Researchers look for specific molecules, regions, or pathways when developing new drug treatments for injury or disease for efficacy. In a new study, an international team led by Kyoto University discovered a new pathway in the arm for motor modules in the spinal cord via the arm.
In the late 90s, researchers proposed that individual cells have biological processes that are modular. Functional modules are then composed of molecules with multiple functions.
Human hands are composed of 27 muscles and 18 joints, coordinated by the nervous system. How coordination occurs is so complex that even supercomputer models struggle to artificially replicate muscle activity in real-time.
Motor Activity and Interneurons
To reduce the complexity of the central nervous system, the team used the motor module hypothesis. The theory states that the brain uses interneural modules from the spinal cord to coordinate movement instead of sending signals to multiple hand muscles.
Therefore, specific movements are the result of various modules that are combined. In 1981, researchers discovered two neuron modules that control leg muscles that formed a pattern of motor activity.
The team wanted to determine if the same motor control method was present in primates' spinal cord. The research would lead to the crucial role of spinal interneurons in motor activity. Moreover, new treatments can be developed for movement disorders, and models can possibly recreate the post-spinal injury of limbs.
Macaque Arm Function Via Spinal Neurons
For the study, three macaques were given cervical spinal cord implants containing electrodes. Various groups of interneurons, or neurons that transmit impulses in between other neurons during movement underwent intraspinal microstimulation (ISMS) while the monkeys were under anesthesia.
ISMS is a technique commonly used to help restore some functions after spinal cord injury such as bladder function. Electrical stimulation can also reanimate paralyzed legs, which usually lose muscle control after severe spinal injury.
Similar to the frog leg study, arm movement was stimulated through neural modules from the spinal cord. The macaques displayed strong movement and found that most muscles in the elbow, wrist, and finger were stimulated.
"This is a very interesting finding for two reasons," said Kazuhiko Seki from Tokyo's National Center of Neurology and Psychiatry's Department of Neurophysiology. "First, it demonstrates a particular trait of the primate spinal cord related to the increased variety of finger movements. Second, we now have direct evidence primates can use motor modules in the spinal cord to control arm movement direction and force magnitude both efficiently and independently."
Unlike the frog study where two groups of interneurons were activated, the team observed a third group of active interneurons in the spinal cord that could alter the force of arm movement. The researchers concluded that there is a path where the brain sends signals to the spinal cord for muscle activity.
For example, the "plan and adapt" type of motor control can be demonstrated by drinking a can of soda. The brain may predetermine the trajectory of the best way to lift the soda and bring it up to the mouth to drink without knowing how heavy the can is.
The INa and INb interneuron groups determined the trajectory while the third module group, Inc, determined the necessary force. During the soda can drinking scenario, the three module groups were activated instead of determining which muscles were needed to perform the action.
In conclusion, the study determined that primate arm movement can be controlled by motor modules in the spinal cord. Further studies can help determine how the treatment of human limb movements can be developed after a spinal cord injury or patients with neurodegenerative diseases that cause movement disorders, stroke, and paralysis.
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