L13 - Muscular System PDF

L13 - Muscular System Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua LEARNING OUTCOMES  Describe the Microscopic anatomy of the skeletal muscle fiber,  Defind sarcomere  Describe the...

L13 - Muscular System Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua LEARNING OUTCOMES  Describe the Microscopic anatomy of the skeletal muscle fiber,  Defind sarcomere  Describe the Muscle proteins. The Muscular System  Muscles are responsible for all types of body movement – they contract or shorten and are the machine of the body  Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle  Organization of Skeletal Muscle Sarcomere: Fascicle: units of Muscle Muscle myofibrils belly a bundle of Fiber: responsible for muscle fibers muscle cell the striated appearance Myofibrils: Myosin: Myofilament: structures thick protein that make filaments filaments that up a muscle make up a Actin: thin fiber sarcomere filaments Coverings of a Skeletal Muscle  Skeletal muscles are organs They contain muscle fibers, nerves, and blood vessels Connective tissue membranes separate each muscle structure  Fascia – layer of fibrous tissue that separates muscles from each other and from the skin Coverings from largest to smallest  Epimysium – covers the entire skeletal muscle  Perimysium – surrounds a bundle of muscle fibers (fascicle)  Endomysium – surrounds a single muscle fiber (cell) Skeletal Muscle Attachments  Epimysium blends into a connective tissue attachment Tendon – cord-like structure  Sites of muscle attachment Bones Cartilages Connective tissue coverings MUSCLE PROTEINS  Skeletal muscle proteins can be divided into three categories:  Myofibrillar,  Sarcoplasmic, and  Stromal proteins MUSCLE PROTEINS  Myofibrillar Proteins  Contractile, structural, and regulatory proteins make up the myofibrillar proteins.  Myosin and actin are contractile proteins that form thin and thick filaments that regulate skeletal muscle contraction and relaxation.  Troponin and tropomyosin are two regulatory proteins. MUSCLE PROTEINS  SARCOPLASMIC PROTEINS:  Desmin and filamin are essential for connecting myofibrils to the sarcolemma and stabilising muscle structure.  STROMAL PROTEINS:  Collagen makes up 90% of connective tissue, along with other fibrous proteins including elastin, laminin, and fibronectin, as well as proteoglycans.  Fibroblasts, macrophages, lymphoid cells, mast cells, and eosinophils are among the cells found in connective tissue. Microscopic Anatomy of Muscle Fiber (muscle cell)  Cells are multinucleate  Nuclei are just beneath the membrane  Sarcolemma – specialized plasma membrane  Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum involved in muscle contraction  Myofibril Bundles of myofilaments Myofibrils are aligned to give distinct bands  Lightband = “I band”  Dark band = “A band”  Sarcomere Contractileunit of a muscle fiber Organization of the sarcomere  Thick filaments = myosin protein  Thin filaments = actin protein  Myosin and actin overlap somewhat in the sarcomere  Myosin filaments have heads (extensions) that can ‘grab’ onto actin forming a crossbridge Physiology of Muscle Contraction  Skeletal muscles must be stimulated by a nerve (motor neuron) to contract Transmission of Nerve Impulse to Muscle  Step 1: Nerve releases a neurotransmitter (acetylcholine)  Step 2: Neurotransmitter causes the muscle cell membrane gates to open  Step 3: Ions (Na+ & K+) exchange places causing the sarcoplasmic reticulum to release Ca2+  Step 4: This release of Ca+ starts the muscle contraction as the actin filaments slide past the myosin filaments The Sliding Filament Theory of Muscle Contraction  Sliding Filament Model - a muscle contracts when the thin filament in the muscle fiber slides over the thick filament Activated by ATP and calcium (Ca+) ions The Sliding Filament Theory of Muscle Contraction  1) An influx of Ca2+ causes thick myosin filaments to form crossbridges with the thin actin filament by exposing the binding site on actin The Sliding Filament Theory of Muscle Contraction  2) The crossbridges change shape as it pulls on filaments which slides towards the center of the sacromere in the power stroke The distance between the Z line decreases, shortening the muscle.  3) The crossbridges detach from the actin filament when ATP bonds to myosin head. The Sliding Filament Theory  4) The myosin head gets ready to bond to actin again using ATP energy The cycle is repeated on another site of actin filament using the stored ATP energy