Karson Phippen BIOL 2420 Vitalis All or Nothing: A Case Study in Muscle Contraction Due: Oct 20 11:59pm Extra c
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Karson Phippen BIOL 2420 Vitalis All or Nothing: A Case Study in Muscle Contraction Due: Oct 20 11:59pm Extra credit: up to 6 pts 2 pts for each part. Read the PDF for the story lines and the associated word banks and data tables. Since you are given incremental pieces of information, it is most interesting to do each question sequentially. You can complete portions of the entire case study (eg what looks most interesting to you) for partial credit I want to acknowledge that this case study was written by graduate students and their advisor at Texas Tech Univ, and I am grateful that they have made it publically available. If you choose to work this case study, it will help you learn the steps of nervous system stimulation of skeletal muscle and the molecular process of muscle contraction by discovering what happens when one of the steps ‘goes wrong’. Note that it is considerable work for the points, yet ought to be a valuable learning experience. Part I – The Tour Questions Use the word bank (see PDF document) to match the appropriate letter to the definitions/descriptions : 1. __(d) Actin_ Thin contractile protein involved in crossbridge formation, comes in filamentous or globular forms. 2. __(i) Synaptic vesicles _ Store neurotransmitters, and following a Ca2+ driven signal, dump neurotransmitters into the synapse. 3. _(n) Synaptic terminal__ The structure at the end of the axon that contains neurotransmitters and vesicles. 4. __(p) Sarcomere _ The functional unit of the muscle fiber that includes the Aband, Iband, Hzone and the Mline. 5. _(a) Sodium__ The ion responsible for depolarizing the muscle membrane by traveling through the nACh receptor, down its electrochemical gradient. 6. _ (m) Ryanodine receptor __ Located on the sarcoplasmic reticulum and once opened, allows Ca2+ flow from the sarcoplasmic reticulum into the sarcoplasm. 7. _(c) Myosin__ Thick filamentous contractile protein involved in crossbridge formation, has a clublike appearance with a “head.” 8. _(e) Acetylcholine (ACh)__ A neurotransmitter derived from choline; responsible for sending the excitatory signal in the neuromuscular junction.
Karson Phippen 9. _(j) T-tubule__ These invaginations allow depolarization of the muscle membrane to quickly penetrate from the sarcolemma to the myofibril. 10. _(g) Motor end plate__ Large and complex terminal formation by which an axon of a motor neuron establishes synaptic contact with a skeletal muscle fiber, transmitting neural impulses to a muscle. 11. _(o) Sarcolemma__ The plasma membrane of a muscle fiber. 12. _(h) Acetylcholinesterase (AChE)__ The enzyme responsible for stopping the ACh signal. Functions by metabolizing ACh into choline, which is recycled, and acetate. 13. _(b)Nicotinic acetylcholine receptor (nACh)__ Responsible for opening a ligandgated Na+/K+ channel in the muscle membrane when the proper ligand binds to it. 14. _(l) Dihydropyridine receptor __ A Ltype calcium channel in the muscle cell membrane, activated upon depolarization, couple depolarization signal to release of calcium. 15. __(f) Depolarization_ An electrical change which brings the relative charge of the inside of the cell more positive; necessary for transmission of electrical impulses within a cell, or from one cell to another. 16. _(k) Sarcoplasmic reticulum __ Modified endoplasmic reticulum, stores and releases calcium. Exercise Using the sliding filament theory draw and explain the process of sarcomere shortening. Start from the point where calcium would interact with troponin. Make sure to include the roles of actin, myosin, and ATP.
Karson Phippen
Part II – Jeff Slater
Questions 1. What symptoms is Jeff experiencing? Dizzines, increase HR, SOB, muscle cramps, unconciousness. 2. What is the role of AChE in the NMJ? Degrades or clears Ach in the synaptic cleft 3. Which of Jeff’s levels are abnormal? AChE Activity percentage is low 4.
What is the mechanism of action of sarin or a sarinlike chemical? Sarin inhibits/inactivates acytlcholinesterase, by covalently bond to the active site and disables its function. Leading to a increase in of Ach in the NMJ
Karson Phippen 5.
How would exposure to a sarinlike poison affect the amounts of Na+ going into the muscle cell? Explain why. The excess Ach would continuatly stimulate Cholinergenic receptors and leave the ion channels open flooding muscle cell with Na+, continually depolarizing the membrane and send a continual signal.
6.
How would exposure to a sarin or sarinlike chemical affect Ca2+ levels inside the sarcoplasmic reticulum? Why? The contiunal electrical signal would continually stimulate DHSR and therfore the Ryanodine receptor, opening the Calcium channels allowing them to continually bind to the troponin.
7.
How do these altered Ca2+ levels affect the position of the actin and myosin filaments? Why/how? Ca2+ Binds to Troponin, which moves the Tropomyosin and exposes the G sites on the actin, this allows the heads of the myosin to form crossbridges causing power strokes, shortening the sarcomere and leading to contraction or constriction.
8.
What needs to happen to Jeff’s postsynaptic membrane to remedy his paralysis? Physiologically what do we need more of, and where? Muscarrinic Ach receeptors need to be antagonized, and Cholinesterases regenerated inorder to be clear the synapse of the ACh. Part III – Sandy Thompson
Questions 1. 2.
What symptoms is she experiencing? Drooping eyelids, Weakness in arms and Legs, diffuculty speaking and swalloing, Muscle fatigue and weakness What levels from her blood work are abnormal? Antibodies for Ach Receptors are present all other results are WNL (within Normal Limits)
Karson Phippen 3.
How would antibodies against ACh receptors affect the neuromuscular junction? AChR antibodies hinder Ach actions, Bind to Ach receptors and destroys them, prevent binding of ACh, remove ACh from the NMJ
4.
How would antibodies against the ACh receptors affect the influx of Na+ into the cell? Inhibit the depolarization and electrical signaling down the t tublues and limit the opening of the Ryanodine receptors
5.
How would antibodies against the ACh receptors affect the levels of Ca2+ inside the sarcoplasmic reticulum? Limited function of Ryanodine receptors, caused by low Ach and signal conduction, would not release the Ca 2+ from the vesicles, causing it to retain the high storage.
6.
What effect does this have on the actin and myosin filaments? The lack of Ca2+ would mean that the Tropomyosin would remain blocking the G sites and inhibit the Myosin from making bonds with Actin, limiting muscle contractions 7. What disorder does Sandy have? How do we treat/manage this diagnosis? Myathenia Gravis MG treatment and management include, blood tranfusion, muscle stregnhers, steroids to reduced the inflamioty response Thymectomy or removal of the thymus gland 8. Using the below table, compare the muscle contraction problems faced by Jeff and Sandy to that of a normal person. Use normal, increased, decreased to complete the table.
SR Ca2+ release
Cross Frequency of bridge muscle formation contraction
ACh
AChE
ACh Receptors Na+ influx
Healthy person
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Jeff
Increased
Inactive
Over Excited
Increased
Increased
Increased
Increased
Sandy
Decreased
Normal Malfunction/ but Clears Inhibited low ACh
Decreased
Decreased
Decreased Decreased