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Bilateral corticospinal projections arise from each motor cortex in the macaque monkey: a quantitative study
Steve Lacroix1, Leif A Havton, Heather McKay
1Department of Neurosciences, University of California-San Diego, La Jolla, California 92093, USA.
The Journal of Comparative Neurology
|April 22, 2004
Summary
Primates possess bilateral corticospinal tract (CST) projections, with a notable percentage of axons projecting ipsilaterally. These bilateral CST pathways may explain fine motor control and offer plasticity for neural regeneration after injury.
Area of Science:
- Neuroscience
- Motor Control
- Primate Anatomy
Background:
- The corticospinal tract (CST) is traditionally viewed as primarily contralateral, crucial for fine motor function in primates.
- Observed functional recovery after unilateral CST lesions suggests potential bilateral projections, either pre-existing or emergent.
- Understanding the full extent of CST projections is vital for comprehending motor control and recovery mechanisms.
Purpose of the Study:
- To anatomically detail the projection and termination patterns of corticospinal motor neurons in rhesus monkeys.
- To quantify the degree of ipsilateral versus contralateral projections within the CST.
- To investigate the potential role of bilateral CST projections in motor control and neural plasticity.
Main Methods:
- Injection of biotinylated dextran amine (BDA) into 126 sites in the right lower extremity primary motor cortex of four rhesus monkeys.
- Quantification and mapping of CST projections and terminations at lumbar spinal cord levels (L1, L4, L7) using serial-section reconstructions.
- Detailed single-axon reconstructions to trace projection pathways and termination sites.
Main Results:
- Approximately 10.1% of CST axons projected ipsilaterally in the lateral CST, while 87.9% projected contralaterally.
- A small but significant proportion (11.2%) of CST axons terminated in the ipsilateral spinal cord gray matter, many near motoneurons.
- Evidence of CST axons crossing the spinal cord midline and terminating in contralateral laminae, including lamina IX.
Conclusions:
- Bilateral projections from a single motor cortex exist within the primate CST, challenging the exclusively contralateral view.
- These ipsilateral projections may contribute to bilateral control of spinal motor neurons and sophisticated fine motor skills.
- Bilateral CST projections offer a potential substrate for neural plasticity and therapeutic strategies in recovery from motor injury.