BIOM 121 1nd Edition Lecture 16 Outline of Last Lecture I Structure of a Muscle Fiber Cell II The Skeletal Muscle Fiber III The Myofibril IV Sarcomere V The Nerve Muscle Relationship Outline of Current Lecture I The Neuromuscular Junction II Myasthenia Gravis III Electrically Excitable Cells IV Skeletal Muscle Fiber Stimulation V Rigor Mortis Current Lecture I II The Neuromuscular Junction Continued a Junctional folds i Acetylcholine receptors 50 million ii Folds increase surface area Myasthenia Gravis a Autoimmune disease b Found in women of ages 20 40 c Body produces antibodies and they bind to acetylcholine receptors and blocks acetylcholine from binding to receptors d Can t get efficient muscle contraction e Large portion of receptors are blocked by antibodies f Diagnosis 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 III IV i Have them look up after 60 seconds one eyelid will start to droop and then eyelid will completely close ii Problems with all types of skeletal movement g Treatments i Cholinesterase inhibitors allows acetylcholine to not be broken down acetylcholine may eventually be able to beat of antibodies ii Immunosuppressive agents suppresses production of antibodies iii Thymus removal thymectomy removes ability to make T cells iv Plasmapheresis remove blood take out plasma where antibodies are and put it back tube used often causes infection Electrically Excitable Cells a Muscle fibers and neurons b Electrophysiology study of the electrical activity of cells c Voltage electrical potential d Resting membrane potential RMP electrical difference for un stimulated cell 90 mV i Chloride and sodium concentrations are high outside the cell ii Potassium and other anions are high in concentration inside the cell iii Anion charges are large and cannot move across cell membrane e Stimulation of cell causes channels to open up potassium goes out sodium comes in and membrane potential becomes very positive this process creates action potential f Graphical representation of action potential i Un stimulated cell 90mV ii Depolarization stimulated cell sodium channels open membrane potential 1 Bigger opening in sodium channels causes steeper slope iii Repolarization sodium channels close and potassium channels open removes a lot of positive charge and membrane potential again becomes negative could also be caused by adding negative charge into cell iv Hyperpolarization potassium channels remain open after the potential reaches resting un stimulated level charge temporarily goes below 90mV v Potassium channels are slow to open and slow to close sodium channels open and close very quickly Skeletal Muscle Fiber Stimulation a Four major phases of contraction and relaxation i Excitation 1 Arrival of a nerve signal action potential a Opens voltage gated calcium channels on synaptic knob b Calcium enters synaptic knob c Binds to synaptic vesicles that contain acetylcholine d Vesicles undergo exocytosis into synaptic cleft e Acetylcholine then binds to receptors 2 Acetylcholine Ach release a Receptor binds to acetylcholine 2 molecules to each receptor b Activates receptor meaning it opens up i Ligand regulated ion gate channel ii End plate potential EPP generated local change in membrane potential c Acetylcholine receptors allow sodium in and potassium out which changes membrane potentials 3 End plate potential action potential enough end plate potentials must add up to get an action potential to fire 4 In action potential open sodium and potassium gates ii Excitation contraction coupling 1 Release of calcium from sarcoplasmic reticulum links excitation to contraction 2 Voltage travels down T tubule and calcium released into sarcoplasm and links with troponin 3 Calcium binding to troponin cause tropomyosin to roll away making actin available to bind to myosin iii Contraction 1 Myosin ATPase enzyme in myosin head hydrolyzes an ATP molecule ATP ADP inorganic phosphate can now use this as energy 2 The head is in a cocked position and is activated 3 In order for myosin to bind to actin it must release the ADP and inorganic phosphate 4 Power stroke pulling of the myosin head requires one molecule of ATP 5 As soon as it release actin ATP binds to myosin 6 If no hydrolysis of ATP occurs then there s no contraction iv Relaxation 1 Occurs when ATP binds back to myosin causing myosin to release actin 2 Nerve stimulation stops 3 ACh release stops 4 Acetyl cholinesterase AChE breaks down Ach 5 Need to get rid of calcium calcium is reabsorbed into sarcoplasmic reticulum and rebinds to protein called calsequestrin 6 Tropomyosin re blocks the active site 7 Muscle fiber returns to its resting length V Rigor Mortis hardening of muscles and stiffening of body a Muscle relaxation requires ATP b In rigor mortis when someone dies there is still some ATP activity going on but cells begin to die and deteriorate releasing all cellular content including calcium c ATP hydrolysis and calcium release causes muscles to contract but with no addition of ATP after that there is no relaxation that follows contraction d Rigor mortis last about 48 hours and then the body begins to relax due to deterioration of all proteins