New Brainstem Pathway Discovered to Control Human Hand Movements

Researchers have identified a previously overlooked neural pathway linking the brainstem and spinal cord that is essential for controlling voluntary hand and arm movements. This discovery reveals an unexpected layer of the nervous system that enables humans to grasp, hold, and manipulate objects.

The study, led by University of California, Riverside (UCR) and published in the Proceedings of the National Academy of Sciences, challenges the long-held scientific belief that fine hand movements are almost exclusively managed by the cerebral cortex. While the cortex is the brain’s advanced command center for conscious thought and voluntary movement, this research demonstrates that evolutionarily older structures in the brainstem also play a vital role.

The Multi-Stage Relay System

The research indicates that signals for voluntary hand movements do not simply travel in a direct line from the brain to the spinal cord. Instead, they move through a multi-stage relay system that integrates cortical signals with the medulla and specific segments of the spinal cord.

The researchers focused on the medulla, which is the lowest portion of the brainstem located just above the spinal cord. The medulla is known to regulate essential functions like heart rate and breathing, but it also serves as a major crossroads for signals traveling between the brain and the body. The study observed activity in two specific regions of the medulla during hand movements.

the study provides the first evidence from human brain activity measurements that two segments of the spinal cord in the neck—cervical levels C3 and C4—act as a relay. These segments facilitate communication between the brainstem and the lower spinal cord, which then directly activates the muscles in the hand.

Comparative Analysis of Humans and Mice

To map this pathway, the team used functional magnetic resonance imaging (fMRI) to compare brain activity in both humans and mice. In the study, mice were trained to press a small lever with their forepaw, while human volunteers squeezed a device with varying levels of force using their fingers.

Despite the vast differences in motor control complexity between the two species, the researchers found striking similarities in how these regions communicate. The same regions of the medulla were consistently active during these tasks in both humans and mice, suggesting that this circuitry is a fundamental mammalian architecture for limb control.

For a long time, we thought fine hand movements in humans were controlled almost entirely by the cortex. What we are observing is that evolutionarily older brainstem structures also play an important role.

Shahab Vahdat, assistant professor of bioengineering at UCR

Implications for Stroke Recovery

The identification of this “backup system” for movement has significant implications for medical rehabilitation, particularly for stroke survivors. Damage to the cortical motor regions often results in a lasting loss of hand and arm function.

Because this brainstem-spinal cord pathway exists alongside the cortical route, it may provide a new target for neuromodulation therapies. By stimulating these surviving circuits, clinicians may be able to help patients bypass damaged areas of the cortex to regain manual dexterity.

These pathways give us additional targets to explore. If we can engage them after a stroke, they may help compensate and restore function in the hands and arms.

Shahab Vahdat, assistant professor of bioengineering at UCR

The discovery suggests that voluntary hand movement is not the result of a single command center, but rather a coordinated effort where signals from the cortex are integrated with brainstem and spinal networks before reaching the muscles.