctual" and spinal cord, and something else and brain. Brain and spinal cord can work independently of each other; that's the point of the observations on paraplegics, with each having its own inputs and outputs beyond the nervous system. The response to a bear (perhaps "instinctual") involves both brain and spinal cord, since the input (seeing the bear) actually arrives in the brain (optic nerve). And the spinal cord is pretty sophisticated, capable of doing a lot of things one wouldn't obviously call instinctual. We'll talk more about what "instinctual" means, but for the moment let's leave it aside and just say that brain and spinal cord are capable of working independently of one another. PG