Jessica Meiman 10/22/14Chapter 9 Learning OutcomesH - Follow the events of excitation-contraction coupling that lead to cross-bridge cycling.Excitation-contraction coupling involve a series of events beginning with the creation of action potential that ultimately results in the sliding of the myofilaments. The creation of action potential is triggered by a nerve impulse that releases the neurotransmitter acetylcholine into the synaptic cleft. The receptor for acetylcholine, located in the sarcolemma, functions as an ion channel and is activated by the presence ofacetylcholine and consequently allows sodium and potassium to cross the membrane. Because there is a greater number of Na ions moving into the cell, local depolarization occurs and nearby voltage gated channels open and continued depolarization occurs as more Na enters the cell. This process generates an action potential, which in turn triggers the release of calcium into the cytosol. The release of the calcium ions is what sets the stage for cross-bridge cycling.I - Define motor unit and muscle twitch, and describe the events occurring during the 3 phases of a muscle twitch.A motor unit consists of a motor neuron and all the muscle fibers that it innervatesand a muscle twitch is the response of the motor neuron due to stimulation; where theresponse is then measured by how much tension or force is produced. The three phases ofa muscle twitch include a latent period, contraction phase, and relaxation phase. The latent period is the period of time following stimulation where excitation-contraction and cross bridge cycling begins but there is no measurable tension. The contraction phase is the period of time where tension rises until it reaches peak value due to active cross bridges. The relaxation phase is the period that is where tension starts to fall as calcium ispumped back to the sarcoplasmic reticulum from the cytosol.N - Compare the gross and microscopic anatomy of smooth muscle cells to that of skeletal muscle cells.Skeletal muscle is distinguished by its striated appearance and contains singe, long, cylindrical multinucleate cells. Skeletal muscle is supplied by nerves and blood that allow for control via the nervous system and also allows for nutrients to reach the musclesalong with wastes removed. Myofibrils are the contractile elements of the muscle cell and are composed of sarcomeres, which have portions of overlapping myofilaments. Myofilaments contain both thick and thin filaments, which contribute to the striations seen in skeletal muscle. A bands, are regions where thick and thin filaments overlap, creating a dark stripe, and I bands, are regions where only thin filaments are found and therefore create a light stripe. Thin filaments contain proteins, tropomyosin and troponin while thick filaments contain myosin. On a less microscopic level, different connectivetissue layers surround each section of muscle beginning with the endomysium, which is found surrounding each muscle fibers. Next is the perimysium, which is found surrounding groups of muscle fibers, and the epimysium found surrounding whole muscles. The most apparent difference in smooth muscle is the lack of striations giving the muscle a “smooth” appearance and consequently a lower ratio of thick to thin filaments and lack of myofibrils. Smooth muscle cells unlike skeletal muscles are small, spindle shaped cells with a centrally located nucleus. Unlike skeletal muscle, they do not contain connective tissue layers that separate fibers from the entire muscle, but instead arearranged in sheets. Neuromuscular junctions are also not present in smooth muscles, along with a much less developed sarcoplasmic