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Phys Exam 1: Ch 6
Contraction of Skeletal Muscle
| Question | Answer |
|---|---|
| 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
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