Diseases of the motor-sensory unit involving the alpha motor neuron,nerve, myoneural junction, and muscles may produce hypotonia (Figure 130.1). Alpha motor neuron disease refers to disorders that affect the alpha motor neuron located in the anterior horn of the spinal cord, in the motor nucleus of the cranial nerves, or at both locations. Nerve disease refers to disorders that affect the alpha motor neuron axons, the sensory axons that carry information from the muscle spindle, or both. Myoneural junction diseases are disorders that affect the junction between the alpha motor neuron axons and muscle fibers. Muscle disorders are diseases that affect the extrafusal muscle fibers.

Figure 130.1.— Schematic representation of the motor-sensory unit system and the possible sites lesions producing hypotonia. AMN: alpha motor neuron; RC: Renshaw cell; DGC: dorsal ganglion cell; EFMF: extrafusal motor fiber; IFMF: intrafusal motor fiber; GMN: gamma motor neuron. Arrows indicate direction of conduction. (5) alpha motor neuron; (6) nerve; (7) myoneural junction; (8) muscle.

The possible sites of injury in neonates with hypotonia due to diseases of the motor-sensory unit system can be represented in a simplified scheme (Figure 130.2 [6-10]) similar to the one used to explain localization in upper motor neuron lesions.

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Figure 130.2.— Schematic representation of the upper motor neuron system [A] and lower motor unit structures [B] demonstrating possible sites of anatomical involvement that lead to generalized hypotonia. Simplified schematic representation of the lower motor system [C] following same numerical order as [B]. Generalized hypotonia may be due to an upper motor neuron lesion at the (1) brain, (2) brainstem, (3) rostral cervical spinal cord, or (5) cerebellum. Generalized hypotonia may result from damage to the lower motor neurons of the arms and the upper motor neuron fibers of the legs in the lower cervical spine area (4). Generalized hypotonia may be due to lower motor neuron lesions at the: (6) alpha motor neuron; (7) nerve; (8) presynaptic myoneural junction; (9) postsynaptic myoneural junction; and (10) muscle. LMN: lower or alpha motor neuron; MNJ: myoneural junction.