This very cool study provides a great example of the stunning complexity and sophistication of the motor control system in coordinating a seemingly simple activity like running.
When we run or walk, our head moves from side to side. Because the human head and spine is basically a bowling ball on top of a stack of hockey pucks, it is hard to balance upright, so any head movement will require a lot of compensatory movement throughout the rest of the body. In other words, even simple walking requires the whole body to quickly cooperate. Part of this cooperation is ensured by sensory hair cells in the inner ear, which monitor changes in the fluid levels of ear canals, and then signal the brain about changes in head position.
Researchers have recently discovered that the sensitivity of these hair cells* is tuned by information from the spinal cord. Whenever the spinal cord initiated a movement related to gait, it immediately sends a signal to the sensory hair cells. This reduces their sensitivity to the movement, and prevents them from sending a barrage of information about the movement to the brain.
Why does that happen? According to one of the researchers: “This feedforward principle is crucial, because it prepares the hair cells to react appropriately to the next movement."
Here's my interpretation: The signal to the ear is basically a way to tell the hair cells – the head is about to move, but it's nothing to worry about - we're just walking here, it's under control by central pattern generators in the spine, no need to report a balance emergency to the brain.
It’s an amazing display of the incredible sophistication of motor control system, and it's wide distribution throughout the body. In the words of the the lead researcher:
The direct impact of input from the spinal cord on the sensitivity of sensory nerve cells in the balance organ demonstrates the importance of interactions between sensory and motor systems, and it underlines the significance of the interplay between different components of the central nervous system – in this case, the spinal cord and the brainstem. Here, evolution has not only come up with an elegant means of anticipating the effects of locomotion on the body but also of compensating for them in an adaptive fashion.
Another cool reminder that movement intelligence lives in dynamic interaction between many parts, and is not localized in any one area.
*Note: The studies were done on frogs not humans. But we share a common ancestor with frogs, and apparently the apple didn't fall too far from the tree in regard to this aspect of our nervous systems.