TY - CHAP M1 - Book, Section TI - Neuromuscular Pathology A1 - Thorne, Leigh B. A1 - Lee, Eric T. A1 - Chahin, Nizar A2 - Reisner, Howard M. PY - 2020 T2 - Pathology: A Modern Case Study, 2e AB - QUICK REVIEWNeuromuscular disorders arise from pathologic abnormalities in muscle (myopathy), the nerve which innervates the muscle (neuropathy), or the junction between the two (neuromuscular junction disorders). The classification of these disorders is very broad encompassing a wide variety of underlying etiologies. The clinical history and physical examination are very important in narrowing the differential diagnosis. Ancillary testing provides an additional layer of granularity to assist in making the diagnosis. This may include a variety of laboratory tests, diagnostic muscle biopsy, electromyography (EMG), or nerve conduction studies. In order to understand muscle pathology, one must understand the organization of skeletal muscle and the histochemical stains performed to help distinguish between different diseases.Organization of Skeletal MuscleMuscle → Fascicles → Fibers → Myofibrils → Myofilaments (actin, myosin)Skeletal muscle is composed of numerous muscle fascicles or bundles separated by connective tissue. Within each fascicle, there are numerous muscle fibers which are also separated by a thin layer of connective tissue. By electron microscopy, one can see the ultrastructural components of the muscle fiber: myofibrils with thick and thin filaments composed of myosin and actin respectively, the contractile proteins of muscle (Figures 20-1 and 20-2).Connective TissueEpimysium: Connective tissue covering the outer surface of the musclePerimysium: Connective tissue between and surrounding muscle fascicles; contains small arteries, veins, and nervesEndomysium: Connective tissue surrounding individual muscle fibers within the fasciclesMuscle FiberElongated multinucleated cellSarcolemma: Plasma membrane surrounding each fiberSarcoplasm: Specialized cytoplasm of a muscle cell containing different organellesNuclei and mitochondria, located just beneath the sarcolemmaMyofibrils: Composed of thick (myosin) and thin filaments (actin) arranged in sarcomeresSarcoplasmic reticulum: Fenestrated membrane system extending between myofibrils responsible for the release and uptake of calcium ions during muscle contraction and relaxationTransverse tubule system: Depolarization occurs through this branched tubular system running transversely along the fiber.Muscle Fiber TypesMuscle fibers are divided into different types based on specific characteristics and can be found in different concentrations in different muscles throughout the body. By light microscopy, muscles form a mosaic or checkerboard pattern of different fiber types.Type 1: Slow-twitch, oxidativeType 2A: Fast-twitch, oxidative-glycolyticType 2B: Fast-twitch, glycolyticType 2C: UndifferentiatedType 1 fibers have the following properties:Loaded with mitochondriaDepend on cellular respiration for ATPFatty acids are the main energy sourceResistant to fatigueRich in myoglobin (red meat)Activated by small diameter, slow conducting motor neuronsSlow twitch fibersMuscles used in activities requiring enduranceType 2 fibers have the following properties:Few mitochondriaRich in glycogenDepend on creatine phosphate and glycolysis for ATP productionLow in myoglobin (white meat)Activated by large diameter fast conducting motor neuronsFast twitch fibersRapid and forceful movement SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accessmedicine.mhmedical.com/content.aspx?aid=1173789409 ER -