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The structures that influence the alpha motor neuron from “above” are in the brain, cerebellum, brainstem, and spinal cord. The structures can be divided in two systems: the pyramidal system and the extrapyramidal system. The term pyramidal system is reserved for neurons housed in the cerebral cortex that make direct contact with the alpha motor neuron and whose axons travel through the pyramids in the medulla (Figure 94.1). The term upper motor neuron is used for the pyramidal system neurons and neurons housed in the cerebral cortex that make direct contact with the alpha motor neurons in the brainstem above the pyramids.



Figure 94.1.
— Schematic representation of the upper motor neuron (represented in blue). Arrows indicate direction of normal signal conduction: (1): neuron in motor cortex; (2): alpha motor neuron.

The term extrapyramidal system refers to all the neurons in the brain that influence the motor-sensory unit without making contact with the alpha motor neuron and neurons in the cerebellum, brainstem, and spinal cord that influence the motor-sensory unit with or without making direct contact with the alpha motor neuron.
The upper motor neurons and the extrapyramidal system are often referred to as the upper motor neuron system (Figure 94.2).

Figure 94.2.Schematic representation of the upper motor neuron system and the motor-sensory unit. Arrows indicate direction of normal signal conduction: (1): motor cortex; (2): basal ganglia; (3): cerebellum; (4): red nucleus; (5): reticular formation; (6): lateral vestibular nucleus.

 

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Brazis, 1990 Carpenter, 1983 By pausing the pointer on abbrevations label will appear. Figure must be centered. intrafusal muscle fiber extrafusal muscle fiber extrafusal muscle fiber gamma motor neuron dorsal ganglion cell alpha motor neuron lateral vestibular nucleus reticular formation red nucleus reticular formation cerebellum motor cortex basal ganglia