Why is the brain wired up the way it is—why does the right cortex control the left side of the body, and vice versa?
—Peter Wilson, London
Mark A. W. Andrews, professor of physiology and director of the Independent Study program at the Lake Erie College of Osteopathic Medicine, provides this explanation:
FROM ANCIENT TIMES, many have asked your question. Greek physician Hippocrates, for example, wondered why trauma on one side of the head caused deficits on the opposite side of the body. Around 100 years ago Spanish neuroanatomist and Nobel laureate Santiago Ramn y Cajal first explained this phenomenon in terms of development of the visual system. Although we now know that animals with rudimentary or no visual systems also show “crossover” neural connections, Ramn y Cajal's explanation did identify important concepts and the stimulus-response arena.
Crossover, or decussation, of neurons within the central nervous system is still not fully understood today. Such a phenomenon arises during embryological development. Recent discoveries indicate that neurons, or nerve cells, get their direction from growth factors with names such as roundabout, commissureless, Sax-3, netrin and sonic hedgehog. And, yes, many animals, including fish, worms, fruit flies and all vertebrates exhibit this decussation of nerve tracts. Where does crossover come from? Scientists are looking for the answers in many places.
Tips from simpler animals. Understanding this structural property of the nervous system can begin with clues from evolutionarily ancient creatures. For example, let us consider the response of a worm to a noxious stimulus. The worm bends away, in the opposite direction of the stimulus, by contracting muscle cells on its opposite, or contralateral, side. To activate those contralateral muscle cells, the neuron signals arising from the ipsilateral (near, or facing) side must cross over.
Thus, decussation across the midline of the body gives an animal a survival advantage. As biologists have learned, once such an appropriate survival mechanism develops, it is maintained through “higher” animals (those that arose later in evolutionary history) unless it somehow becomes disadvantageous.
Perception clues. As visual sensory systems evolved, their neurons also developed crossover communication pathways. In most vertebrates, because of the head structure, the eyes are independent and see separate visual fields. That is, the visual inputs from their left and right eyes are completely different, and the brain stitches the views together into a coherent scene. The entire optic nerve crosses the midline to help an animal survive if it sees a danger.
Think of a fish swimming along. Now imagine that a predator suddenly appears near the fish's right side. Light reflected from the predator enters the eye, forming an image on the retina. That image then crosses over via the optic nerve, and the nervous system reacts: muscles on the contralateral side shorten. This effect makes the fish move in a direction opposite that of the stimulus (the sight of a predator).
The situation gets more complex in animals with front-directed eyes and stereoscopic vision, such as humans. The architecture of crossed nerve pathways still exists. But in such cases, only half the nerve impulses from each eye are sent across the midline to help in stereoscopic vision.
Two-sided response. Let us consider what happens in an animal that has limbs. In legless animals such as fish and worms, the impulses sent out by the motor nerves to control muscles do not have to cross the midline. Only the sensory signal crosses, causing muscle activity on the side it crossed to—thus no need to recross. When limbs are present, however, not only does the contralateral side respond, but the ipsilateral side can also respond. To allow this flexibility, motor nerves cross over back to the original side of the stimulus. In other words, with development of limbs, motor nerves as well as sensory nerves decussate. Thus, your brain's left hemisphere primarily controls your right arm and leg, and the right side handles your left arm and leg.
Scientists have also hypothesized that crossed nerve tracts, with their inherent structural asymmetry, might be the result of differential development of the two sides of the brain. The functional asymmetry of the two sides of the brain could help explain the left hemisphere's emphasis on communication, analytical thought and directing movement and the right hemisphere's specialization in dealing with sensory information, spatial relations and creativity.
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