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9.1: Introduction to Muscle Tissue
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When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs. These are skeletal muscles, so-named because most of them move the skeleton. But there are two other types of muscle in the body, with distinctly different jobs.
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9.2: Overview of Muscle Tissues
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Muscle is one of the four primary tissue types of the body, and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (Figure 10.1.1). All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane.
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9.3: Skeletal Muscle
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Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely; muscles work to keep joints stable.
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9.4: Muscle Fiber Contraction and Relaxation
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The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber. The local membrane of the fiber will depolarize as positively charged sodium ions (Na+) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the T-tubules.
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9.5: Types of Muscle Fibers
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There are three main types of skeletal muscle fibers. Slow oxidative fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers. Lastly, fast glycolytic fibers have fast contractions and primarily use anaerobic glycolysis - these fatigue more quickly than the others.
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9.6: Development and Regeneration of Muscle Tissue
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Most muscle tissue of the body arises from embryonic mesoderm. Paraxial mesodermal cells adjacent to the neural tube form blocks of cells called somites. Skeletal muscles, excluding those of the head and limbs, develop from mesodermal somites, whereas skeletal muscle in the head and limbs develop from general mesoderm. Somites give rise to myoblasts. A myoblast is a muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, or myotube.