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Muscle Tissue

Muscle Tissue. http://photos.demandstudios.com. http://www.histol.chuvashia.com. Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used.

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Muscle Tissue

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  1. Muscle Tissue http://photos.demandstudios.com http://www.histol.chuvashia.com

  2. Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used. Mappings of the lecture slides to the 12th and 13th editions are provided in the supplements.

  3. Outline • Overview • Skeletal muscle • Contraction and relaxation • Neuromuscular junction • Muscle metabolism • Control of muscle tension • Types of skeletal muscle fibers • Physical exercise • Cardiac muscle and • Smooth muscle • Regeneration • Aging

  4. Overview

  5. Muscle Tissue • Muscle tissue consists of elongated cells known as muscle fibers, or myocytes, that use ATP to generate mechanical force known as tension. • Muscle tension produces movements, maintains body posture, and generates heat. • Muscle tissues consist of three types—skeletal, cardiac, and smooth. Chapter 4, page 134

  6. Skeletal Muscle • Skeletal muscle tissue is usually attached to the bones of the skeleton. • The tissues have striations (stripes) of alternating light and dark bands that are visible under a light microscope. • Skeletal muscle movement is considered voluntary because it can con-tract through conscious, willful control. Table 4.9a Chapter 4, page 134

  7. Skeletal Muscle (continued) • Depending on the skeletal muscle, the muscle fibers can vary in length from a few centimeters (cm) to 30 to 40 cm. • The fibers are approximately cylindrical in shape, and each has many nuclei located near its periphery. • The fibers are usually positioned parallel to one another within a whole skeletal muscle. Table 4.9a Chapter 4, page 134

  8. Skeletal Muscle (continued) http://upload.wikimedia.org

  9. Cardiac Muscle • Cardiac muscle tissue, which is also striated, forms most of the wall of the heart. • Cardiac muscle is considered to be involuntary because its contrac-tions are not consciously controlled. • Cardiac muscle fibers are branched, and usually have one centrally-located cell nucleus. Table 4.9b Chapter 4, page 134

  10. Cardiac Muscle (continued) • Intercalated discs, found only in cardiac muscle fibers, hold the heart muscle tissues together through billions of vigorous contractions dur-ing a lifetime. • These microscopic discs are irregular, transverse thickenings of the sarcolemma that connect the ends of cardiac muscle fibers to one another. • Gap junctions enable the rapid conduction of muscle action potentials so that all of the muscles fibers that chambers (atria and ventricles) of a healthy heart contract in unison. Table 4.9b Chapter 4, page 139

  11. Cardiac Muscle (continued) http://upload.wikimedia.org http://www.answers.com

  12. Smooth Muscle • Smooth muscle tissue is found in the walls of hollow internal structures and the iris of the eye. • Internal structures include the blood vessels, airways, lower esophagus, stomach, intestines, gall bladder, and urinary bladder. • The tissue is not striated, hence the term, smooth. • Smooth muscle activity is usually involuntary, although there are some exceptions, as discussed during the lecture module on the autonomic nervous system. Table 4.5c Chapter 4, page 139

  13. Smooth Muscle (continued) • Smooth muscle fibers areshort, and thickest in their middle and taper-ing at the ends. • Gap junctions connect the individual fibers, enabling the muscle fibers to contract in unison. Table 4.5c Chapter 4, page 139

  14. Smooth Muscle (continued) http://www.answers.com http://upload.wikimedia.org

  15. Functions of Muscle Tissue • Producing body movements • Stabilizing body positions • Moving and storing substances • Generating heat Chapter 10, page 292

  16. Producing Body Movements • Body movements require the functions of skeletal muscles, bones, and joints. • The movements are controlled by systems and pathways in the brain and spinal cord. • Whole body movements include activities such as walking, running, bicycling, and swimming. • Localized body movements include holding a pencil, playing a musical instrument, and nodding one’s head. Chapter 10, page 292

  17. Stabilizing Body Positions • Skeletal muscle contractions also stabilize the joints and maintain body posture. • Postural muscles contract continuously in an awake person such as the neck muscles that hold the head upright. Chapter 10, page 292

  18. Moving and Storing Substances • Smooth muscle contractions, known as peristalsis, propel food through the digestive tract. • The contractions of ring-like bands of smooth muscle called sphincters prevent the outflow of the contents of a hollow organ, such as from the stomach. • The temporary storage of food in the stomach and fluid in the urinary bladder is possible since sphincters close-off the outlets to the organs. Chapter 10, page 292

  19. Moving and Storing Substances (continued) • Cardiac muscle contractions allow blood to be pumped through the blood vessels in the systemic and pulmonary circulatory systems. • Smooth muscles in the walls of blood vessels adjust their diameters to regulate blood flow. Systemic circulation is the general circulation of the blood through the body, as opposed to the pulmonary circulation of the blood from the heart to the lungs. (http://www.answers.com) Chapter 10, page 292

  20. Generating Heat • Muscle contractions generate heat by a process called thermogenesis. • Much of the heat from skeletal muscle contractions is used in maintain-ing body temperature of about 98.6oF (37oC). • Shivering, an involuntary contraction of skeletal muscles, increases the rate of heat production. Chapter 10, page 292

  21. Properties of Muscle Tissue • Electrical excitability • Contractility • Extensibility • Elasticity Chapter 10, page 292

  22. Electrical Excitability • Muscle fibers and neurons have a biophysical property known as elec-trical excitability. • Both respond to certain types of stimuli by generating electrical signals known as action potentials. • Action potentials are propagated along the plasma membrane of mus-cle fibers and the axons of neurons due to voltage-gated ion channels. Biophysics = an interdisciplinary science that uses the methods of physics and physical chemistry to study biological systems. (http://en.wikipedia.org) Propagate = travel or spread. Chapter 10, page 292

  23. Electrical Excitability (continued) • The two classes of stimuli that trigger action potentials in muscle fibers are: • Chemical stimuli such as neurotransmitters released by neurons, hormones in the blood, and local changes in acid-base balance (pH). • Autorhythmic electrical signals from pacemaker cells of the heart. Auto- = the prefix for self. Chapter 10, page 292

  24. Contractility • Contractility is the ability of muscle fibers to contract in response to a muscle action potential. • A contracting skeletal muscle generates tension when it pulls on its attachment points of the skeleton. • If the tension is sufficient to overcome the resistance of an object to be moved, the entire skeletal muscle shortens and movement occurs. Chapter 10, page 293

  25. Extensibility • Extensibility is the ability of muscle to stretch to a certain degree with-out being damaged. • Muscle fibers can contract even when stretched, although the tension they generate will be lessened (this is due to the length-tension rela-tionship, to be discussed). • Smooth muscle is usually subject to the greatest amount of stretching such as a stomach full of food. Chapter 10, page 293

  26. Elasticity • Elasticity is the ability of muscle fibers to return to their original length after contraction or extension. • Muscle fibers, as we will discuss, have special properties to maintain elasticity. Chapter 10, page 293

  27. Skeletal Muscle

  28. Structural Hierarchy • Whole muscle • Muscle fibers • Myofibrils • Sarcomeres • Thin and thick filaments Try to keep this top-to-bottom hierarchy in mind as we cover the upcoming slides.

  29. Structure • A whole muscle has many muscle fibers positioned parallel to each other. • The diameter of mature skeletal muscle fibers ranges from 10-to-100 m (microns). • A typical length is about 10 cm, although some are as long as 30-to-40 cm. 1 m (micron) = 1.0 x 10-6 meters (m). Chapter 10, page 295 Figure 10.2

  30. Muscle Fiber http://www.crossfitoakland.com

  31. Structure (continued) • During embryonic development, a muscle fiber fuses from 100 or more mesodermal cells known as myoblasts. • A skeletal muscle fiber can have as 100 or more individual nuclei. • Muscle fibers lose almost all ability to undergo further cell division once fusion has occurred. • The number of muscle fibers are established before birth, and most last a lifetime. Mesoderm = one of the three primary germ cell layers—the other two are the ectoderm and endoderm—in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin. (http://www.medterms.com) Chapter 10, page 295

  32. Skeletal Muscle Repair • A few myoblasts persist in mature skeletal muscle as satellite cells. • Satellite cells retain a capacity to fuse with one another in damaged muscle fibers to regenerate a very limited number of muscle fibers. • The number of muscle fibers that can be regenerated from satellite cells cannot compensate in instances of substantial muscle damage. • Damaged muscle tissue instead undergoes fibrosis, the replacement of muscle fibers by scar tissue. Chapter 10, page 295

  33. Muscle Growth • Skeletal muscle growth after birth is due to hypertrophy, an enlarge-ment of the existing muscle fibers. • Human growth hormone (hGH) and other hormones stimulate an increase in the size of the skeletal muscle fibers in childhood and at puberty. • Certain androgens, a class of hormones that includes testosterone, promote enlargement of skeletal muscle fibers through protein syn-thesis. • This process begins at puberty. Contrast hypertrophy with: Hyperplasia = an increase in the number of fibers. Chapter 10, page 295

  34. Sarcolemma and Transverse Tubules • The nuclei of a skeletal muscle fiber are located just beneath the sar-colemma, the name for the plasma membrane of a muscle cell. • Thousands of invaginations known as transverse (T) tubules tunnel from the surface of the sarcolemma toward the center of each muscle fiber. • The tubules, which open to the outside of the muscle fiber, are filled with interstitial fluid that surrounds cells. Invagination = folded inward. Interstitial fluid = extracellular fluid that fills the microscopic spaces between cells. Chapter 10, page 295 Figure 10.2

  35. Sarcolemma and T Tubules (continued) • Muscle action potentials that trigger muscle contractions propagate (travel) along the sarcolemma, and into the interior of muscle fibers via the T tubules. • The arrangement assures that a muscle action potential excites all parts of a muscle fiber simultaneously for smooth, precise contrac-tions of the fiber. Chapter 10, page 295 Figure 10.2

  36. Sarcoplasm • Inside the sarcolemma is the sarcoplasm, the cytoplasm of muscle fiber. • The contains a large amount of glycogen, a macromolecule composed of glucose molecules, for use in the regeneration of ATP. • The sarcoplasm also contains myoglobin, a protein found only in muscle fibers. Figure 10.2 Chapter 10, page 295

  37. Myoglobin and Mitochondria • The myoglobin binds oxygen (O2) molecules that passively diffuse into the muscle fibers from the interstitial fluid. • Myoglobin releases O2 when needed by the mitochondria for ATP production in muscle fibers. • The mitochondria, which are arranged in rows in each muscle fiber, are adjacent to the motor proteins to provide a continuous source of ATP for contraction. Chapter 10, page 295

  38. Myofibrils • When viewed under a light microscope, the sarcoplasm of a muscle fiber appears to contain very thin threads. • The threads, known as myofibrils, are the contractile organelles of a muscle fiber. • They are about 2 m in diameter, and extend the length of the muscle fiber. • Their striations, or bands, make an entire skeletal muscle fiber appear striated. Figure 10.2 Chapter 10, page 295

  39. Sarcoplasmic Reticulum • The sarcoplasmic reticulum (SR) is a fluid-filled system of sacs that surround the myofibrils. • The SR is similar in structure to the smooth endoplasmic reticulum in non-muscle cells. • The SR stores calcium ions (Ca2+), which are needed for triggering muscle contractions. Figure 10.2 Chapter 10, page 295

  40. Filaments • Even smaller structures, known as thin filaments and thick filaments, are found in the myofibrils. • Thin filaments are about 8 nm and thick filaments are about 16 nm in diameter. • Both filaments are 1-to-2 um long. 1 nm (nanometer) = 1.0 x 10-9 meters. Figure 10.2 Chapter 10, page 298

  41. Filaments (continued) • A thick filament is paired with two thin filaments—each is made-up of a motor protein. • The filaments reside in a compartment called a sarcomere, the basic contractile unit of a myofibril. • The thin and thick filaments overlap one another to a certain extent, depending on whether the muscle is contracted, relaxed, or stretched. • The discs, bands, zones, and lines within a sarcomere are shown in Figure 10.3. Figure 10.3 Chapter 10, page 298

  42. Discs, Bands, Zones, and Lines http://www.ks.uiuc.edu

  43. Discs, Bands, Zones, and Lines (continued) • Z-discs—narrow plates of dense protein that separate the two ends of adjacent sarcomeres. • A band—dark, middle portion of sarcomeres where the thin filaments overlap the thick elements. • I band—lighter portion of sarcomeres that contain thin filaments but no thick filaments. • H zone—light, narrow region at the midline of the sarcomeres that con-tains thick filaments but no thin filaments. • M line—dark, narrow region in the center of the H zone that contains proteins to maintain the position of the thick filaments at the center of the sarcomeres. Chapter 10, page 298 Table 10.1

  44. Muscle Proteins • Myofibrils have three types of proteins—contractile or motor, regula-tory, and structural. • Contractile or motor proteins generate the mechanical force for muscle contractions. • Regulatory proteins switch the contraction process on and off. • Structural proteins keep the thin and thick filaments in proper alignment. Table 10.2 Chapter 10, page 299

  45. Muscle Proteins (continued) • Contractile or motor proteins—myosin and actin. • Regulatory proteins—tropomyosin and troponin. • Structural proteins—titin, -actin, myomesin, nebulin, dystrophin, and others. Table 10.2 Chapter 10, page 299

  46. Contractile or Motor Proteins • Myosin is a contractile or motor protein in muscle tissues. • Contractile proteins produce movement by converting ATP to mech-anical energy. • Each myosin molecule is shaped like two golf clubs twisted together, with two individual heads and a combined tail. • About 300 molecules of myosin form the thick filament in a sarcomere. Figure 10.4 Chapter 10, page 299

  47. Contractile or Motor Proteins (continued) • The primary molecule that make-ups the thin filaments in a sarcomere is actin. • Individual actin molecules are joined to form a filament twisted into a helix shape. • Each actin molecule has a myosin-binding site where a myosin head can attach itself. Figure 10.4 Chapter 10, page 299

  48. Regulatory Proteins • Thin filaments also contain the regulatory proteins, tropomyosin and troponin. • In a relaxed muscle fiber, the myosin heads are blocked from binding to actin since strands of tropomyosin cover the myosin-binding sites. • The tropomyosin strands are held in place by troponin molecules. Figure 10.4 Chapter 10, page 300

  49. Structural Proteins • Muscle fibers have about a dozen different structural proteins that contribute to the alignment, stability, elasticity, and extensibility of the myofibrils. • Titin is the third most abundant protein in skeletal muscle fibers (after myosin and actin). • Titin connects the Z disc to the M line of the sarcomere to align and stabilize the exact position of the thick filament in relationship to the two thin filaments. Chapter 10, page 300

  50. Structural Proteins (continued) • Titin can be stretched at least four times its resting length and still recoil without damage. • This stretch accounts for much of the elasticity and extensibility of the myofibrils within a muscle fiber. Chapter 10, page 310

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