Excitation Contraction Coupling Claudia Stanescu Ph D Office Hours in Gittings 108 Tue 10 11am and Thurs 1 2pm or by appointment Objectives 1 Diagram and label the key components of the neuromuscular junction 2 List in order the events that occur at the neuromuscular junction leading to the electrical stimulation of a skeletal muscle fiber 3 Describe the role of AChE in the cessation of the electrical stimulation of a muscle fiber Objectives 4 Discuss the role of calcium in the regulation of contraction and relaxation of skeletal muscle Explain how calcium provides the link between the electrical activation of muscle fibers and initiation of the contraction cycle 5 List in order the events of the contraction cycle Be able to link these events with the excitation of the muscle fiber Terminology Excitation electrical signal is transmitted from a motor neuron to the muscle fiber Excitation Contraction Coupling the events that connect excitation to contraction Contraction the events that cause the sarcomere and the muscle fiber to shorten Relaxation the events that cause the sarcomere and the muscle fiber to return to resting length Sequence of Events cell in nervous system 1 Electrical signal transmitted from a motor neuron to a skeletalcellmuscle fiber Excitation in muscle system 2 Triggers release of Ca2 from SR EC Coupling sarcoplasm reticulum 3 Ca2 binds to troponin on the thin filament thereby permitting crossbridges to form Text crossbridge cycling results in tension development Contraction 4 Removal of Ca2 resequestration into SR results in Relaxation The Electrical Signal Excitation between nervous system and muscular system Neuromuscular Transmission 1 Activation of skeletal muscle is voluntary requires a signal from the central nervous system CNS this signal results in an electrical impulse along a motor neuron that arrives at the Neuromuscular Junction spinal cord injuries cannot control legs because signals come from brain and cannot exit to muscles muscles REQUIRE electrical signal from brains electrical signal action potential where they meet Ch 12 don t have to know all details motorneuron comes from specific part of brain travels to spinal cord synapses connects to another neuron transfers to hand motorneuron Structural Organization of Skeletal Muscle forms connection voltage data Ca channels open allows Ca to enter motor neuron causes them to undergo exocytosis acetylcholine specific neuron nerve impulse change in voltage generates action potential initiated by sodium ACh doesn t enter cell Neuromuscular Junction Excitation Steps 1 Action potential arrives at the synaptic end bulb of motor neuron and causes opening of voltage gated Ca2 channels 2 Synaptic vesicles containing the neurotransmitter Acetylcholine ACh undergo exocytosis 3 ACh is released into the synaptic cleft and binds to ACh receptors on the sarcolemma motor end plate 4 ACh receptors open and allow Na to enter the muscle fiber generating an action potential on the sarcolemma 5 ACh is quickly broken down to Acetate and Choline by Acetylcholine Esterase AChE Neuromuscular junction animation Neuromuscular Junction captioned http www youtube com watch v JcUwsF pxrbU Release of Ca2 from the SR Excitation Contraction Coupling 1 AP runs along sarcolemma continues into T tubules 2 Triggers release of Ca2 from SR 3 Ca2 diffuses into sarcoplasm and myofibrils 4 Ca2 binds to troponin on thin filament myosin binding site on actin gets exposed 5 Crossbridges form tension is generated starts contraction Excitation contraction coupling animation Excitation Contraction Coupling captioned http www youtube com watch v ant19jZvnI Relaxation Sequestration of Ca2 1 When APs stop arriving at the NMJ the trigger to release Ca2 from the SR stops 2 Active Ca2 transporters in the SR membrane pump Ca2 back into the SR 3 Cytoplasmic Ca2 decreases 4 As Ca2 cyto falls Ca2 comes off troponin the myosin binding sites on actin get covered by tropomyosin 5 Crossbridge cycling stops and tension drops 6 Titin brings the sarcomere back to resting position ATP transports Ca across gradient KNOW THIS Review events http www blackwellpublishing com matthews myosin html