Beyond the Brain: How our spinal cord works to maintain hand position
Figure 1. Illustration of the human spinal cord, shown in yellow. Image source: Bruce Blaus, Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine.
If you recall the last time that you touched something really hot, your hand probably pulled away quickly. In fact, it probably pulled away before you even realized how hot the object was. This is called the ‘withdrawal reflex’ and is just one of many complex actions that the spinal cord can perform without you having to think about them. The spinal cord connects our brain to the rest of our body and vice versa. However, instead of simply being a connection, the spinal cord is completely responsible for certain actions like the withdrawal reflex. Since nerve signals from our limbs take longer to reach the brain than the spinal cord, some actions are performed by networks of cells around the spinal cord, known as spinal circuits. Spinal circuits become active in situations when it is important to react quickly, as in the case of the withdrawal reflex: touching a hot surface activates temperature and pain receptors in the skin, which send nerve signals to the spinal circuit activating the withdrawal reflex. As a result, this reflex works to quickly move limbs away from the hot surface to minimize injury. Even though we know that spinal circuits control these important reflexes, we still don’t know everything that the spinal cord can do. Recently, however, researchers from Western University accidentally discovered a previously unknown function of spinal circuits.
The discovery came when Jeff Weiler, a post-doctoral researcher at Western’s Brain and Mind Institute, was investigating how the brain maintains the position of hands in space. . Dr. Weiler, in collaboration with Paul Gribble and Andrew Pruszynski, was trying to understand how the brain accomplishes this task.
Instead, the researchers noticed that arm muscles were reacting to the perturbation more quickly than expected if the brain was responsible. The time it took for the arm muscles to activate was much shorter than the time it would take for sensory signals from the arm to travel up to the brain and then back down to the muscles. This indicated that circuits in the spinal cord were involved: “We looked at the data and we [realized that] the spinal cord is able to do this”, said Dr. Weiler. This inspired the researchers to explore other potential functions of the spinal cord. “[We decided to] do a whole series of experiments to probe what are the functional capacities of that particular circuit”.
In terms of their arm perturbation study, Dr. Weiler and colleagues suspected that the stretch reflex was doing more than just responding to individual arm muscles being stretched. They hypothesized that the stretch reflex coordinates arm muscles in a sophisticated way to maintain hand position. To test this hypothesis, the team came up with a series of experiments in which participants were asked to place their arm in a robotic exoskeleton. This specialized apparatus holds your arm in a specific orientation and can flex or extend shoulder, elbow and wrist muscles, all while you grip a handle to maintain hand position over a target position (see Figure 1 below).
During the experiments, participants started with their hand position at the home target (red dot on the diagram) and the robotic exoskeleton would perturb the arm in various ways while participants tried to maintain hand position at the target location. Arm muscle activity was recorded and the timing of the muscle response was used to determine whether the spinal cord was involved – responses between 25 and 50 milliseconds (about half as long as the blink of an eye) are produced by spinal circuits. By flexing and/or extending the wrist and elbow joints in different experiments, the researchers found that the spinal cord was producing sophisticated responses to move the hand back toward its position before the perturbation. This is particularly fascinating because it implies that spinal circuits are figuring out how to coordinate the movement of the elbow and wrist in order to get the hand back to the starting position – a function much more complex than a simple reflexive response to stretched muscles.
Figure 2. Illustration of a participant wearing a robotic exoskeleton during an experiment. The initial hand position at the home target (red dot) is shown at the top while the various arm perturbations that the device forces on the experiment’s arm are shown at the bottom. Image modified from Weiler, Gribble, and Pruszynski (2019) Nature Neuroscience.
The researchers also found that the instructions given to participants about whether or not to resist the perturbation had no impact on the stretch reflex – a person’s hand started moving back toward its initial position after a perturbation regardless of their intention to resist the perturbation or not. This automatic response lines up well with what we already know about spinal circuits – that these circuits are reflexive and not dependent on a person’s conscious intent to move (which would require input from the brain). All in all, the spinal stretch reflex appears to be capable of much more than was traditionally thought. We now know that the spinal stretch reflex can perform complex computations that allow us to maintain hand position in space without input from the brain.
Dr. Weiler believes that the discovery of sophisticated spinal circuits is important in shaping our understanding of the role of the spinal cord in movement. “I think our result can now push people away from this ‘brain does everything’ aspect of motor control and think of motor control as a process that evolved in a hierarchy of neural centres, with the spinal cord, brainstem, and cortex all working together to help drive adaptive behaviour.”
With a less brain-centric view of the motor system, researchers may uncover other fascinating functions that of the spinal cord. This change in focus could also be useful clinically, as Dr. Weiler points out, “if the brain is not entirely responsible for adaptive behaviour, from a therapeutic or rehab standpoint, maybe [clinicians] should devote part of [their] rehab procedures to other areas of the nervous system.”
While Dr. Weiler and colleagues had originally set out to understand how the brain was involved in maintaining hand position, the findings of this study have instead added to mounting evidence that the spinal cord can generate these sophisticated responses to sensory information without input from the brain. As scientists continue to expand the horizons of our knowledge, studies like this one remind us that sometimes a fascinating finding is waiting to be discovered in a place we overlooked.
Original Research Article:
Weiler, J., Gribble, P.L. & Pruszynski, J.A. (2019) Spinal stretch reflexes support efficient hand control. Nat Neurosci 22, 529–533. http://dx.doi.org/10.1038/s41593-019-0336-0