Exploring Cross-Cord Reflexes: Revolutionizing Post-Stroke Rehabilitation?

Understanding the complex neural mechanisms underlying motor recovery in post-stroke rehabilitation is crucial for developing effective therapies. One concept gaining attention is the role of cross-cord reflexes and the concept of restoring motor function, especially in managing spasticity and hemiparesis. Cross-cord reflexes refer to the neural communication between the spinal cord's left and right sides, influencing movement coordination and muscle tone across the body. The suprasegmental control, involving the brainstem and cortical structures, plays a pivotal role in modulating these reflexes, making it a key area of interest in neurorehabilitation. 

Cross-cord reflexes involve neural signals crossing from one side of the spinal cord to the other. This phenomenon is essential in maintaining balance and coordination, particularly when unilateral motor control is compromised, as in the case of a stroke. Studies have shown that stroke survivors often experience disruptions in cross-cord reflex pathways, contributing to motor deficits such as spasticity and muscle rigidity (Lemon, 2008).

 

These reflexes are not purely local; they are influenced by higher neural centers, particularly the motor cortex, corticospinal tract, and brainstem structures like the pontomedullary reticular formation (PMRF). This suprasegmental control allows for fine-tuning cross-cord reflexes, enabling more coordinated movement and muscle tone regulation (Pierrot-Deseilligny & Burke, 2012).

 

The pontomedullary reticular formation (PMRF) is another critical structure that modulates cross-cord reflexes. Located in the brainstem, the PMRF influences both excitatory and inhibitory spinal pathways, playing a significant role in regulating postural tone and locomotion (Matsuyama & Drew, 2000). In stroke patients, the balance between excitatory and inhibitory signals from the PMRF is often disrupted, contributing to abnormal muscle tone and impaired voluntary movement. Rehabilitation strategies that restore this balance can improve motor function (Edgley, 2001).

The corpus callosum, a large bundle of neural fibers connecting the brain's two hemispheres, facilitates communication between the motor cortices. In stroke patients, especially those with unilateral damage, the contralesional hemisphere can exert abnormal levels of inhibition on the ipsilesional hemisphere, exacerbating motor deficits (Stinear et al., 2013). This phenomenon, known as peri-callosal inhibition, can limit the ability of the affected hemisphere to activate motor pathways, including those involved in cross-cord reflexes.

Rehabilitation techniques such as constraint-induced movement therapy (CIMT) and transcranial magnetic stimulation (TMS) have been explored to reduce abnormal interhemispheric inhibition. These interventions aim to enhance the affected hemisphere's ability to regain control over spinal reflexes and improve motor recovery (Taub et al., 2006).

When properly modulated by suprasegmental structures, cross-cord reflexes can play a significant role in post-stroke motor recovery. Rehabilitation strategies must consider the local spinal reflexes and the broader neural networks that influence them. Techniques such as specific (quadrupedal) bilateral movement therapy, robot-assisted gait training, functional electrical stimulation (FES), and proprioceptive neuromuscular facilitation (PNF) have been shown to engage both the spinal cord and suprasegmental centers, promoting recovery by enhancing cross-cord reflex function (Mehrholz et al., 2017).

Moreover, recent research suggests that motor imagery and mirror therapy, which activate similar neural circuits as actual movement, can facilitate the recovery of cross-cord reflexes by promoting cortical reorganization and improving the balance between excitation and inhibition in spinal reflex pathways (Thieme et al., 2018). These interventions highlight the importance of integrating cognitive and physical rehabilitation techniques to achieve optimal motor recovery.

The concept of cross-cord reflexes and their suprasegmental control (the cross-cord effect) offers valuable insights into post-stroke rehabilitation. These reflexes, modulated by higher neural structures such as the motor cortex, corticospinal tract, and PMRF, are essential for coordinating movement and regulating muscle tone. Stroke disrupts this delicate balance, leading to spasticity and impaired voluntary movement. However, by targeting both the spinal and cortical aspects of motor control, rehabilitation strategies can harness the potential of cross-cord reflexes to improve motor outcomes. As research in this area continues to evolve, it holds promise for developing more effective and individualized rehabilitation programs for stroke survivors.

Lemon, R. N. (2008). Descending pathways in motor control. Annual Review of Neuroscience, 31(1), 195-218.

Matsuyama, K., & Drew, T. (2000). Vestibulospinal and reticulospinal control of posture and locomotion. Current Opinion in Neurobiology, 10(6), 752-757.

Mehrholz, J., Thomas, S., Werner, C., Kugler, J., Pohl, M., & Elsner, B. (2017). Electromechanical-assisted training for walking after stroke. Cochrane Database of Systematic Reviews, 5, CD006185.

Nielsen, J. B., Brittain, J. S., Halliday, D. M., Marchand-Pauvert, V., Mazevet, D., & Conway, B. A. (2007). Reduction of common motoneuronal drive in stroke patients. Brain, 130(2), 636-650.

Pierrot-Deseilligny, E., & Burke, D. (2012). The circuitry of the human spinal cord: Its role in motor control and movement disorders. Cambridge University Press.

Shapiro, A. (2020). Stroke-induced changes in corticospinal excitability and motor function. Journal of Neurological Science, 413, 116788.

Stinear, C. M., Barber, P. A., Coxon, J. P., Fleming, M. K., & Byblow, W. D. (2013). Priming the motor system enhances the effects of upper limb therapy in chronic stroke. Brain, 131(8), 2225-2232.

Taub, E., Uswatte, G., & Pidikiti, R. (2006). Constraint-induced movement therapy: A new family of techniques with broad application to physical rehabilitation. Journal of Rehabilitation Research & Development, 32(3), 236-252.

Thieme, H., Morkisch, N., Rietz, C., Dohle, C., & Borgetto, B. (2018). The efficacy of movement representation techniques for treatment of limb pain—A systematic review and meta-analysis. Journal of Pain Research, 11, 953-961.