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The upper motor neuron system is more a functional concept than an anatomical structure. The term refers to all the neurons in the brain (Figure 103.1[1]), cerebellum (Figure 103.1[2]), and brainstem (Figure 103.1[3]) that directly or indirectly convey information to the motor neurons in the brainstem and in the anterior horns of the spinal cord (lower motor neurons). The term upper motor neuron system includes the axons of these neurons. These axons travel through the brain and brainstem to make contact with the motor neurons in the brainstem and through the brain, brainstem, and spinal cord (Figure 103.1[4]) to make contact with the motor neurons in the anterior horn of the spinal cord.
The neurons of the upper motor neuron system are located in the cerebral cortex (Figure 103.1[1]), basal ganglia (Figure 103.1[2]), cerebellar nuclei (Figure 103.1[3]), red nuclei (Figure 103.1[4), reticular formation (Figure 103.1[5]), and the lateral vestibular nucleus (Figure 103.1[6]). Neurons from the cerebral cortex and the brainstem (red nucleus, reticular formation, and lateral vestibular nucleus) connect directly with the lower motor neurons. Neurons from the basal ganglia and the cerebellum also influence the lower motor neurons but they do so indirectly by connecting with cortical or brainstem neurons that make direct contact with the motor neurons.

Figure 103.1. Schematic representation of the upper motor neuron system and the muscle motor-sensory unit. [1]: motor cortex; [2]: basal ganglia; [3]: cerebellum; [4]: red nucleus; [5]: reticular formation; [6]: lateral vestibular nucleus; [7]: axons from extrapyramidal neurons; [8]: intertesial neurons; [9]: alpha motor neuron; [10: gamma motor neuron; [11]: dorsal ganglion cell; (A) brain; (B) cerebellum; (C) brainstem; (D) spinal cord.


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gamma motor neuron dorsal ganglion cell alpha motor neuron intrafusal muscle fiber extrafusal muscle fiber extrafusal muscle fiber spinal cord brainstem lateral vestbular nucleus cerebellum brain reticular formation red nucleus reticular formation motor cortex the basal ganblia Pause pointer over abbrebiations to show labels. Figure must be centered.