TAILIEUCHUNG - Chapter 023. Weakness and Paralysis (Part 3)

Lower Motor Neuron Weakness This pattern results from disorders of cell bodies of lower motor neurons in the brainstem motor nuclei and the anterior horn of the spinal cord, or from dysfunction of the axons of these neurons as they pass to skeletal muscle (Fig. 232). Weakness is due to a decrease in the number of muscle fibers that can be activated, through a loss of α motor neurons or disruption of their connections to muscle. Loss of γmotor neurons does not cause weakness but decreases tension on the muscle spindles, which decreases muscle tone and attenuates the stretch reflexes. | Chapter 023. Weakness and Paralysis Part 3 Lower Motor Neuron Weakness This pattern results from disorders of cell bodies of lower motor neurons in the brainstem motor nuclei and the anterior horn of the spinal cord or from dysfunction of the axons of these neurons as they pass to skeletal muscle Fig. 232 . Weakness is due to a decrease in the number of muscle fibers that can be activated through a loss of a motor neurons or disruption of their connections to muscle. Loss of ymotor neurons does not cause weakness but decreases tension on the muscle spindles which decreases muscle tone and attenuates the stretch reflexes elicited on examination. An absent stretch reflex suggests involvement of spindle afferent fibers. Figure 23-2 Afferent neuron Muscle spindle intrafusal fibers Alpha and gamma motor neurons Source Faud AS Kaspar DU Breunw ld E Hausar SL Longo DU Jemeson JL LoscalEO J Hnrris cut s Principles of Intern Medicine 17th Edition http i w w w. e cees s me d i tin e. co m Copyright Th McGraw-Hill Companies Inc. All rights rasar Lower motor neurons are divided into a and y types. The larger a motor neurons are more numerous and innervate the extrafusal muscle fibers of the motor unit. Loss of a motor neurons or disruption of their axons produces lower motor neuron weakness. The smaller less numerous y motor neurons innervate the intrafusal muscle fibers of the muscle spindle and contribute to normal tone and stretch reflexes. The a motor neuron receives direct excitatory input from corticomotoneurons and primary muscle spindle afferents. The a and y motor neurons also receive excitatory input from other descending upper motor neuron pathways segmental sensory inputs and interneurons. The a motor neurons receive direct inhibition from Renshaw cell interneurons and other interneurons indirectly inhibit the a and y motor neurons. A tendon reflex requires the function of all illustrated structures. A tap on a tendon stretches muscle spindles which are tonically .

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