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The “motor unit”consists of a lower motor neuron(anterior horn cell or neuron in cranial nerve nuclei), its axon and the muscle fibers innervated by it. The number of muscle fibers innervated varies from a few fibers(oculo-motor muscles) to several hundreds(extremity muscles). Muscles fibers of one motor unit are scattered in a wide area in a random fashion(“checkerdboard”).
Diseases can be classified as involving: A. Motor neuron B. Peripheral nerves C.Neuro muscular junction D. Muscles
A. DISEASES OF MOTOR NEURON. -Etiology of most of the motor neuron dis.(AML, progressive muscular atrophy) is not known, but may be caused by several agents: adriamycin, vincristine, aluminium perikaryon(neuronal cell body) primarily affected w/loss of microtubules and nuclear displacement + increased cytoplasmic neurofilaments or tangled masses of neurofila- ments. Viral infections(Polyomyelitis, Herpes encephalitis, Varicella-zoster) may also affect the motor neuron or sensory ganglia.
B. DISEASES OF PERIPHERAL NERVES I. Axonal degeneration(axonal neuropathy) a. Wallerian degeneration(crush/cut along a myelinated fiber) b. Axonal degeneration caused by other diseases (less axoplasm leakage+ more inflammation + chronic evolution): 1.Proximal axonal degeneration: intoxic. w/ IDPN(BB-Iminodipropionitrite) shrinkage of distal axons due to focal proximal blockage. 2.Distal axonal degeneration: earliest changes occur in the most distal portion of axons
DISEASES OF PERIPHERAL...(cont.) Many diseases w/earlier changes in the most distal portion of axons slow spread to proximal structures perikaryon cannot support the terminal axon, vgr.: --Hereditary neuropathies. -HSMN I (Charcot-Marie-Tooth)disease, the MOST common of these, inherited as autosomal- dominant is usually present in childhood/early adulthood(PMP 22 gene/17p11.2-p12 locus), characterized by progressive atrophy of leg mus- cles,foot drop/deformed feet w/less sensory defect
DISEASES OF PERIPHERAL...(cont.) --Hereditary neuropathies... -HSMN II(CMT2A) also AD, with similar manifestations than CMT I but without nerve enlargement and presentation at a la- ter age. Linked to chromosome 1p35-p36. -HSMN III(Dejerine-Sottas disease) is an AR condition that begins slowly in early childhood w/delayed acquisition of motor skills and involvement of muscles of limbs and trunk enlar gement of nerves easy to detect. Genetic heteroge nicity(PMP 22, MPZ, PRX and EGR2).
DISEASES OF PERIPHERAL...(cont.) --Acquired metabolic/toxic neuropathies. -Diabetic peripheral neuropathy: w/symme tric neuropathy involving distal sensory/mo tor nerves + dysfunction of autonomic nervous system(20-40% of cases).It can also present as a single peripheral or cranial neuroneuropathy(oculomotor nerve). -Metabolic/nutritional: chronic liver disease,resp. insuff., renal failure, thiamine def.,Vit.B12,B6,E. Chronic alcoholism, etc.