Lower Motor Neuron Disease

Chapter 101 Lower Motor Neuron Disease


The lower motor neuron (LMN) is comprised of the cell body (found in the brainstem or spinal cord) and the axon that contributes to either the spinal or cranial nerves. The LMN terminates on skeletal muscle fibers at the neuromuscular junction (NMJ). The LMN, NMJ, and skeletal muscle fibers together make up the motor unit.1

The upper motor neuron (UMN) system originates in the cerebral cortex and brainstem, with axons contributing to the corticospinal, rubrospinal, reticulospinal, or vestibulospinal tracts. The UMN is confined to the central nervous system and terminates on the LMN. Strictly defined, the UMN is a premotor neuron because it results only in movement and muscle tone through its actions on the LMN.2

The paretic or paralyzed patient may have disease of the brain, spinal cord, nerve, NMJ, or skeletal muscle. The neurologic examination and its interpretation are the first and most important steps in localizing the lesion. The first question asked is whether the clinical signs result from dysfunction of the UMN system or of the motor unit. Disorders of the UMN system typically result in spasticity and exaggerated segmental reflexes. Disorders of the motor unit result in flaccidity, diminished segmental reflexes, and rapidly progressing muscle atrophy (over 1 to 2 weeks). This chapter will describe how to recognize diseases of the motor nerve (neuropathy), NMJ (junctionopathy), and muscle (myopathy), how to confirm an anatomic diagnosis, and how to treat specific disorders of the motor unit.


Disease of the motor unit results in flaccid paresis or paralysis. A short-strided gait is apparent when appendicular muscles are involved. Postural reactions and segmental reflexes frequently are diminished. Muscle atrophy is common, although involvement of specific muscle groups may result in specific dysfunction including incontinence, dysphagia, dysphonia, regurgitation, and abnormal facial expressions. In order to generate a list of appropriate differential diagnoses, the location of the lesion (nerve, NMJ, muscle) must first be determined using information from the clinical examination, clinicopathologic tests, electrodiagnostic testing, and/or biopsy.


Cranial Mononeuropathies

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Lower Motor Neuron Disease

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