Finely tuned nervous systems allowed birds and mammals to adopt smoother strides

Since the 1900s, neuroscientists have known that the peripheral nervous systems of tetrapods (four-footed animals) vary greatly, but how these differences affect the way that animals walk, run, or move has not been well understood. Now, a study published in the Journal of Experimental Biology, authored by a New York Institute of Technology anatomy professor, suggests that neuromuscular adaptations in mammals and birds may have allowed them to become more nimble than reptiles and amphibians.

Golgi tendon organs in reptiles and amphibians vs. birds and mammals
[Credit: Michael Granatosky]

"This research could explain why tigers have a much smoother walk than crocodiles, which lumber and drag their abdomens, and perhaps one reason why today's humans have evolved to walk with such uniform steps," says lead author Michael Granatosky, Ph.D., assistant professor of Anatomy at New York Institute of Technology College of Osteopathic Medicine (NYITCOM).

Tetrapods have small receptors in their muscles called Golgi tendon organs, which protect muscles from forces during locomotion (walking) and other physical activity. When muscle tension becomes dangerous, these receptors signal the nervous system to produce reflexes that release tension and prevent injury.

Amphibians and reptiles, which diverged from early tetrapods before mammals and birds, have freeform Golgi tendons located further from the muscle-tendon junction, suggesting that they detect stress across the entire muscle. In contrast, birds and mammals have encapsulated Golgi tendon organs set directly at the muscle-tendon junction, signifying an ability to detect tension in precise muscle areas, which would allow for more controlled motion. Now, researchers pose that birds and mammals owe their agile strides to these finely tuned receptors.