Muscle Tissue

1. Muscle Tissue Anatomy 32 Chapter 10

2. Muscle tissue- muscle tissue alone is not an organ, but a muscular structure such as your bicep is an organ. A muscular structure incorporates tendons, fascia, blood vessels (connective tissue), and nerves (nervous tissue). There are three types of muscle tissue: skeletal, cardiac, and smooth.

3. a. Functions- there are four general functions: 1. movement-skeletal and smooth muscle aid in movement or bones and fluids. 2. posture maintance- skeletal muscles contract to maintain the body in a sitting or standing position. 3. joint stabilization- tendons that cross over joints stabilize joint as the muscle tone (constant low level contraction) places tension on the tendon. 4. heat generation- muscular contractions generate heat influencing body temperature. b. Specialization- just like epithelial and connective tissue have distinguishing characteristics so does muscle tissue: 1. Able to contract-when long cells shorten simultaneously a pulling force is created that contracts the muscle- reduce the overall size. Contractions may cause movement or stabilization. 2. Able to extend- at the end of a contraction the muscle may return to its original length by relaxing or extended with the aid of an opposing muscle. 3. Able to become excitable- muscle cells respond to nerve impulses by contracting. 4. Elasticity- muscle may be stretched beyond its normal size and recoil back to it’s normal size.

4. C. Classification- muscles are classified based on structure and function. 1. Skeletal muscle- this is known as multinucleated, striated, voluntary muscle that attaches to bones and causes the skeleton to move. 2. Cardiac muscle- this is known as binucleated, branched, striated involuntary muscle that makes up the wall of the heart. 3. Smooth muscle-this is known as non-striated involuntary muscle that assists in the movement of internal viscera. It has the capacity to stretch widely and contract powerfully.

6. II. Skeletal Muscle- arranged in bundles and attached to bones by the skeleton it allows the body to move. Below is a description of its basic structure.

7. A. Basic features of skeletal muscle 1. Connective tissue and fascicles- Three sheaths composed of connective tissue help to organize a muscle. These sheaths extend out of the muscle area to join the ligaments, they help to transfer the contracting force of the fiber over to the bone so the joint is moved. a. The outer-most-layer is epimysium= a sheath of dense connective tissue that externally surrounds the entire muscle. b. Bundles within the muscle called fascicles are surrounded by a sheath of fibrous connective tissue called perimysium. c. A sheath of reticular fibers called endomysium surrounds the bundles within the fascicles.

8. 2. Nerves and blood vessels- both structures enter the muscle as single units and then branch at areas of connective tissue. Nerves supply the excitability signals and blood vessels bring in nutrients and remove waste.

10. 3. Muscle attachments- the origin is the side of a muscle that attaches to a bone and when the muscle contracts it does move that bone. As the tendon on the other side of that muscle crosses over a joint it connects into an insertion. The joint it crosses will move when the muscle contracts. Some muscles cross over two or more joints, when they contract both joints move. Tubercles, trochanters, and crests are raised areas where muscles attach to bones as a response to the pulling force the bone thickens.

11. B. Microscopic and functional anatomy of skeletal muscle tissue-This is a description of the arrangement of filaments that make up muscle fiber. 1. The skeletal muscle fiber-long cylindrical, large multinucleated cells created by the fusion of embryonic muscle cells. The nuclei lie on the sides of the cell right below the sarcolema (cells membrane)

12. 2. Myofibrils and sarcomeres- the visible stripes result from the arrangement of myofibrils in the sarcoplasm. Myofibrils are contractile cellular organelles made up of rows of sarcomeres and are smaller than muscle fibers (muscle cell) but larger than myofilaments (protein strands the make up myofibrils). Myofibrils may be surrounded by mitochondria and sarcoplasm.

14. a. Sarcomeres: basic contraction unit formed by the arrangement of thin myofibrils called actin and a thick myofibril called myosin. The sarcomere boundary is at the Z line (Z discs). Actin extends from the z line to the middle of the sarcomere. In the middle of the sarcomere and in between actin strands lies myosin strands. The myosin length, including the area where actin and myosin overlap, is the A-band. The H-zone is where only myosin is present and the I band is the region where only actin is present. Down the middle of the sarcomere lies the M-line.

17. 3. Mechanisms of contraction-the sliding filament theory explain how a muscle contraction happens: a. The contraction is initiated by the release of calcium from the terminal cisternae once a nervous impulse is received. b. Calcium ions bind to troponin on actin. The troponin-tropomyosin complex moves exposing the myosin binding sites. c. The ATP (energy molecule) provides power for myosin to move by “energizing” the molecule and changing its conformation (cocked postion). d. The myosin head attaches to the actin filament (cross bridge) and in a swivel action pulls the actin filament (power stroke) . e. A second ATP binds the myosin head causing it to release actin and return to a cocked position. f, The myosin binds multiple times pulling towards the center further and further down the actin strand. As the filaments slide past each other the overall size shortens. g. The released calcium isre-absorbed by the terminal cisternae. h. With the contraction the Z-lines come closer to each other, the I and H bands become smaller, the A band and M line remain the same.

20. 4. Muscle extension- after a contraction the muscle can be stretched to its original position by the action of an opposing muscle. 5. Muscle fiber length and the force of contraction-each sarcomere or muscle have an optimal length. The more it is stretched the greater the ability to produce contraction forces as long as it is not past the stretch limit which would not allow it to contract. There is a limit to how much a muscle contracts or stretches and it is monitored by joint structure. 6. The role of Titin- molecules in sarcomeres that resist overstretching and attach to myosin. 7. Sarcoplasmic Reticulum and T tubules- the smooth ER of muscle cells (muscle fibers) is called sarcoplasmic reticulum and run longitudinal around myofibrils. It stores calcium which are released for contraction and reabsorbed. T-tubules a continuation of a sarcolemma is a triad.

21. 8. Types of skeletal muscle fiber-all muscles in the body contain the three types of fibers but each varies in its proportions (this is genetically determined). These fiber proportions influence the muscles performance capabilities. a. red slow-twitch fibers- high in myoglobin content and in mitochondria, and in vascularization. Not easily fatigued as long as there is a good oxygen supply, can contract continuously, do not generate a lot of power. b. white fast-twitch fibers- low myoglobin content and in mitochondria larger in diameter, greater capability to produce power. c. Intermediate fast-twitch fibers- this fiber type is an intermediate in all aspects between red and white.

22. III. Cardiac muscle- myocardium is the muscle of the heart wall, it contracts to pump blood. A. Muscle cells are not fused they connect by cell junctions called intercalated discs and have a branching pattern. Each cell has one to two nuclei in the center. The cells have sarcomeres which makes the tissue look striated.

23. B. This tissue has an abundance of mitochondria to prevent fatigue. Contractions are also triggered by calcium ions. Not all cells are innervated, cells can independently have rhythmic contractions. The length of the cells is proportionally related to the force it produces when it contracts

25. IV. Smooth Muscle- A tissue formed by uninucleated spindle shaped cells found in six areas of the body: blood vessel walls, respiratory tract, digestive tubes, urinary organs, reproductive organs, and the eye.

26. A. It exist as two layers with fibers running perpendicular to each other. One layer, the longitudinal layer is parallel to the axis, the circular layer is perpendicular. As they alternate contractions they shorten and constrict the organ.

27. B. There are no striations and no sarcomeres. Calcium ions signal contraction and the forces are not high. It can contract for a long time before fatiguing. Typically cells are not individually innervated and contraction may be signaled by stretching or hormones.

29. V. Disorders of muscle tissue A. Muscular Distrophy- disorder in which muscle is destroyed and replaced with connective tissue, it is first diagnosed during childhood. Duchenne muscular dystrophy- muscle degeneration from pelvis to cranium, predominantly in men, person usually lives to be 20 years old. It is related to the lack of production of a protein that influence cell structure. Genetherapy and stem cell research suggest possible cures. B. Myofascial pain syndrome-overused or strained postural muscles cause tightening of muscle bands that twitch when the skin is touched. This affect about half the population and is treated with anti-inflammatory drugs and stretching. C. Fibromyalgia-chronic pain syndrome of lower back or neck muscles, affects 2% of people, mostly women, treated with antidepressants, exercise, and pain relievers. It is not truly a muscle disorder.

30. VI. Muscle tissue throughout life 1. Mesoderm cells called myoblast fuse to form skeletal muscle tissues or join at cellular ends to form cardiac and smooth muscle. 2. Cardiac muscle contracts by the week 3 and skeletal muscle by week 7 of development! 3. Mitosis= skeletal muscle stops dividing once formed but has limited regenerated capacity in case of injury, cardiac muscle stops dividing by age 9, and smooth muscle divides as needed and has great regenerative abilitiy. 4. Muscle tissue is replaced with connective tissue as one ages. This is called sarcopenia and is reversible with exercise