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CAUSES OF HYPOTONIA

Generalized hypotonia occurs with neuromuscular disorders, systemic illnesses, and connective tissue abnormalities.

Neuromuscular Disorders
Neuromuscular 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.

The 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.

 

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