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
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
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.