occurs with neuromuscular disorders, systemic illnesses, and connective
disorders are the most frequent causes of generalized hypotonia. Neuromuscular
disorders produce generalized hypotonia by decreasing the spring-like
properties of the striated muscle fiber. A decrease in the spring-like
properties of the striated muscle fiber results from a physical or functional
disruption of the contractile elements of the muscle fiber (muscle pathology)
or from abnormal neurological input to the muscle contractile elements
(nervous system pathology).
Nervous system pathology localized
at different levels of the neuroaxis may produce hypotonia. Nervous system
pathology involves structures that either convey information to the alpha
motor neurons, convey information from the alpha motor neurons to the
muscles, or both.
The axons of the alpha motor
neurons become the motor nerve fibers that ultimately synapse with the
striated muscle fibers at the myoneural junction. The term lower motor
neuron (Figure 92.1) will be used to refer to the alpha motor neurons
in the anterior horn of the spinal cord and in cranial nerve motor nuclei,
their axons, and terminal endings (presynaptic region of the myoneural
junction). The terms alpha motor neuron and lower motor neuron are interchangeable.
Figure 92.1.— Schematic representation of the
lower motor neuron: (1): alpha motor neuron; (2): axon; (3): presynaptic
region of the myoneural junction.
term motor unit refers to the lower motor neuron and the muscle fibers
(extrafusal) that it innervates (Figure 92.2).
Figure 92.2.— Schematic representation of the
motor unit: (1): alpha motor neuron; (2): axon; (3): presynaptic region
of the myoneural junction; (4): extrafusal muscle fiber.