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Phys Exam 1: Ch 6

Contraction of Skeletal Muscle

Describe the structure of skeletal muscle 1. Connected to bone at each end/side; 2. made of numerous muscle fibers (cells), organized into fascicles; 3. each subdivision is covered w/connective tissue
Describe the composition of a muscle, from smallest fiber to largest myofilaments --> myofibril --> muscle fiber --> fascicle --> muscle
Describe the structure of a muscle fiber Numerous muscle cells are joined end to end, producing a multi-nucleated cell
What is the membrane that covers the muscle cell called? Sarcolemma
What does the membrane that covers the muscle, fascicle, and fiber combine to form? Tendon
Describe myofibrils 1. Cell organelles that almost fill the sarcoplasm of each muscle fiber; 2. each myofibril is a bundle of myofilaments; 3. is subdivided perpendicularly into sarcomeres
What is cytoplasm called in a muscle cell? Sarcoplasm
What is a sarcomere? the unit of contraction; extend from Z-disc to Z-disc; are ~2mm long
What are Z-discs? 1. Made of protein and connective tissue; 2. anchor the myofilaments w/in a sarcomere; anchor the sarcomeres to each other
What is titin? 1. A very large, linear, multi-folded protein; 2. holds the myofilaments in alignment; 3. a spring-like protein attaching the M-line and myosin bundle to the Z-disc
Compare actin vs. myosin as pertains to how they attach to the Z-discs Thin actin filaments are attached to the Z-disc and overlapping the thicker myosin filaments
Describe what happens to the actin & myosin when contraction occurs the actins and myosins increase their overlap --> shortening the sarcomere
Describe the black & white electron micrograph image of contracted striated muscle fibers as shown on the slides Look on the slides
What are the 6 molecular components of contraction myosin, actin, titin, tropomyosin, troponin, Ca ions
Describe myosin 1. the myosin myofilament contains several hundred myosin molecules; 2. composed of 2 chains, mostly wound into a long tail w/2 short flexible heads; 3. each head has a region that binds to the actin molecule, and a region that acts as an ATPase
Describe actin 1. a double-stranded helix (F-actin) made up of G-actin; 2. each G-actin contains an "active site" where it binds w/a myosin head; 3. the base of each actin filament is embedded in the Z-disc; the free end of each actin extends b/n the myosin filaments
Describe tropomyosin a long fiber wrapping around the F-actin, blocking the active sites on the G-actin
Describe troponin a complex of 3 spheres anchoring the tropomyosin to the F-actin
Describe the actin complex 1. An F-actin helix w/active sites on each G-actin; 2. Tropomyosin wraps around the actin helix blocking the active sites; 3. the troponin ("3 spheres") anchor the tropomyosin
Describe a myofilament that is ready for contraction (see also image on slides) Each myosin molecule has a hinge at the base of its cross-bridge and is aligned in a regularly staggared array w/in the bundle of filaments
Describe the initiation of muscle contraction 1. AP reaches the end of an axon; 2. ACh is released at NMJ; 3. ACh initiates an AP in the sarcolemma; 4. AP causes release of Ca from sarcoplasmic reticulum (SR); 5. Intracellular Ca initiates muscle contraction
Describe the contraction mechanism (see also image on slides) Ca binds to troponin; Troponin pivots tropomyosin off actin active sites; Myosin heads bind to exposed active sites; Binding causes myosin head to tilt toward shaft of myosin; ATP is hydrolysed, energy resets myosin head; Process repeats when Ca present
Which specific action of the myosin causes shortening of the sarcomere? The myosin head will move and be reset by an ATP back to its original position, where it will bind w/another actin. Bending & resetting pulls on actin past myosin, shortening the sarcomere.
Describe the process of terminating contraction 1. Ca pump moves Ca back into SR; 2. When troponin is no longer exposed to Ca, tropomyosin moves back over active sites; 3. Myosin heads can no longer bind to actin, and contraction ends
Describe the action of titin during contraction 1. Titin is folded up by contraction; 2. At end of contraction, unfolding of titin pushes Z-discs back apart, reducing the myosin-actin overlap back to starting position
What causes rigor mortis? 1. Death = no more ATP produced (body is limp); 2. SR breaks down, leaks Ca into muscle (body is stiff); 3. Muscle fiber itself starts to break down (body is limp again)
Created by: hclark86