Neurological Disorders of Muscles

Classification of Muscle Disorders

• Myogenic diseases/myopathies: affect skeletal muscle fibers directly (e.g. muscular dystrophy)
• Neuromuscular transmission diseases: affect the neuromuscular junction/synapse
• Neurogenic diseases/neuropathies: affect ventral horn motor neurons that project to skeletal muscles

Subcategories of Neurogenic Diseases

• Motor neuron diseases: affect the cell body of motor neurons
• Peripheral neuropathies: affect the axon and/or myelin sheaths of motor neurons

Sensory Neuropathies

• Affect transmission of proprioceptive signals from muscles, joints, and skin to somatosensory cortices and cerebellum
• Can lead to difficulties with accurately coordinating voluntary limb movements and uncontrollable involuntary limb movements
• Rare and usually due to an unusual autoimmune response

Myasthenia Gravis

• A neuromuscular junction disease resulting in failure of signal transmission
• Causes include:
  • Autoimmune: production of antibodies that attack post-synaptic ACh receptors on skeletal muscle cells
  • Congenital: transfer of ACh receptor antibodies across the placenta
  • Inherited: gene mutations affecting ACh production and/or signalling
• Results in reduced number of ACh receptors or reduced interaction between ACh and receptors, leading to reduced muscle activation

Guillain-Barré Syndrome (GBS)

• An autoimmune disorder with rapid onset of muscle weakness
• Caused by immune system damaging the peripheral nervous system
• Specifically, attacks the myelin sheaths of lower motor neuron axons

Symptoms of Lower Motor Neuron Diseases

• Muscle weakness
• Muscle atrophy (e.g. wasting)
• Decreased tendon reflexes
• Decreased muscle tone
• Fasciculations (i.e. twitching of the muscle)

Symptoms of Upper Motor Neuron Diseases

• Muscle weakness, but little/no atrophy or paralysis
• Increased tendon reflexes
• Increased muscle tone (spasticity)
• Babinski reflex present

Neurogenic vs. Myopathic Diseases

• Neurogenic diseases: affect motor neuron cell body, axon, and/or neuromuscular junction
• Myopathic diseases: affect the muscle directly
• Both have muscle weakness and atrophy as common symptoms
• Important to determine whether disease is neurogenic or myopathic in patients with muscle weakness

Distribution of Muscle Weakness

• Neurogenic: progressive denervation of skeletal muscles starting distally (e.g. hands and feet)
• Myopathic: muscle weakness and atrophy affecting proximal limbs and trunk initially, leading to myotonia (i.e. inability of muscles to relax) and cramps