In motor neuron diseases, there is a prolonged period of time before any clinical symptoms begin to appear. During this time, distal axonal degeneration, or “dying back” axonopathy, begins to occur before the onset of clinical symptoms and motor neuron death. This preclinical degeneration is a hallmark of motor neuron diseases in both animal models and human patients. Generally, in muscles with mixed fiber types, distal degeneration occurs in fast-fatigable α–motor axons innervating type IIb muscle fibers before axons innervating slow, type I muscle fibers. We investigated whether the “dying back” axonopathy in a pure fast-fatigable α–motor axon nerve is a length dependent process. The lateral thoracic nerve (LTN) exclusively consists of motor nerves that innervate the very thin cutaneous maximus muscle (CMM) that solely contains type II neuromuscular synapses. We characterized the LTN and CMM synapses both morphologically and physiologically in the superoxide dismutase 1 (SOD1) mutant mouse model of amyotrophic lateral sclerosis (ALS). By 60 days of age, there was a significant “dying back” phenomenon at the caudal region while the rostral region remained intact. The longer axons innervating the caudal region appear to be more susceptible to degeneration in the SOD1 mouse indicating that the axonal degeneration of motor neurons innervating type II fibers is a length dependent process. Additionally, we identified how the simplicity of the LTN-CMM system offers a better method to investigate axon degeneration in an ALS mouse model and may be used to investigate possible therapeutic compounds for axon protection and regeneration.

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