An animal's ability to move - like the kicking of a developing baby or the crawling and walking of insects - is intrinsic, not dependent on sensory stimulation, says a University of Toronto neurobiologist.
"All animals, from worms to humans, have rhythmic movements that underlie locomotion," says Max Suster, a post-doctoral fellow at U of T at Mississauga and lead author of a paper in the March 14 issue of Nature. "The question is whether this ability is built into the neurons of central nervous systems or whether sensory input from the outside world helps organize those movements so that they are suitable to real life. Our research suggests that the development of embryonic motor systems is largely intrinsic to central nervous systems and not dependent on sensory cues like touch or smell."
Suster and University of Cambridge professor Michael Bate examined the development of rhythmical movements in a type of fruit fly known as Drosophila. They compared fly embryos that received sensory input to those deprived of stimulation through methods of genetic engineering.
"We discovered, somewhat surprisingly, that the ones without sensory references still performed enough movements to get out of their egg case and crawl," says Suster. "The rhythmic movements were essentially the same, regardless of whether the embryo received stimuli or not. The amazing thing is that it was demonstrated so clearly, something that we haven't seen before."
Movements generally have certain rhythmic properties to them and it has been known for some time that central pattern generators - specialized groups of neurons in the nervous system - produce these rhythmic movements, which are essential for all forms of locomotion and reflexes, says Suster.
He says all animals, including humans, have fundamentally the same kinds of rhythmic movements. "We all depend on these types of circuits or central pattern generators. What the research tells us is that even when embryos are deprived of sensory references, they still develop rhythmic movements. And that leads us to the suggestion that these movements are, to a large extent, coded in the intrinsic properties of neurons within the central nervous system, so much so that they are sufficient to drive the movements."
However, the researchers also discovered that a lack of sensory information - while not necessary for coordinated movement in the embryo - did influence movement in the larval stage. Once the embryos hatched from their shells, they exhibited inappropriate movement such as moving backwards, swinging their heads frequently and crawling upside down. This strongly suggests that sensory information plays a crucial role in directional movement and that without such information, animals are unable to adapt to their surroundings, search for food and survive, Suster says.
This research was supported by the Venezuelan National Academy of Sciences and a grant from the Wellcome Trust.
U of T Public Affairs
U of T at Mississauga zoology department