The upper motor neuron system can be simplified as having three neurons: an upper motor neuron and two upper motor neuron helpers (Figure 96.1). The upper motor neuron is housed in the cerebral cortex and makes direct contact with the alpha motor neuron (pyramidal tract). The two upper motor neuron helpers differ in their site of origin and type of connections (extrapyramidal component). One upper motor neuron helper (#1) makes direct contact with the alpha motor neuron but it differs from the upper motor neuron because it is housed in the brainstem rather than in the cortex. The other upper motor neuron helper (#2) does not make contact with the lower motor neuron. This second upper motor neuron helper is in the cerebellum and makes contact with the upper motor neuron through a neuron in the brainstem.

Figure 96.1.— Schematic representation of the neurons of the upper motor neuron system.

For the purpose of remembering the possible sites of pathology in neonates with generalized hypotonia it is not necessary to account for the upper motor neuron helper #1. However, it is important to understand the relation between the upper motor neuron and the upper motor neuron helper #2, and the regions of the central nervous system they occupy (Figure 96.2).

Figure 96.2.— Schematic representation of the relation between relevant regions of the central nervous system and the upper motor neuron system in generalized hypotonia. The axons of the upper motor neurons cross to the opposite side; those for the cranial nerve muscles cross at the upper levels of the brainstem whereas those for the spinal nerve muscle cross at the lower medulla. The neurons and axons of the upper motor neuron system are represented by the red area and lines. Also represented in this figure is the upper motor neuron helper #2 illustrated as dark pink squares in the cerebellum.