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SC BIOL 460 - Excitation of a Skeletal Muscle Cell

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BIOL 460 1st Edition Lecture 15Outline of Last Lecture I. Skeletal Muscle a. Myofibersi. A-bandsii. I-bandsb. Mechanism of muscle contractionc. Cross-bridge cycling Outline of Current Lecture I. Excitation of Muscle CellsII. Length/Tension RelationshipIII. Motor Unit Current LectureExcitation of muscle cells1. Caused by somatic motor neurons2. Synapse – neuromuscular junction3. Has large terminal bouton4. 1 neuromuscular junction per muscle fiber5. sarcolemma of muscle fiber at neuromuscular junction is called motor end plate6. trigger for contraction – calcium, which bonds to troponin7. need 10^-6 M of Ca2+ for contraction8. relaxation occurs when there isn’t a high enough Ca2+ concentration9. Ca2+ comes from sarcoplasmic reticulum10. Relaxation established by calcium pumps11. SR wraps around every myofibril in each cellThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.12. Most of Ca2+ found in terminal cisternae13. T-tubule (transverse) runs from one side of muscle fiber to the othera. Formed from invagination of sarcolemmab. Go around myofibrilsc. Have lumen that is continuous with extracellular environment14. Receptors on motor end plate are cholinergic – nicotinica. Open ion channels and generate EPSPb. EPSP on motor end plate – end plate potentialc. Edges of motor end are analogous to axon hillockd. Opens K+/Na+ VGC if at thresholde. Causes action potential, conducted across sarcolemmaf. VGC extend into PM lining t-tubulesg. Propagates AP deep into the muscle fiber15. Getting Ca2+ into sarcoplasma. Minor way – Ca2+ VGC in sarcolemma (especially in t-tubules)b. Opens as a result of a conformation change in Ca2+ VGCc. Most Ca2+ comes from SRi. Exits via release channels (2 types)ii. Mechanically coupled Ca2+ release channels1. Electromechanical release mechanism2. Mechanically coupled with Ca2+ VGC of sarcolemma3. Conformation change of Ca2+ VGC is mechanically coupled to RC –the physical changes causes a change in the release channel4. Most Ca2+ in skeletal muscleiii. Ligand gated Ca2+ release channels1. Ca2+ in sarcoplasm released by electromechanical release mechanism2. It bonds to ligand gated Ca2+ release channels3. Ca2+ induces release of more Ca2+iv. As long as somatic motor neuron is releasing ACh, muscle contractions will continuev. Without AP, Ca2+ pumps have advantage and pump calcium into SR, drops molarity to below 10-6, relaxing the musclevi. ATP necessary for contraction (cross bridge cycling) and relaxation (Ca2+ pumps) – lack of ATP causes constant contraction (Rigor Mortis)Length/Tension relationship1. Optimum length generating most tension2. Too short, no sliding possible, no contraction3. No cross bridge overlap, nothing to grab, no contraction4. Optimal length – thin barely overlaps, cross bridges grab on, greatest amount of sliding (normal length in vivo)Motor Unit1. Skeletal muscles only have fibers organized into motor units2. 1 somatic motor neuron + all of the muscle fibers that it innervates3. axons of somatic motor neurons have collateral branches4. all muscle fibers of a motor unit contract in unison in an all-or-none fashion – as forcefully as possible or not at all5. skeletal muscles work via a graded contraction6. this is done by recruitmenta. Strength of graded contraction based on how many motor units are activatedb. Innervation ratio – number of fibers that make up a motor uniti. Smallest 1:23 (extrinsic ocular muscles, great precision)ii. Largest 1:2000 – coarser movements, large leg musclesc. Motor units are not all the same sizei. Calf mscule/gastroenemius muscle – average IR is 1:1000, some are muchbigger or smaller (1:2000, 1:200)ii. Cell body of somatic motor neuron that supplies motor unit 1:2000 innervation ratio is much bigger that that of one that supplies motor unit of 1:200iii. Less decrement of EPSP via cable properties, so much more likely that thesmaller motor unit will respond at a lower threshold


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SC BIOL 460 - Excitation of a Skeletal Muscle Cell

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