Neuromuscular Junction

Neuromuscular Junction

1. Terminal Axon

   • Motor End Plate of Muscle Fiber
     • 4) Receptors open up ligand-gated cation channels
       • 5) Depolarization of sarcolemma membrane via t-tubules
         • Sodium ions enter muscle fiber
         • Potassium ions leave muscle fiber
         • goes from -70 mv--> +40 mv
           • 6) Action potential fired along length of the muscle fiber
             • travels length of the myotube
     • Muscle fiber/myotube
       • Sarc. Reticulum
         • Storage of calcium ions
           • Resting muscle = HIGH [Calcium]
           • ATP-fueled CA2+ pumps/channels
       • Sarcoplasm (membrane)
       • Myofibril
         • Made of 2 myofilaments
           • Cross Bridge Interaction
             • Actin
               • Has myosin binding sites
               • Thick strand
             • Myosin
               • Double Headed, Thin strand
               • covered by Tropomyosin
                 • Calcium binding sites
               • Myosin heads activated via ATP
                 • ATP from Creatine Phosphate
                   • most abundant source of ATP for skeletal muscles
                   • Enzyme Creatine Kinase
                     • Break phosphates into ATP
                     • Build ATP back up together
                 • Break it into ADP and P
                   • Muscle chance to relax until ATP formed again
     • t-tub
       • 7) take action potential deep into the individual muscle cells
         • 8) Causes sarcoplasmic Reticulum to release calcium into sarcoplasm
           • DHP receptors cause release of calcium
             • DHP--> voltage-dep calcium channel
           • 9) Calcium rushes off and binds to tropomyosin
             • Conformational Change
               • 10) Tropomyosin moves away
                 • 11) Actin binds to Myosin
                   • Actin pulls itself along= MUSCLE CONTRACTION
                     • Contraction END: when Calcium ions actively transported back to sarcoplasmic reticulum

2. Neuron

   • 1) Action Potential arrives From Neuron
     • 2) CA 2+ ions diffuse into the dendrite
       • go into presynaptic end
         • via open voltage-gated calcium channels
       • Calcium influx
         • synaptic vesicles ready to fire AcH
         • AcH released into the synaptic cleft
           • released via exocytosis
             • 3)Diffuses across and binds to receptors on motor end plate
               • presynaptic AcH binding--> + feedback loop
                 • preps more Ach release
                   • sustain/more muscle contraction
                   • AcH removal
                     • Reuptake
                       • Diffuses back across cleft
                     • Broke down
                       • Acetylcholinesterase
                         • Acetyl Acid
                         • Choline

3. All muscle types use calcium and ATP for contraction