WINDSOR UNIVERSITY SCHOOL OF MEDICINE

1. WINDSOR UNIVERSITYSCHOOL OF MEDICINE Skeletal Muscle Physiology-Excitation-contraction coupling Dr.VishalSurender.MD.

3. Excitation-contraction coupling • It is the relationship between stimulation of the motor-neuron that innervates the muscle fibers and the production of a contraction. Neuromuscular Junction • Skeletal muscle cells contract as a result of impulses from • motor neurons. • The place where a motor neuron stimulates a muscle cell • is called a neuromuscular junction.

4. Skeletal Muscle Innervation

5. Motor neuron axon Synaptic Cleft Synaptic Vesicles containing acetylcholine Sarcolemma AChE Axon terminal Postjunctional folds With nicotinic receptors. T -Tubule Terminal Cisternae & Sarcoplasmic Reticulum Motor End Plate Sarcomere

6. - - + + + - - + - + - + - + + + - + - + - + Neuromuscular Transmission: Step by Step Depolarization of terminal opens Ca channels Nerve action potential invades axon terminal

7. Binding of ACh opens channel pore that is permeable to Na+ and K+. ACh binds to its receptor on the postsynaptic membrane ACh is released and diffuses across synaptic cleft. ACh ACh Ca2+ ACh Ca2+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ ACh Na+ Na+ Na+ Na+ Na+ Na+ Na+ Ca2+ induces fusion of vesicles with nerve terminal membrane. Nerve terminal K+ K+ K+ K+ K+ Outside Muscle membrane Inside K+ K+ K+ K+ K+ K+ K+ K+

8. Neuromuscular Junction Ca2+ Depolarization Depolarizing muscle cell membrane Na+ Ca2+  Ca2+ Na+ Na+

9. Depolarisation T-tubule Ca2+ SR Ca2+ SR Muscle Contraction

10. Sarcoplasmic Reticulum Myoplasm Myoplasm Ca2+ Ca2+ Ca2+ T tubule Voltage-dependent Ca channels Ca2+ Ca2+ Ca2+ Elevation of Intracellular Calcium Ry-R(Ryanodine- Receptor)Ca release channel Sarcoplasmic Reticulum

11. EPP Electrical Events at the Neuromuscular Junction -> End Plate Potential (EPP) VNa The movement of Na+ and K+ depolarizes muscle membrane potential (EPP) 0 Muscle Membrane Voltage (mV) Threshold Presynaptic terminal -90 mV VK Time (msec) Presynaptic AP Outside Muscle membrane Inside Voltage-gated Na Channels ACh Receptor Channels Inward Rectifier K Channels

12. ACh Choline ACh ACh Choline ACh Acetate ACh Meanwhile ... ACh is hydrolyzed by AChE into Choline and acetate Choline resynthesized into ACh and repackaged into vesicle Choline is taken up into nerve terminal ACh unbinds from its receptor so the channel closes Nerve terminal Outside Muscle membrane Inside

13. Lets tie all this together • Information leaves motor areas of CNS • Travel down spinal cord to muscle via motorneurone • Depolarisation of axon terminal leads to ACh release • Leads to muscle depolarisation which passes down the t tubule

14. Release of Ca2+ from the SR • Ca2+ binds to troponin • Crossbridge binds to actin • Contraction • SR removes Ca2+ • Inhibition of binding • Relaxation.

15. Somatic motor neuron releases ACh at neuro- muscular junction. Net entry of Na+ through ACh receptor-channel initiates a muscle action potential. 1 2 (a) 1 Axon terminal of somatic motor neuron ACh Muscle fiber potential Action K+ Action potential 2 Na+ Motor end plate Sarcoplasmic reticulum T-tubule Ca2+ DHP receptor Tropomyosin Z disk Troponin Actin M line Myosin head Myosin thick filament Excitation-Contraction Coupling

16. Action potential in t-tubule alters conformation of DHP receptor. DHP receptor opens Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm. 3 4 5 Ca2+ binds to troponin, allowing strong actin- myosin binding. (b) 4 3 Ca2+ Ca2+ released 5 7 6 Myosin thick filament M line Distance actin moves Actin filament slides toward center of sarcomere. Myosin heads execute power stroke. 6 7 Excitation-Contraction Coupling

17. Neuromuscular Transmission • Properties of neuromuscular junction • 1:1 transmission: A chemical transmission which is designed to assure that every presynaptic action potential results in a postsynaptic one • Neuromuscular Transmission Can Be Altered by Toxins and Drugs. • The NMJ is a site of considerable clinical importance

18. Clinical Chemistry Related compounds are useful in the neuroscience research Suberyldicholine is a synthetic neuromuscular agonist. Ach is the natural agonist at the neuromuscular junction. Carbachol and related compounds are used clinically for GI disorders, glaucoma, salivary gland malfunction, etc. Tubocurarine competes with ACh for binding to receptor- but does not open the pore. Tubocurarine and other, related compounds are used to paralyze muscles during surgery. So tubocurarine is a neuromuscular blocking agent. Tubocurarine is the primary paralytic ingredient in curare. Carbachol is a synthetic agonist not hydrolyzed by acetylcholinesterase.

19. Anticholinesterase Agents • Anticholinesterase (anti-ChE) agents inhibit acetylcholinesterase 1. Normal: AChCholine + Acetate AChE 2. With anti - AchE: AChCholine + Acetate anti - AChE • prolong excitation at the NMJ.

20. Anti-ChEagents • Clinical applications (Neostigmine, Physostigmine)-Myasthenia gravis. • Insecticides (organophosphate ) • Nerve gas (e.g. Sarin)

21. NMJ Diseases • Myasthenia Gravis • Autoimmunity to ACh receptor • Fewer functional ACh receptors • Decreased NM transmission • Lambert-Eaton syndrome • Autoimmunity directed against Ca2＋channels • Reduced ACh release • Decreased NM transmission