Bilateral Corticospinal Projections Arise from Each Motor Cortex in the Macaque Monkey: A Quantitative Study

The corticospinal projection is considered to influence fine motor function through nearly exclusively contralateral projections from the cortex in primates. However, unilateral lesions to this system in various species are frequently followed by significant functional improvement, raising the possibility that bilateral projections of this pathway may exist or emerge after injury. To examine the detailed anatomy and projections of the corticospinal motor neurons in rhesus monkeys (n = 4), we injected the high-resolution anterograde tracer biotinylated dextran amine (BDA) into 126 sites centered about the right lower extremity (LE) primary motor cortex. Projection and termination patterns were quantified at lumbar levels L1, L4, and L7 and mapped by using serial-section reconstructions. Notably, a mean of 10.1 ± 0.6% (± SEM) of corticospinal tract (CST) axons descended in the lateral CST ipsilateral to the cortical BDA injection, and 87.9 ± 1.0% of total CST axons projected in the contralateral lateral CST. The ipsilateral ventral CST contained only 1.0 ± 0% of all projecting CST axons, whereas the contralateral ventral CST contained 0.3 ± 0.2% of all axons. In addition, a minor dorsal column CST projection was identified. Measurement of BDA-labeled terminals in the spinal cord gray matter revealed that 11.2 ± 2.2% of CST axons terminated ipsilateral to the side of cortical injection, and the remainder terminated contralaterary. As previously reported, most CST axons terminated in spinal cord laminae V-VIII, as well as the laterodorsal motoneuronal group of lamina IX (which innervates distal extremity muscles). Notably, many CST axons crossed the spinal cord midline (mean 19.9 ± 4.9 axons per 40-μm-thick section). Detailed single-axon reconstructions revealed that most ipsilaterally projecting lateral CST axons terminated in ipsilateral gray matter. Notably, we found that the bouton-like swellings of many ipsilateral CST axons descending in the dorsolateral tract were located within Rexed's lamina IX, in close proximity to motoneuronal somata. Thus, bilateral projections of corticospinal axons originating from a single motor cortex could contribute to bilateral control of spinal motor neurons and to the highly evolved degree of fine motor control in primates. Furthermore, bilateral CST projections from a single motor cortex could represent a potential source of plasticity after injury, as well as a target of therapeutic effort in neural regeneration strategies.