1 Postural Control in Multiple Sclerosis: Stability and Complexity Analyses!Neural signals: The neuromuscular junction! University of Massachusetts Amherst Department of Kinesiology Questions ① Brief review of muscle structure ② Nerve stimulus and the neuromuscular junction © 2013 Pearson Education, Inc. Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Bone Tendon Epimysium Epimysium Perimysium Endomysium Muscle fiber in middle of a fascicle Blood vessel Perimysium wrapping a fascicle Endomysium (between individual muscle fibers) Muscle fiber Perimysium Fascicle Microscopic anatomy of a skeletal muscle fiber. Photomicrograph of portions of two isolated muscle fibers (700x). Notice the obvious striations (alternating dark and light bands). Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber. Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Cross-sectional view of a sarcomere cut through in different locations. I band thin filaments only H zone thick filaments only M line thick filaments linked by accessory proteins Outer edge of A band thick and thin filaments overlap Nuclei Dark A band Light I band Fiber Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band Sarcomere Z disc I band M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Myosin filament Actin filament A band M line2 Microscopic anatomy of a skeletal muscle fiber. Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber. Sarcolemma Mitochondrion Myofibril Nucleus Light I band Dark A band Sarcolemma: plasma membrane Sarcoplasm: cytoplasm of muscle cell -> large # glycosomes and hemoglobin Microscopic anatomy of a skeletal muscle fiber. Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band A band I band M line Sarcomere Muscle contraction: (1) Sarcomere shortens (2) I-band shortens (3) H-zones disappear (4) A-bands move closer together but length stays the same Microscopic anatomy of a skeletal muscle fiber. Enlargement of one sarcomere (sectioned length- wise). Notice the myosin heads on the thick filaments. Z disc Sarcomere M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Sliding filament model: during contraction myosin and actin slide past each other intermittently through cross-bridges Muscle structure • Muscle contraction is controlled by the nerve initiated electrical impulses that travel along the sarcolemma • T-tubules: continuations of sarcolemma; -> signal travels to the deepest parts of muscle • Sarcoplasmic Reticulum: regulates intracellular calcium3 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Part of a skeletal muscle fiber (cell) Myofibril Sarcolemma I band A band I band Z disc H zone Z disc M line Sarcolemma Triad: • T tubule • Terminal cisterns of the SR (2) Tubules of the SR Myofibrils Mitochondria Tubules of SR: blue T-tubules: white Questions ① Brief review of muscle structure ② Nerve stimulus and the neuromuscular junction The phases leading to muscle fiber contraction. (1 of 2) Phase 1 Motor neuron stimulates muscle fiber (see Figure 9.8). Action potential (AP) arrives at axon terminal at neuromuscular junction ACh released; binds to receptors on sarcolemma Ion permeability of sarcolemma changes Local change in membrane voltage (depolarization) occurs Local depolarization (end plate potential) ignites AP in sarcolemma The phases leading to muscle fiber contraction. (2 of 2) Phase 2: Excitation-contraction coupling occurs AP travels across the entire sarcolemma AP travels along T tubules SR releases Ca2+; Ca2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins4 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released. (1 of 3) Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Synaptic vesicle containing ACh Synaptic cleft Junctional folds of sarcolemma Action potential arrives at axon terminal of motor neuron. Voltage-gated Ca2+ channels open. Ca2+ enters the axon terminal moving down its electochemical gradient. Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis. ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma. Axon terminal of motor neuron Fusing synaptic vesicles Sarcoplasm of muscle fiber ACh 1 2 3 4 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released. (2 of 3) ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential -> action potential -> Ca2+ release 5 Postsynaptic membrane ion channel opens; ions pass. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction. Degraded ACh ACh Acetylcholinesterase Ion channel closes; ions cannot pass. 6 ① Action potential arrives at axon terminal ② Voltage-gated Ca2+ channels open ③ Release of Ach through by exocytosis ④ Ach diffuses across synaptic cleft ⑤ Binds to receptors: Na+ into muscle cell; K+ out of cell; endplate potential –> action potential –> Ca2+ release ⑥ Ach is broken down by enzyme Acetylcholinesterase5 The phases leading to muscle fiber contraction. (2 of 2) Phase 2: Excitation-contraction coupling occurs AP travels across the entire sarcolemma AP travels along T tubules SR releases Ca2+; Ca2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins 