The early nerve gets the brain space

An odd little animal that uses the touch of its sensitive nose to experience the world is teaching biologists how nerves stake their claim to territory in the brain. Kenneth C. Catania of Vanderbilt University in Nashville finds that sensory nerves of the star-nosed mole may race to occupy brain space early in development. He makes the controversial suggestion that nerves in other animals lay claim to disproportionately large areas of brain surface by showing up early.

A small ray (arrow) on each side of the star-nosed mole’s star has the greatest tactile representation in the brain. Catania

In the embryonic star-nosed mole, the developing fovea (circle) covers more area than any of the other rays (shown on right) of that side of the star. Nature Neuroscience

“The sensitivity of the [star-nosed mole’s] nose is startling,” says Catania. Although less than half an inch across, the fleshy star contains endings of more than 100,000 large nerve fibers. In contrast, only 17,000 such fibers extend to the surface of a person’s hand. Catania says their high neural density probably gives star-nosed moles an extremely detailed “picture” of the world they live in.

The neural input that comes from the most sensitive patch of the human retina, a tiny area called the fovea, gets most of the brain area devoted to vision. Similarly, Catania found that the input from two small appendages, or rays, of this star-nosed mole’s elaborate schnozzle takes 25 percent of the area in the mole’s brain that represents the star. Catania calls each of the two rays a tactile fovea.

Altogether, about half of the animal’s cortex that processes touch information is dedicated to the star.

“We’ve known for a long time that you’ll have magnification of the sensory space in the system of most importance to the animal,” says Daniel Feldman of the University of California, San Diego, who studies the brain patterns of nerves carrying input from rat whiskers.

What Catania found for the first time, says Feldman, is that the mole’s brain seems to allocate its real estate according to which nerve endings develop first.

Another hypothesis has the sensory system divvying up space in the brain according to a genetic plan, “like a blueprint for a house,” says David Rapaport of the University of California, San Diego, who has studied eye development. In yet another view, the most active nerve cells have the upper hand in staking out brain space.

Among the tactile nerve cells in the mole embryo’s star, those from the foveal rays are the first to become active, reach the cortex, and take up space in the brain, Catania reports in the April Nature Neuroscience. He recorded electrical impulses from brain areas that process information from each ray and tracked the development of the rays’ input.

“Amazingly, the development of the tactile fovea in the star closely follows the developmental sequence in the retinal fovea,” says Catania. The parallel growth patterns of the touch and visual nerve endings suggest that specialized sensory systems share basic features of development, says Catania.