Muscles and Muscle Tissue

1. Muscles and Muscle Tissue Chapter 9

2. Overview of Muscle Tissue • There are three types of muscle tissue • Skeletal muscle • Cardiac muscle • Smooth muscle • These muscle tissues differ in the structure of their cells, their body location, their function, and the means by which they are activated to contract

3. Overview of Muscle Tissue • All skeletal and smooth muscle cells are elongated and are referred to as muscle fibers • Muscle contraction depends on two types of myofilaments, actin and myosin • All prefixes of myo or mys and sarco reference muscle

4. Skeletal Muscle Tissue • Skeletal muscle tissue appears as distinct skeletal muscle that attach to the skeletal system • Skeletal muscle has obvious striations • It is a voluntary muscle under conscious control

5. Cardiac Muscle Tissue • Cardiac muscle occur only in the heart • The muscle is striated but involuntary • Cardiac fibers are short, fat, branched and interconnected • Cardiac muscle cells are interlocked by intercalated discs and function as a single unit

6. Smooth Muscle Tissue • It is found in the walls of hollow organs such as the stomach, urinary bladder, and intestines • It has no striations • It is not subject to voluntary control

7. Differences in Contractions • Skeletal muscle can contract rapidly but tire easily and must be rested • Skeletal muscle contractions vary in force depending on use • Cardiac muscle contracts at a steady rate but can accelerate to cope with demand • Smooth muscle contracts in steady, sustained contractions and continues on tirelessly

8. Muscle Functions • Muscle performs four important functions in the body: • Producing movement • Maintaining posture • Stabilizing joints • Generating heat

9. Producing Movement • Movement results from skeletal muscle contraction • Skeletal muscle are responsible for all locomotion and manipulation • Allows you to interact or react with your external environment • It controls eye movement, facial expression (skeletal); circulation (cardiac), and moves gas, liquids, and solids through organs (smooth)

10. Maintaining Posture • Skeletal muscles are utilized constantly to maintain sitting, standing, and moving postures • Postural muscle develop to compensate for the never ending pull of gravity • Our developmental milestones as an infant are our initial victories over gravity • Curves of the spinal column are shaped by the interplay of skeletal muscle and gravity

11. Stabilizing Joints • Skeletal muscle provide the dynamic stability of joints • Many joints are poorly reinforced by ligaments and connective tissue • Many joints have noncomplementary surface which do not contribute to stability

12. Generating Heat • Muscles generate heat as they contract • The heat generated is vitally important to maintain normal body temperature • Skeletal muscle generates most of the heat because it represents 40% of body mass • Excess heat must released to maintain body temperature

13. Functional Characteristics • Excitability or irritability • It has the ability to respond to a stimulus • Contractility • It has the ability to shorten forcibly • Extensibility • Muscle fibers can be stretched • Elasticity • Resume its normal length after being shortened

14. Skeletal Muscle

15. Anatomy of a Skeletal Muscle • Each skeletal muscle is a discrete organ with thousands of fibers • Muscle fibers predominate the tissue but it also contains, blood vessels, nerve fibers, and connective tissue

16. Connective Tissue Wrappings • Each muscle fiber is wrapped by fine sheath of areolar connnective called endomysium • Several fibers are gathered side by side into bundles called fascicles • Each fascicle is bound by collagen a fiber layer called the perimysium

17. Connective Tissue Wrappings • Fascicles are bound by a dense fibrous connective tissue layer called the epimysium • The epimysium surrounds the entire muscle • External to the epimysium is the deep fascia that binds muscles into functional groups

18. Connective Tissue Wrappings • All the connective tissue layers are continuous with one another as well as with the tendons that join muscles to bone • When muscle fibers contract they pull these connective tissue sheaths which in turn transmit the force to the bone to be moved • Connective tissues supports each cell

19. Nerve and Blood Supply • Normal activity of skeletal muscle is totally dependent on its nerve and blood supply • Each skeletal muscle fiber is controlled by a nerve ending (neuromuscular junction) • Contracting muscle fibers use huge amounts of energy which requires a continuous supply of oxygen and nutrients • In general, each muscle is served by an artery and one or more veins

20. Attachments • Most muscles span joints and have at least two attachments an origin and an insertion • Origin • Attachment of a muscle that remains relatively fixed during muscular contraction • Generally a more proximal or axial location • Insertion • Attachment of a muscle that moves during muscular contraction • Generally a more distal or appendicular attachment

21. Attachments • Direct attachments have the epimysium attaching directly to the periosteum of the bone or perichondrium of a cartilage • Indirect attachments have the epimysium attaching to a tendon or an aponeurosis • Temporalis has both muscle attachments

22. Contraction of Skeletal Muscle • The principles of contraction of a muscle cell can be generalized to the entire muscle • The force exerted is called tension, the resistance to the force is called the load • A contracting muscle does not always shorten (isometric or isotonic) • Skeletal muscle can contract with varying force for different periods of time which enhances its efficiency

23. The Motor Unit • Each muscle is served by at least one motor nerve which contains hundreds of motor neuron axons • As a nerve enters a muscle it branches into a number of axonal terminals, each of which forms a neuromuscular junction with a single nerve fiber • A motor neuron and all the muscle fibers it supplies is called a motor unit

24. The Motor Unit • When a motor neuron transmits an electrical impulse, all the muscle fibers that it innervates respond by contracting • The average number of muscle fibers per unit is 150, but it ranges from 4 to several hundred

25. The Motor Unit • Muscles that exert very fine control have small motor units (eyes, fingers) • Large muscles of locomotion and weight bearing have large motor units and as a consequence have less precise control

26. The Motor Unit • The muscle fibers in a unit are not clustered together but rather are spread throughout the entire muscle • Stimulation of a single unit causes a weak contraction of the entire muscle • This allows control of the intensity of the contraction

27. Skeletal Muscle Fiber • Skeletal muscle fibers are long and cylindrical • These cells are huge • Diameter of 10-100 m up to 10 times average cell size Length is phenomenal for a cell - from several centimeters to dozens of centimeters in long muscles

28. Skeletal Muscle Fiber • These cells actually form by the fusion of hundreds of embryonic cells • Because of its development skeletal muscle fiber contains many nuclei • Nuclei lie at the cell periphery, just deep to the sarcolemma

29. Myofibrils and Sarcomeres • Under the microscope stripes called striations are visible in skeletal muscle fibers • These striations result from the internal structure of long rods called myofibrils within the sarcoplasm • Note that fibrils are to be distinguished from fibers and filaments

30. Myofibrils and Sarcomeres • Myofibrils are unbranched cylinders that are present in large numbers making up 80% of the sarcoplasm • Myofibrils can be conceptualized as specialized contractile cellular organelles unique to muscle fibers

31. Myofibrils and Sarcomeres • Different myofibrils in a fiber are separated and surrounded by narrow regions of sarcoplasm that contain rows of mitrochondria and glycosomes that supply energy for muscle contraction

32. Myofibrils and Sarcomeres • Distinguishing individual myofibrils is histologically difficult because the striations of adjacent myofibrils line up almost perfectly

33. Myofibrils and Sarcomeres • A myofibril is a long row of repeating segments called sarcomeres • The sarcomere is the basic unit of contraction in skeletal muscle • The boundaries at each end of the sarcomere are called z discs

34. Myofibrils and Sarcomeres • Attached to each Z disc and extending toward the center of the sarcomere are many fine myofilaments called thin (actin) filaments, which consist primarily of the protein actin, although they contain other proteins as well

35. Myofibrils and Sarcomeres • In the center of each sarcomere and overlapping the inner ends of the thin filaments is a cylindrical bundle of thick (myosin) filaments

36. Myofibrils and Sarcomeres • Myosin filaments are mostly myosin and some ATPase enzymes that split ATP to release energy required for muscle contraction • Both ends of a thick filament are studded with knobs called myosin head or cross bridges

37. Types of Skeletal Muscle Fiber • Not all skeletal muscle fibers are alike as they vary on the type of contractions they produce • Muscle fiber can be divided by the strength, speed, and endurance of the contraction to which they contribute • Specifically the fibers are referred to as red slow twitch, white fast twitch and intermediate fast-twitch fibers

38. Types of Skeletal Muscle Fiber • Red slow twitch are relatively thin fibers • They are named for the abundant myoglobin (oxygen binding pigment) in their sarcoplasm • Red fibers obtain their energy from aerobic (oxygen requiring) reactions and thus have relatively large numbers of mitrochondria (the site of aerobic metabolism) and a rich blood supply from an extensive network of capillaries

39. Types of Skeletal Muscle Fiber • Red slow twitch fibers contract slowly, are resistant to fatigue as long as oxygen is present • Deliver prolonged contractions • Used in many of the postural muscles of the axial skeleton • Because their fibers are thin, slow twitch fibers do not generate much power

40. Types of Skeletal Muscle Fiber • White fast twitch fibers are pale because they contain little myoglobin • The fibers are about twice the diameter of red slow twitch fibers, they contain more myofibrils and generate more power • The fibers depend on anaerobic pathways (no oxygen used) to make ATP

41. Types of Skeletal Muscle Fiber • They contain few mitrochondria or capillaries but have many glycosomes containing glycogen as a fuel source • White fast twitch fibers contract rapidly and tire quickly • This fiber type is common in the muscle of the upper limbs • Used to lift heavy objects for brief periods

42. Types of Skeletal Muscle Fiber • Intermediate fast twitch are sized between the other two fiber types • Like white fibers they contract quickly; like slow twitch they are oxygen dependent and have a high myoglobin content and a rich supply of capillaries • Because they are intermediate fibers they depend largely on aerobic metabolism, and are less fatigue resistant

43. Types of Skeletal Muscle Fiber • They are more powerful than red fibers, but not as strong as white • This type of fiber is abundant in the muscles of the lower limbs • Used to move the body for long periods of time in activities like walking and jogging

44. Types of Skeletal Muscle Fiber • Because muscles contain a mixture of the three fiber types, each muscle can perform different tasks at different times • For example the Gastrocnemius muscle can be used for sprinting, walking and as a postural muscle • Although everyone’s muscles contain mixtures of the three fiber types, some people have relatively more of one type • These differences are genetically controlled

45. Next Section • Turn to “Smooth Muscle” on page 255 of your text • You are not responsible for the section on sliding filament theory

46. Smooth Muscles • Smooth muscle lacks the courser connective tissue seen in skeletal muscle • Small amounts of endomysium is found between smooth muscle fibers

47. Smooth Muscles • Smooth muscles are organized into sheets of closely apposed fibers • These sheets occur in the walls of all but the smallest blood vessels and in the walls of hollow organs of the respiratory, urinary digestive and reproductive tracts

48. Smooth Muscles • In most cases two sheets of muscles are present with their fibers aligned at right angle to each other • These forms the longitudinal (long axis) and circular (encircling) layer • These two layers squeeze the contents of the organ

49. End of Chapter Chapter 9