Abstract
Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input. Therefore, both the cortex and the spinal cord are involved. The cortex has special importance for motor neuron diseases, in which initiation and modulation of voluntary movement is affected. Amyotrophic lateral sclerosis (ALS) is defined by the progressive degeneration of both the upper and lower motor neurons, whereas hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS) are characterized mainly by the loss of upper motor neurons. In an effort to reveal the cellular and molecular basis of neuronal degeneration, numerous model systems are generated, and mouse models are no exception. However, there are many different levels of complexities that need to be considered when developing mouse models. Here, we focus our attention to the upper motor neurons, which are one of the most challenging neuron populations to study. Since mice and human differ greatly at a species level, but the cells/neurons in mice and human share many common aspects of cell biology, we offer a solution by focusing our attention to the affected neurons to reveal the complexities of diseases at a cellular level and to improve translational efforts.
Highlights
Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input
Once the upper motor neuron integrates all input coming from many different sources, it may tilt the balance towards generating an action potential, which will be carried long distances towards the spinal cord targets
Even though upper motor neurons are mostly considered as one neuron type, they can be grouped based on their target innervation patterns in the spinal cord
Summary
Movement is one of the most complicated tasks the human body performs. It involves many different neuron and cell types located both in the cerebral cortex and the spinal cord. The upper motor neurons, which are referred to as corticospinal motor neurons (CSMN) in mice and Betz cells in humans, have a unique importance They receive many different levels and types of cortical input from long distance projection neurons such as thalamocortical neurons, callosal projection neurons, as well as local circuitry neurons [1,2]. The motor neuron circuitry with the involvement of numerous different types of neurons in the brain that converge onto upper motor neurons, with non-neuronal cells that play significant roles in synapse formation and maintenance, with the very many different types of spinal motor neurons and the high-level complexity of muscle innervation patterns is one of the most complex systems in our bodies
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