![]() Thalamacortical neurons are important in carrying cognitive and sensory information to CSMN via neuronal networks that include cerebellum and basal ganglia ( Clasca et al., 2012). While details are still emerging about the timing and extent of CSMN regulation, anatomical studies suggest that the major excitatory input to CSMN is mediated by neurons located in layer II/III and layer V of the motor cortex ( Thomson and Lamy, 2007 Shepherd, 2011). Therefore, to emphasize the importance of cortical input to the motor neuron circuitry, we think the word “motor” is necessary when naming these long distance projection neurons of the cerebral cortex.ĬSMN are heavily modulated by local neuron circuitry and long distance projection neurons, including, but not restricted to, thalamocortical neurons and callosal projection neurons (CPN) (Figure 1A). Without CSMN, especially in humans, the connection between the cerebral cortex and the spinal cord would be greatly impaired. This distinct ability allows them to act as the “spokesperson” of the cerebral cortex for the motor function. We prefer using the name corticospinal motor neurons (CSMN) due to their unique ability and function and to emphasize their role within the motor neuron circuitry.ĬSMN are special neuron populations in our cerebral cortex that can collect, integrate, translate, and transmit both the excitatory and the inhibitory cortical inputs as one single message to long distance spinal cord targets. They are also referred to as the upper motor neurons, corticospinal neurons, and corticospinal projection neurons. These neurons, known as Betz cells in humans, are located in layer V of the motor cortex. These neurons are characterized by: (1) a large pyramidal cell body, (2) a single apical dendrite that extends toward layer I displaying major branching and arborization, especially within layer II/III, (3) numerous basal dendrites arising from the basolateral surface, and most impressively (4) a very long axon that projects toward spinal cord targets ( Molnar and Cheung, 2006 Ozdinler and Macklis, 2006 Molyneaux et al., 2007). Among all other neuron types in the cerebral motor cortex, however, one neuron population stands out with its unique abilities and function. Since cognitive abilities are reflected in our actions, it is unreasonable to think that only one neuron type in the brain would be responsible for movement. It is this circuitry that helps define us as human beings by giving us a unique advantage to build and create tools and to express ourselves. The output of neuron function is manifested by muscle contraction leading to precise movement of the legs, arms, and hands. Voluntary movement is initiated, modulated, and controlled via a very complicated neural network, called the motor neuron circuitry, which includes neurons and cells that are located both in the cerebral cortex and the spinal cord. Our expertise in the precise control of fine movement sets us apart from other mammals. Such studies hold promise for building long-term effective treatment solutions in the near future. Here, we will review recent developments and current applications of novel strategies that reveal the cellular and molecular basis of CSMN health and vulnerability. However, visualizing and identifying these vulnerable neuron populations in the complex and heterogeneous environment of the cerebral cortex have proved challenging. Detailed cellular analyses are crucial to gain a better understanding of the pathologies underlying CSMN degeneration. In addition, CSMN death results in long-term paralysis in spinal cord injury patients. CSMN degeneration has an immense impact on motor neuron circuitry and is one of the underlying causes of numerous neurodegenerative diseases, such as primary lateral sclerosis (PLS), hereditary spastic paraplegia (HSP), and amyotrophic lateral sclerosis (ALS). ![]() 3Cognitive Neurology and Alzheimer's Disease Center, Feinberg School of Medicine, Northwestern University, Chicago IL, USAĬorticospinal motor neurons (CSMN) have a unique ability to receive, integrate, translate, and transmit the cerebral cortex's input toward spinal cord targets and therefore act as a “spokesperson” for the initiation and modulation of voluntary movements that require cortical input.Lurie Cancer Center, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA 1Davee Department of Neurology and Clinical Neurological Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. ![]()
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