Early evolution of the motor cortex included development of connections to brainstem reticulospinal neurons; these projections persist in primates. In this study, we examined the organization of corticoreticular connections in five macaque monkeys (one male) using both intracellular and extracellular recordings from reticular formation neurons, including identified reticulospinal cells. Synaptic responses to stimulation of different parts of primary motor cortex (M1) and supplementary motor area (SMA) bilaterally were assessed. Widespread short latency excitation, compatible with monosynaptic transmission over fast-conducting pathways, was observed, as well as longer latency responses likely reflecting a mixture of slower monosynaptic and oligosynaptic pathways. There was a high degree of convergence: 56% of reticulospinal cells with input from M1 received projections from M1 in both hemispheres; for SMA, the equivalent figure was even higher (70%). Of reticulospinal neurons with input from the cortex, 78% received projections from both M1 and SMA (regardless of hemisphere); 83% of reticulospinal cells with input from M1 received projections from more than one of the tested M1 sites. This convergence at the single cell level allows reticulospinal neurons to integrate information from across the motor areas of the cortex, taking account of the bilateral motor context. Reticulospinal connections are known to strengthen following damage to the corticospinal tract, such as after stroke, partially contributing to functional recovery. Extensive corticoreticular convergence provides redundancy of control, which may allow the cortex to continue to exploit this descending pathway even after damage to one area.

SIGNIFICANCE STATEMENT The reticulospinal tract (RST) provides a parallel pathway for motor control in primates, alongside the more sophisticated corticospinal system. We found extensive convergent inputs to primate reticulospinal cells from primary and supplementary motor cortex bilaterally. These redundant connections could maintain transmission of voluntary commands to the spinal cord after damage (e.g., after stroke or spinal cord injury), possibly assisting recovery of function.

运动皮层的早期进化包括与脑干网状脊髓神经元连接的发展；这些预测在灵长类动物中持续存在。在这项研究中，我们使用来自网状结构神经元（包括已识别的网状脊髓细胞）的细胞内和细胞外记录检查了五只猕猴（一只雄性）的皮质网状连接组织。评估了对双侧初级运动皮层 (M1) 和辅助运动区 (SMA) 不同部分的刺激的突触反应。观察到广泛的短潜伏期激发，与快速传导通路上的单突触传递兼容，以及较长的潜伏期反应可能反映了较慢的单突触和寡突触通路的混合。有高度的收敛性：56% 的来自 M1 输入的网状脊髓细胞在两个半球都接收到来自 M1 的投射；对于 SMA，相应的数字甚至更高 (70%)。在从皮层输入的网状脊髓神经元中，78% 的神经元接收到来自 M1 和 SMA 的投射（无论半球如何）；83% 的来自 M1 输入的网状脊髓细胞接收到来自多个测试 M1 位点的投射。这种在单细胞水平上的融合允许网状脊髓神经元整合来自皮层运动区域的信息，同时考虑到双侧运动环境。众所周知，网状脊髓连接会在皮质脊髓束受损后加强，例如中风后，部分有助于功能恢复。广泛的皮质网络会聚提供了控制的冗余，

意义声明网状脊髓束 (RST) 与更复杂的皮质脊髓系统一起为灵长类动物的运动控制提供了一条平行通路。我们发现从双侧初级和辅助运动皮层到灵长类动物网状脊髓细胞的广泛收敛输入。这些冗余连接可以在损伤后（例如，中风或脊髓损伤后）维持自愿命令向脊髓的传输，可能有助于功能恢复。