What Crosses Our Minds When Danger's Afoot
Monday, June 13, 2005
One of the more mysterious and less-explored observations about human anatomy is also one of the oldest, going all the way back to the Father of Medicine.
"If the wound be situated on the left side [of the head], the convulsion attacks the right side of the body," Hippocrates noted in the 4th century B.C. The Greek physician recognized that trauma on one side of the head could cause a seizure limited to or, more likely, starting in the limbs on the opposite side of the body.
About A.D. 150, a physician named Aretaeus the Cappadocian extended this observation in a truly remarkable way.
He noticed that if the right side of the head was severely damaged, the left side of the body would be paralyzed. However, if the damage was in the right side of the spinal cord instead, the paralysis would be on the same side. He then came up with an explanation.
"The cause of this is the interchange in the origins of the nerves, for they do not pass along on the same side . . . until their terminations," he wrote, according to an account in the 1994 book "Origins of Neuroscience" by medical historian Stanley Finger. Each nerve "passes over to the other side from that of its origin, [separating from] each other in the form of the letter X."
This crossing over of the nerves that govern the muscles was debated through the Middle Ages. But by the early 1700s, its existence was certain, and even the site of the crossover was known. It is in the part of the brainstem known as the medullary pyramids. Much later, research established that a similar "midline crossing" occurs with nerves carrying sensation, although at a different site. The sense of touch or pain in the left hand is perceived by the right side of the brain, and vice versa.
This arrangement is most highly developed in primates. At least 75 percent of the "motor" fibers that start on one side of the human brain end up on the opposite side of the spinal cord, where they stimulate the nerve cells that actually drive the muscles. In animals lower on the evolutionary tree, a smaller percentage of nerve fibers cross over. Nevertheless, it is a general design feature -- and a tricky one to build into the animal.
Like cars on a crowded freeway, the tips of nerve fibers growing down from the brain as a fetus develops must be kept to one side of the spinal cord for a certain distance, at which point most are forced to cross the median strip and continue down the lanes on the opposite side of the cord.
The body accomplishes this by making a half-dozen or more substances that direct the growth of motor nerves down the cord. Some act as chemical guard rails, keeping the nerve tips away from the midline. Others function as molecular state troopers, diverting the mass of nerves across the midline at a designated spot.
All told, it is an amazing feat of evolution. The question is: Why does it exist?
The answer appears to be that this design is the natural result of two things: the physics of optical lenses and the emergence of limbs as a means of locomotion.
Earlier this year, Denis Jabaudon, a Swiss neuroscientist from Geneva University Hospital who is now at Harvard Medical School, explored this question in the journal Lancet Neurology. He extended and refined an explanation first offered 107 years ago by the Spanish anatomist Santiago Ramon y Cajal.