BIOL 460 1st Edition Lecture 17 Outline of Last Lecture I Skeletal Muscle a Myofibers i A bands ii I bands b Mechanism of muscle contraction c Cross bridge cycling Outline of Current Lecture I Excitation of Muscle Cells II Length Tension Relationship III Motor Unit Current Lecture Excitation of muscle cells 1 Caused by somatic motor neurons 2 Synapse neuromuscular junction 3 Has large terminal bouton 4 1 neuromuscular junction per muscle fiber 5 sarcolemma of muscle fiber at neuromuscular junction is called motor end plate 6 trigger for contraction calcium which bonds to troponin 7 need 10 6 M of Ca2 for contraction 8 relaxation occurs when there isn t a high enough Ca2 concentration 9 Ca2 comes from sarcoplasmic reticulum 10 Relaxation established by calcium pumps 11 SR wraps around every myofibril in each cell These 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 cisternae 13 T tubule transverse runs from one side of muscle fiber to the other a Formed from invagination of sarcolemma b Go around myofibrils c Have lumen that is continuous with extracellular environment 14 Receptors on motor end plate are cholinergic nicotinic a Open ion channels and generate EPSP b EPSP on motor end plate end plate potential c Edges of motor end are analogous to axon hillock d Opens K Na VGC if at threshold e Causes action potential conducted across sarcolemma f VGC extend into PM lining t tubules g Propagates AP deep into the muscle fiber 15 Getting Ca2 into sarcoplasm a Minor way Ca2 VGC in sarcolemma especially in t tubules b Opens as a result of a conformation change in Ca2 VGC c Most Ca2 comes from SR i Exits via release channels 2 types ii Mechanically coupled Ca2 release channels 1 Electromechanical release mechanism 2 Mechanically coupled with Ca2 VGC of sarcolemma 3 Conformation change of Ca2 VGC is mechanically coupled to RC the physical changes causes a change in the release channel 4 Most Ca2 in skeletal muscle iii Ligand gated Ca2 release channels 1 Ca2 in sarcoplasm released by electromechanical release mechanism 2 It bonds to ligand gated Ca2 release channels 3 Ca2 induces release of more Ca2 iv As long as somatic motor neuron is releasing ACh muscle contractions will continue v Without AP Ca2 pumps have advantage and pump calcium into SR drops molarity to below 10 6 relaxing the muscle vi ATP necessary for contraction cross bridge cycling and relaxation Ca2 pumps lack of ATP causes constant contraction Rigor Mortis Length Tension relationship 1 2 3 4 Optimum length generating most tension Too short no sliding possible no contraction No cross bridge overlap nothing to grab no contraction Optimal length thin barely overlaps cross bridges grab on greatest amount of sliding normal length in vivo Motor Unit 1 2 3 4 Skeletal muscles only have fibers organized into motor units 1 somatic motor neuron all of the muscle fibers that it innervates axons of somatic motor neurons have collateral branches all muscle fibers of a motor unit contract in unison in an all or none fashion as forcefully as possible or not at all 5 skeletal muscles work via a graded contraction 6 this is done by recruitment a Strength of graded contraction based on how many motor units are activated b Innervation ratio number of fibers that make up a motor unit i Smallest 1 23 extrinsic ocular muscles great precision ii Largest 1 2000 coarser movements large leg muscles c Motor units are not all the same size i Calf mscule gastroenemius muscle average IR is 1 1000 some are much bigger 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 1 200 iii Less decrement of EPSP via cable properties so much more likely that the smaller motor unit will respond at a lower threshold stimulus
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