Skeletal Muscle Histology PDF

Skeletal muscle histology Prepared by Dr. Ahmed A. Morsi Assist Prof of Histology & Cell Biology Learning objectives By the end of this lecture, you will be able to:- 1.Enumerate 3 types of muscle cells 2.Describe the neuromuscular junction 3.Recognize the structure of s...

Skeletal muscle histology Prepared by Dr. Ahmed A. Morsi Assist Prof of Histology & Cell Biology Learning objectives By the end of this lecture, you will be able to:- 1.Enumerate 3 types of muscle cells 2.Describe the neuromuscular junction 3.Recognize the structure of skeletal muscle 4.Define triad system & its role 5.Know the ultrastructure of myofibrils & contractile proteins 6.Explain the sequence of nerve stimulation of a skeletal muscle  Innervation of skeletal muscle Skeletal muscle is supplied by motor nerve (voluntary). Within the muscle ,The motor nerve divides into several nerve endings, each one dilates to form a terminal swelling which synapses with an individual muscle fiber forming (neuromuscular junction/ motor end plate).  Structure of neuromuscular junction (MEP) Presynaptic side (nerve terminal): contains mitochondria, numerous synaptic vesicles containing (acetycholine) Synaptic cleft: space Postsynaptic side (sarcolemma of muscle fiber): are thrown into deep junctional folds (rich in acetylcholine receptors so, increase the surface area) Structure of skeletal muscle  Histological structure  Transverse section (TS): Skeletal muscle is composed of muscle fibers, held together by CT that is organized as follow:-  Epimysium: dense CT the surround the whole muscle.  Perimysium: less dense CT that surround groups of muscle fibers forming bundles or fascicle.  Endomysium: delicate highly vascular CT that surround the individual muscle fiber 7 Longitudinal section (LS):  The skeletal muscle fibers are long, run parallel without branching and show clear transverse striations. Nuclei: Multiple flat peripherally located. The muscle fibers are separated by connective tissue. Skeletal Muscle Cells= Fiber  Cytoplasmic organelles & inclusions of skeletal muscle cell Myofibrils: contractile units Sarcoplasmic reticulum: well developed smooth endoplasmic reticulum highly organized as tubular network around myofibrils. Mitochondria: numerous Myoglobin: O2 binding pigmented protein Glycogen & lipid granules: Triad Tubular system (T- system):  Triad Tubular system (T- system): They are long finger like invaginations of the sarcolemma into sarcoplasm, at intervals, forming transverse (T) tubules to encircle every myofibril. Each T tubule is flanked, in both sides by wide terminal cisternae of the sarcoplasmic reticulum (SR). The 3 components, the T tubule in the center and the 2 terminal cisternae of SR are called the triad system of the skeletal muscle  Role of Traid system T tubule → spreading of the nerve impulse to the interior of the muscle fiber so, all myofibrils are stimulated at the same time → allow uniform contraction. Sarcoplasmic reticulum: Ca pump, releasing Ca into sarcoplasm during depolarization (contraction) and its squestration (return back) during relaxation  Myofibrils They are parallel contractile units of skeletal muscle formed of contractile proteins (myofilaments) Contractile proteins: 1.Thin filaments: actin 2.Thick filaments: myosin 3.Tropomyosin: associated with actin filament 4.Troponin: associated with actin filament  EM of myofibril (Arrangement of the contractile proteins ) Each myofibril show alternating dark and light bands which are arranged in the same level in adjacent myofibrils → transverse striations. The dark band (A band) shows central pale region called H zone The light band ( I band) is bisected by Z disc  The dark (A) band → actin and myosin filaments The light (I ) band → actin filament only The H zone → thick filaments only Z line is a dense membrane bisect the I band Sarcomere is a portion of a myofibril between 2 successive Z line. It is considered as the functional unit of contraction. Skeletal muscle contraction Skeletal muscle contraction Clinical application: Myasthenia gravis Definition: Autoimmune disease characterized by fluctuating weakness of striated muscles that worsens with repeated activity. Pathophysiology: of weakness Autoimmune dysfunction production of autoantibodies directed against acetycholine receptors (Anti-AchR) decrease in the Ach receptors at the post synaptic membrane impairment of the neuromuscular transmission Changes that occur in the bands during contraction  Changes that occur in the bands during contraction The I band → decrease in size (narrow) The H zone → shorten (disappear) The A band → remain constant Sarcomere → shorten The length of actin and myosin filaments → not changed RECOMMENDED REFERENCES, WEBSITES