Clicker questions • Clicker channel: 70 • Response Ware Session ID: psio201What is this? A. A myofibril B. A myofilament C. A sarcomere D. The sarcoplasmic reticulum (SR) A. B. C. D.79%1%16%4%Name this Ca2+-sensitive protein A. Actin B. Myosin C. Tropomyosin D. Troponin A. B. C. D.1%87%10%2%What is this area of the sarcomere called? A. A band B. H zone C. M line D. Zone of overlap A. B. C. D.7%91%1%2%Lecture 19: Excitation-Contraction Coupling and Muscle Relaxation Office hours: Tuesday and Thursday 1-2pm in Gittings 108 or by appointment Claudia Stanescu, Ph.D.Objectives (pp. 304-308, sec. 10.3): 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 acetylcholine esterase (AChE) in the cessation of the electrical stimulation of a muscle fiber 4. Discuss the role of calcium in the regulation of contraction and relaxation of skeletal muscle. 5. List in order the events of muscle contraction from excitation, to excitation contraction coupling, to contraction and lastly, relaxation.How is cross bridge cycling controlled?Sequence of Events 1. Electrical signal transmitted from a motor neuron to a skeletal muscle fiber (Excitation) 2. Triggers release of Ca2+ from SR (Excitation Contraction Coupling) 3. Ca2+ binds to troponin on the thin filament, thereby permitting crossbridges to form crossbridge cycling results in tension development (Contraction) 4. Removal of Ca2+ (resequestration into SR) results in RelaxationThe ‘Electrical Signal’ - Excitation 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 the junction between the motor neuron and a muscle fiberStructural Organization of Skeletal MuscleNeuromuscular JunctionNeuromuscular 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=JcUwsFpxrbURelease 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=a-nt19jZvnI SR T-tubuleRelaxation - Sequestration of Ca2+ 1. When Action Potentials 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 positionSummary of StepsSuggested Learning Activities • Draw the neuromuscular junction including the following: synaptic end bulb, synaptic cleft, motor end plate, synaptic vesicles, ACh receptors, calcium channels. • Watch the two animations provided in this lecture with your notes in front of you. Pause the videos and review each step described. • List the steps of excitation, excitation contraction coupling, contraction and relaxation in