Featured Research

from universities, journals, and other organizations

Genes Or Environment: What Shapes The Sensory Homunculus?

March 31, 2000
Harvard Medical School
A single gene expressed in the brain can change a long-standing icon of basic neuroscience that was until now thought to be shaped mostly by neural input from the body's periphery, Harvard Medical School researchers have found.

Protein shown to help build body maps raises questions about individual differences in function

Related Articles

Boston, MA March 30, 2000 -- A single gene expressed in the brain can change a long-standing icon of basic neuroscience that was until now thought to be shaped mostly by neural input from the body's periphery, Harvard Medical School researchers have found.

The sensory homunculus is the familiar textbook caricature of a human being spread out improbably over the surface of the brain. It depicts how much space the brain allocates to our different body parts when it comes to feeling out the world around us. Its wildly uneven proportions are generally thought to arise in response to the activity of sensory neurons feeding information to the brain. In the April Nature Neuroscience, however, researchers led by John Flanagan, HMS associate professor of cell biology, report that a protein previously known to establish maps of the visual world in lower brain centers also plays a prominent role in imprinting a proper body map on the brain's cortex.

The work is intriguing because previous research in other areas has correlated representation in the brain with functional ability¯suggesting, in essence, that more important things get more brain space. "That we can modify this with one genetic change was totally unexpected," says Flanagan.

His group has not yet analyzed how the mice's distorted body map affects them, but he says that generally "it is not too much of a stretch to think that if you change the scale of representation in the cortex, you change behavior."

Most parents marvel at how their children differ in their interests. Where could those differences come from? Genetics partly addresses this question by trying to link genes to specific behaviors, such as a propensity for adventurousness. "But in terms of these maps, there is no previous evidence for a genetic basis to how they could be controlled," says Flanagan.

Long-established species differences in sensory maps already reflect differences in functional importance and, by extension, ability. In mice, for example, the whiskers and snout take up most of the sensory brain space, whereas monkey brains dedicate large areas to the hands and feet. Flanagan speculates that in humans genetic variations in how much space the brain devotes to each body part might help explain the difference between an average person and, say, a gifted musician. The current work does not negate the importance of neural activity by incoming neurons to determining brain maps. It says for the first time, however, that the cortex also has a hand in divvying up brain space, and that this influence is genetic.

Two 1995 papers by German researchers illustrate this joint influence of genetics and the brain's environment. Using magnetic resonance imaging, one study found differences in certain auditory brain areas between musicians with perfect pitch and nonmusicians. These differences, the authors write, develop around the 30th week of pregnancy, and might therefore be genetic. The other study showed that string players had a larger cortical representation of the digits on their left hand but not their right hand than did nonmusicians, and that this difference probably arose when the person began to play.

Pierre Vanderhaeghen, then a postdoc in Flanagan's lab, started this project to learn whether a family of mapping proteins called ephrins did in the cortex what they were known to do in the visual system and elsewhere in the brain. That question was controversial, in part because the developing cortex looks uniform. Indeed, scientists did not even know whether it contained mapmaking molecules, and if so, where. How the sensory homunculus develops is not well understood, and previous work in this area had focused on the incoming neural connections. It argued that densely innervated areas, including the lips and tongue, sent large cohorts of neurons into the brain, and that their intense activity enabled them to claim a large territory in cortical layer 4, which handles incoming sensory information. The cortex itself was thought to have only a small instructive role, if any.

Vanderhaeghen's experiments suggested there must be more to this. First he found that ephrin-A5 was expressed in the mouse's somatosensory cortex in a gradient¯high at the top of the head, low at the sides. Then he found a matching gradient of a receptor for ephrin-A5, EphA4, on incoming neurons from the thalamus, the last relay station of tactile information from the body's surface. Both gradients appear during the developmental stage when these thalamic neurons penetrate the cortex and make connections in layer 4.

Presumably, ephrin-A5 is one of several gradients that overlap to define points in the brain, much as an x-, y- and z-axis define points in a 3-D mathematical graph. The idea is that incoming neurons negotiate this crossfire of labels with the combination of receptors they carry. In the end, each neuron finds its proper spot in a spatial pattern that reflects the outside world.

The molecular mechanism by which this occurs is poorly understood. Yet researchers do know that the ephrins act by repulsion¯that is, neurons studded with many EphA4 receptors avoid areas of dense ephrin-A5 ligand.

Next Vanderhaeghen measured the precise area devoted, in layer 4, to the mouse's whiskers, its most important tactile organ. He found that mice lacking ephrin-A5 had strangely distorted whisker fields, giving some whiskers expanded space while squishing together others.

The overall effect resembles differences in world maps drawn up according to criteria such as oil resources: countries' neighbor-neighbor relationships remain accurate but suddenly Norway looks oddly large next to little Sweden and Finland. Similarly, the mice's body map is basically intact but shifted in scale (see image). That was surprising, because in the visual system ephrins are known to help incoming neurons faithfully maintain these neighbor-neighbor relationships as they make connections in the brain. Without ephrin labels, their spatial order disintegrates, and the neurons grow into the wrong areas altogether. Lastly, lest someone feel misrepresented by the homunculus, Flanagan adds that it is not about mere sensitivity, but ability to resolve tactile information. We do feel pain when pinched in the ribs or lower leg, but would not use these parts to read Braille.

Story Source:

The above story is based on materials provided by Harvard Medical School. Note: Materials may be edited for content and length.

Cite This Page:

Harvard Medical School. "Genes Or Environment: What Shapes The Sensory Homunculus?." ScienceDaily. ScienceDaily, 31 March 2000. <www.sciencedaily.com/releases/2000/03/000331084002.htm>.
Harvard Medical School. (2000, March 31). Genes Or Environment: What Shapes The Sensory Homunculus?. ScienceDaily. Retrieved March 28, 2015 from www.sciencedaily.com/releases/2000/03/000331084002.htm
Harvard Medical School. "Genes Or Environment: What Shapes The Sensory Homunculus?." ScienceDaily. www.sciencedaily.com/releases/2000/03/000331084002.htm (accessed March 28, 2015).

Share This

More From ScienceDaily

More Health & Medicine News

Saturday, March 28, 2015

Featured Research

from universities, journals, and other organizations

Featured Videos

from AP, Reuters, AFP, and other news services

S. Leone in New Anti-Ebola Lockdown

S. Leone in New Anti-Ebola Lockdown

AFP (Mar. 28, 2015) — Sierra Leone imposed a three-day nationwide lockdown Friday for the second time in six months in a bid to prevent a resurgence of the deadly Ebola virus. Duration: 01:17 Video provided by AFP
Powered by NewsLook.com
These Popular Antibiotics Can Cause Permanent Nerve Damage

These Popular Antibiotics Can Cause Permanent Nerve Damage

Newsy (Mar. 27, 2015) — A popular class of antibiotic can leave patients in severe pain and even result in permanent nerve damage. Video provided by Newsy
Powered by NewsLook.com
WH Plan to Fight Antibiotic-Resistant Germs

WH Plan to Fight Antibiotic-Resistant Germs

AP (Mar. 27, 2015) — The White House on Friday announced a five-year plan to fight the threat posed by antibiotic-resistant bacteria amid fears that once-treatable germs could become deadly. (March 27) Video provided by AP
Powered by NewsLook.com
House Ready to Pass Medicare Doc Bill

House Ready to Pass Medicare Doc Bill

AP (Mar. 26, 2015) — In rare bipartisan harmony, congressional leaders pushed a $214 billion bill permanently blocking physician Medicare cuts toward House passage Thursday, moving lawmakers closer to resolving a problem that has plagued them for years. (March 26) Video provided by AP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.


Breaking News:

Strange & Offbeat Stories


Health & Medicine

Mind & Brain

Living & Well

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News


Free Subscriptions

Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile

Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?

Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins