Neuroanatomy, Motor Neuron

While the term “motor neuron” evokes the idea that there is only one type of neuron that conducts movement, this is far from the truth. In fact, within the classification of a “motor neuron,” there lies both upper and lower motor neurons, which are entirely different in terms of their origins, synapse points, pathways, neurotransmitters, and lesion characteristics. Overall, motor neurons (or motoneurons) comprise various tightly controlled, complex circuits throughout the body that allows for both voluntary and involuntary movements through the innervation of effector muscles and glands. The upper and lower motor neurons form a two-neuron circuit. The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body. Understanding the difference between upper and lower motor neurons, as well as the pathway that they take, is crucial to being able to not only diagnose these neuronal injuries but also localize the lesions efficiently.