Term
| Describe the structure of skeletal muscle. |
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Definition
| A muscle is surrounded by the epimysium membrane within which are muscle fascicles. Each fascicle is surrounded by a perimysium membrane. Each fascicle contains thousands of myofibrils, each surrounded by an endomysium membrane, and they contain multiple nuclei and mitochondria. Inbetween the myofibrils is the sarcoplasm. The sarcoplasm also contains the sarcoplasmic reticulum. Each myofibril contains 1500 myosin (thick) filaments and 3000 actin (thin) filaments interweaved. |
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Term
| What is a sarcomere and describe its structure. |
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Definition
| A sarcomere is the basic unit of a myofibril. It is divided into several bands/zones. At either end is a Z disk, followed by the I band (only thin filaments), followed by the A band (thin & thick), followed by the H zone (A band part with only thick). The middle of the sarcomere is called the M line (only thick). |
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Term
| During striated muscle contraction, what areas of the sarcomere contract? |
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Definition
| The H zone of the A band and the I band. |
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Term
| What keeps the sarcomere structure in tact, ie. anchors the filaments? |
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Definition
| The cytoskeleton holds everything in place. Contains several agents to do so: alpha-actinin anchors actin (thin) filaments to Z disks, while titin (elastic protein) binds myosin (thick) filaments to the Z disks. Dystrophin-glycoprotein complex anchors the thin filaments to the cytoskeleton. |
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Term
| What differentiates a small motor unit from a large? |
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Definition
| Small motor units are nerves that stimulate fewer muscle fibers (greater control) while Large motor units stimulate many muscle fibers (less control, more "brute force") |
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Term
Describe the process of Excitation-Contraction Coupling for Skeletal Muscle. |
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Definition
- Action potential in presynaptic motor axon terminals.
- Influx of Ca2+ into axon terminal
- Acetylcholine released from nerve into synaptic cleft.
- Acetylcholine binds and diffuses into membrane of muscle cell through ACh receptors.
- Sodium/Potassium channels open flooding muscle. Initiates an action potential in muscle membrane.
- Action potential propagates through muscle fiber by Transverse Tubule
- Dyhydropyridine Receptor senses action potential, talks to Ryanodine receptor causing Sarcoplasmic Reticulum to dump calcium contents into muscle.
- Calcium binds to troponin C causing the tropomyosin to shift revailing active sites on actin filaments.
- Myosin heads on the thick filaments bind to revealed active sites and slide along causing contraction.
- Calcium pumped back into SR via calcium pumps, ending contraction.
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Term
| Describe the process for sliding filaments. |
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Definition
- Myosin Hydrolizes ATP and undergoes a conformational change into a high energy state.
- Myosin head binds to actin at the active binding site, energy released, ADP and Pi dissociate from myosin.
- Relaxation of myosin molecule rotates the head, pulling thin filament in sliding motion
- New ATP binds to myosin head causing it to release from the active binding site.
- Repeat
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Term
| What are the three sources of energy for muscle contraction? |
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Definition
- Direct phosphorylation of ADP using creatine phosphate (1 mole/ATP per mole creatine phosphate). Extremely fast and limited.
- Anaerobic Glycolysis through glucose (2-3 moles of ATP per mole glucose). Very fast and limited.
- Oxidative Phosphorylation. 32 moles ATP per mol glucose. Slow but nearly unlimited.
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Term
| What are the different types of Myosin? |
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Definition
- Type 1 (Slow oxidative): Slow rate of ATP hydrolysis. Primarily dependent on oxidative phosphorylation. Blood glucose and free fatty acids are also major energy sources. Muscle glycogen is slowly depleted.
- Type 2B (Fast Glycolytic): Fast ATP hydrolysis. Primarily dependent on glycolysis for ATP needs. Muscle glycogen is rapidly depleted.
- Type 2A (Fast Oxidative): Fast ATP hydrolysis. Depends on oxidative phosphorylation. Infrequent in humans.
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Term
| Describe the structure of Cardiac Muscle |
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Definition
| Cardiac muscle is a synctium of many muscle cells (individual cells working in unison). Striated, similar to skeletal muscle. Has more mitochondria however due to higher energy need, and the need to maintain a high level of oxidative phosphorylation. Rich capillary suplpy (1 capillary per fiber) for oxygen, and a more developed T-tubular system. Less developed Sarcoplasmic Reticulum - contraction requires extracellular Ca2+ |
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Term
| How does the Excitation-Contraction coupling for cardiac muscle differ from skeletal? |
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Definition
- Action potential in skeeltal muscle is caused by fast sodium channel. In Cardiac muscle its caused by both that and slower calcium channels.
- Calciumchannels are opened by voltage difference caused by sodium. Slower to open but remain so longer (0.7 sec)
- The influx of extracellular calcium triggers release of calcium from Sarcoplasmic Reticulum.
- Cardiac ECC is Calcium-Induced-Calcium-Released as opposed to Skeletal Muscle's Sodium-Induced-Calcium-Released
- The combination of sodium/calcium maintains a longer period of depolarization (plateau)
- Strength of cardiac contraction depends on extracellular concentration of Ca2+
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Term
| How is calcium handled during repolarization of cardiac muscle? |
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Definition
- Ca2+ ATPase on Sarcoplasmic Reticulum regulated by phospholamban
- Sodium-Calcium Exchanger
- Ca2+ ATPase on sarcolemmal membrane
- Ca2+ influx into mitochondria
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Term
| What are the two types of smooth muscle and where can they be found? |
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Definition
Multi-unit: airways, iris, ciliary and piloerector muscle Single-unit: visceral muscle |
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Term
| Describe the structure of smooth muscle. |
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Definition
| Composed of much smaller cells that skeletal. Not striated, no myofibrils or Z disks. Actin filaments attached to dense bodies, some to cell membrane, held in place by a scaffold of proteins (cytoskeleton). Some of the dense bodies are attached between cells for intracellular communication. No T-tubules. Composed of more actin and less myosin, no troponin. Regulated by calcium through calmodulin. Prolonged contraction time of 1-3 seconds. Requires less energy for contraction. |
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Term
| How does the Excitiation Contraction Coupling of Smooth Muscle differ from Skeletal? |
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Definition
- Smooth muscle lacks neuromuscular junctions. Instead innervated through varicosities (swollen areas containing vesicles). Autonomic fibers branch on top of muscle sheet forming diffuse junctions that secrete neurotransmitters in proximity to muscle cells.
- No troponin, instead uses calmodulin to regulate contraction.
- Longer contraction time 1-3 seconds (30x longer than skeletal)
- Requires less energy for contraction.
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Term
| What hormones affect smooth muscle contraction? |
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Definition
| Angiotensis II, norepinephrine, epinephrine, vasopressin, endothelin |
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