Two feet or four, software is the same

Standing fully erect and balancing on only two feet gives humans a strange strut that sets them apart from all other mobile critters. Yet the basic motor commands that direct a human stride may also get other animals moving, a new study suggests.

Although legged vertebrates come in many different shapes and sizes and exhibit a wide variety of walking styles, they may all employ a similar nerve system, located in the spine, to coordinate the muscle activity needed for locomotion, neurophysiologist Francesco Lacquaniti of the University of Rome Tor Vergata and colleagues report in the Nov. 18 Science.

Networks of spinal nerve cells, called central pattern generators, contain all the necessary information to time the muscles for the step cycle, says neuroscientist Sten Grillner of the Karolinska Institute in Stockholm,  who was not involved in the study. The networks still need to be turned on by the brain, but once triggered, the spinal nerves handle locomotion all on their own. A message to start moving gets generated in the spinal cord and travels down the nerve pathway to specialized nerve cells that deliver the message directly to muscle fibers.

The central pattern generators are so autonomous that, in some cases, cats can still walk after having their spinal cords severely damaged. It doesn’t work the same in humans, who typically suffer permanent paralysis after significant spinal shock.

“CPGs are still a little bit mysterious,” says Lacquaniti. But he hopes that recognizing the similarities in nerve-muscle interactions between different species will help physicians design better approaches to rehabilitating damaged spinal cords and developing robotic prosthetics.

Many herd animals take their first steps a few minutes after being born, and several species of bird walk directly after hatching. But it takes a human baby a lot longer to learn how to walk unassisted, leading some scientists to believe that there must be something fundamentally different about the human gait. But even though the musculature and biomechanics in humans might be different, “the motor pathways are strikingly similar across species,” says Lacquaniti.

In fact, newborn rats and newborn humans had nearly identical neurological stepping profiles, the researchers found. And adult rats, cats, macaques and guinea fowl demonstrated motor nerve patterns that very closely resembled those of stepping human toddlers. “Our findings suggest that all types of vertebrate locomotion may derive from an ancestral neural network,” Lacquaniti says.

To compare the nerve activity of human locomotion to that of felines, rodents, birds and other primates, the researchers used strategically placed electrodes to measure the electrical activity in 24 walking muscles in newborns (who were held up and allowed to make the walking motion), toddlers, preschoolers and adults.

The motor nerves of newborns exhibited two patterns of electrical activity that appeared as long waves on a monitoring device called an electromyograph. These same two electrical patterns were present in older participants, but so were two new patterns — not waves, but flat lines with sharp peaks.

“In adults, we saw much shorter pulses of neural activity because they time muscle contraction at precise phases of the gait cycle,” says Lacquaniti. Adults get the most out of each stride, minimizing muscle use to save energy, something human newborns haven’t yet learned how to do. The longer waves of nerve activity seen in babies are evidence of prolonged muscle contractions, “a very uneconomical way to move,” says Lacquaniti.

Adults don’t totally abandon the less efficient nerve patterns that first get them stepping, but refine such patterns and overlay new ones to improve the timing of the step cycle and maximize energy use while walking around. Newborns can only partially support their body weight and swing their limbs, but adults use additional nerve pathways to roll the foot heel-to-toe on the ground, seamlessly decelerating and accelerating once again.

Once perfected, the human gait is a marvel of physics, Lacquaniti says. Standing straight aligns all the leg and hip joints, reducing the torque around each hinge, which basically means less force is required to move human legs. The human walking motion isn’t a completely ideal pendulum (some energy is wasted), but adults have learned how best to exploit their alternately swinging legs, using the inertia of body motion to take the burden off leg and hip muscles.

There are disadvantages to walking on two legs — primarily a lack of balance — but the ability to amble and keep hands free for other tasks trumps instability issues. It’s possible that human locomotion evolved primarily to free hands for carrying tools, food and children, Lacquaniti says. As a result of such multitasking, humans now depend more on commands from the brain to coordinate locomotion.

Being more reliant on the brain’s commands may be a major reason why humans don’t regain motor function after a spinal injury the same way cats can — an unfortunate by-product of human locomotive evolution, suggests Lacquaniti. But the most important thing this study reveals is that the human walking style is simply an elaboration on a common plan of vertebrate locomotion, with central pattern generators as the shared foundation.

Continuing to study the nerve patterns that drive locomotion in other animals could be the key to getting patients with spinal cord damage or conditions like cerebral palsy walking again.