The Muscular System

1. The Muscular System Textbook Chapter 8

2. The Muscular System • Muscles are organs composed of specialized cells that use chemical energy, obtained from nutrients, to pull on structures to which they are attached • There are three types of muscles: skeletal, smooth, and cardiac muscle • We will focus on skeletal muscles

3. The Muscular System • Muscles are responsible for all types of body movement—they contract, or shorten, to make the body move

4. Functions of Muscles • Support the body • Allow for movement by making bones and other body parts move • Maintain constant body temperature • Assist in movement of cardiovascular veins and lymph vessels • Protect internal organs and stabilize joints

5. Body System Connections • Integumentary System • The skin increases heat loss during skeletal muscle activity • Lymphatic System • Muscle action pumps lymph through the lymphatic vessels

6. Structure of a Skeletal Muscle • A skeletal muscle is an organ of the muscular system that is composed of skeletal muscle tissue, nervous tissue, blood, and other connective tissue

7. Structure of a Skeletal Muscle:Connective Tissue Coverings • Layers of connective tissue enclose and separate all parts of a skeletal muscle • Fascia (dense connective tissue) separates all individual muscles from each other

8. Structure of a Skeletal Muscle:Connective Tissue Coverings • Epimysium is a layer of connective tissue that closely surrounds each skeletal muscle • Perimysium separate the muscle tissue into small sections called fascicles • Fascicles are bundles of skeletal muscle fibers • Endomysium is a thin covering of connective tissue around a muscle fiber

10. Skeletal Muscle Attachments • Epimysium blends into a connective tissue attachment • Tendon- cord-like structure • Sites of muscle attachment • Bones • Cartilages • Connective tissue coverings

11. The Muscular System:Tendonitis • In tendonitis, a tendon becomes painfully inflamed and swollen following injury or repeated stress of athletic activity • The tendons most commonly affected are those associated with the joint capsules of the shoulder, elbow, and hip and those that move the hand, thigh, and foot

12. Structure of a Skeletal Muscle:Skeletal Muscle Fibers • A skeletal muscle fiber is a single cell that contracts in response to stimulation and then relaxes when the stimulation ends • Skeletal muscle fibers contain many threadlike myofibrils that lie parallel to one another

13. Structure of a Skeletal Muscle:Skeletal Muscle Fibers • Myofibrils play an important role in muscle contraction • They contain two kinds of protein filaments (myofilaments): • Thin filaments called actin • Thick filaments called myosin • The organization of the myofilaments is what produces alternating light and dark bands of a skeletal muscle fiber

15. Structure of a Skeletal Muscle:Skeletal Muscle Fibers • The bands of skeletal muscle result from a repeating pattern of units called sarcomeres

16. Organization of Skeletal Muscle Fascicle: a bundle of muscle fibers Sarcomere: units of myofibrils responsible for the striated appearance Muscle belly Muscle fiber: muscle cell Myosin: thick filaments Myofilament: protein filaments that make up a sacromere Myofibrils: structures that make up a muscle fiber Actin: thin filaments

17. Structure of a Skeletal Muscle:Muscle Strains • Muscle fibers and their associated connective tissues are flexible but can tear if overstretched • This is called a muscle strain • The severity of the injury depends on the degree of damage • Mild strains—only a few muscle fibers are injured and fascia remains intact. Loss of function is minimal • Sever strains—many muscle fibers and fascia tear. Muscle function may be completely lost. Painful and produces discoloration and swelling

18. Structure of a Skeletal Muscle:Neuromuscular Junction • Remember that neurons paly a role in communication within the body • Motor neurons are neurons that control the muscles in the body • Muscle fibers contract when they receive a stimulus from a motor neuron

19. Structure of a Skeletal Muscle:Neuromuscular Junction • Each skeletal muscle is connected to the end of a motor neuron • The neurons communicate with the muscle fibers through neurotransmitters (chemicals released by the nervous system) • The connection between a motor neuron and the muscle fiber it controls is called a neuromuscular junction

20. Skeletal Muscle Contraction • A muscle fiber contraction occurs when myosin binds to actin and exerts a pulling force • The result is a movement within the myofibrils in which the actin and myosin slide past one another, increasing the area of overlap • This shortens the muscle fiber, which then pulls on the body part that it moves

21. Skeletal Muscle Contractions • Myosin contains “heads” projecting from one end • Actin contains binding sites for the myosin heads to attach • Actin is often shaped like a double helix—proteins troponin and tropomyosin are part of this helix • A myosin head can attach to a binding site on actin and pull • The myosin then releases the actin and combines with a binding site further down the actin filament

22. Skeletal Muscle Contraction • The sliding filament model of muscle contraction is based on these actin-myosin interactions • The filaments do not change length, they slide past one another, with the thin filaments moving toward the center of the sarcomere • Acetylcholine serves as the stimulation needed for muscle contraction to begin • Once acetylcholine breaks down and the stimulus to the muscle fiber ends, the muscle relaxes

23. Body System Connections • Skeletal System • Bones provide attachments that allow skeletal muscles to cause movement • Digestive System • Skeletal muscles are important in swallowing. The digestive system absorbs the nutrients needed for muscle contraction

24. Skeletal Muscle Contraction:Steps of Muscle Contraction • Impulse travels down a motor neuron • The neuron releases Acetylcholine (ACh) • ACh binds to receptors in the muscle fiber • An impulse travels over the surface of the muscle fiber • Calcium channels open • Calcium binds to troponin (actin) • Tropomyosin moves to expose binding sites • Myosin heads attach to the actin filaments • Actin is pulled toward the center of the sarcomere • The muscle fiber produces a pulling force

25. Skeletal Muscle Contraction:Muscle Fatigue • Fatigue is when muscles that have been exercised strenuously for a prolonged period may have decreased ability to contract • Occasionally, muscles become fatigued and cramps at the same time • A cramp is a painful condition in which a muscle undergoes a sustained involuntary contraction • Occurs when extracellular fluid somehow trigger uncontrolled stimulation of the muscle

26. Muscular Responses • One way to observe muscle contraction is to remove a single muscle fiber from a skeletal muscle in the lab • Threshold Stimulus • Certain strength of stimulation applied to the muscle fiber to begin contraction

27. Muscular Responses:Recording of a Muscle Contraction • The response of a single muscle fiber to a single impulse is called a twitch • A twitch consists of a period of contraction and a period of relaxation. These can be recorded in a myogram • The twitch has a brief delay between the time of stimulation and the beginning of contraction (latent period) • 2 milliseconds in humans

28. Muscular Responses:Recording of a Muscle Contraction

29. Muscular Responses:Recording of a Muscle Contraction • The various movements we need to preform daily activities requires contraction of multiple muscle fibers simultaneously • Muscle fibers vary in contraction speed • Fatigue-resistant slow twitch • Fatigueable fast twitch

30. Muscular Responses:Recording of a Muscular Contraction • Contractions of whole muscles enable everyday activities, but the force generated by contractions must be controlled • EX. Holding a cup • The degree of tension reflects: • The frequency at which the muscle fibers are stimulated • How many fibers take part in the overall contraction

31. Muscular Responses:Summation • When a muscle fiber is exposed to a series of stimuli of increasing frequency, it is unable to relax between twitches • When the force of the individual twitches combines, it is called summation

32. Smooth Muscle • The contraction of smooth muscle is very similar to the contraction of skeletal muscle • There are some important structural and functional differences between smooth and skeletal muscles

33. Smooth Muscle:Smooth Muscle Cells • Smooth muscles are elongated with tapered ends • Also contain thin and thick filaments, but they are randomly organized • Not striated

34. Smooth Muscle:Smooth Muscle Cells • There are 2 kinds of smooth muscle cells: • Multiunit smooth muscle • Visceral smooth muscle

35. Smooth Muscle:Smooth Muscle Cells • In multiunit smooth muscle, the cells are separate from one another • Found in the irises of the eyes and in the walls of blood vessels • Typically contract only in response to stimulation by neurons or certain hormones

36. Smooth Muscle:Smooth Muscle Cells • Visceral smooth muscle is made up of sheets of cells in close contact with one another • This is the more common type of smooth muscle • Found in the walls of hollow organs (like the stomach, intestines, bladder)

37. Smooth Muscle:Smooth Muscle Cells • Visceral smooth muscle displays rhythmicity—a pattern of repeated contractions • Due to self-exciting cells that deliver spontaneous impulses periodically into the surrounding tissue • When one cell is stimulated, it causes stimulation in surrounding cells • Ripple effect

38. Smooth Muscle:Smooth Muscle Cells • Rhythmicity and transmission of impulses are responsible for the wave-like motion (peristalsis) • Peristalsis helps force the contents of certain organs along their length (i.e. the intestines)

39. Smooth Muscle:Smooth Muscle Contraction • Smooth muscle is slower to contract than skeletal muscle • Can maintain a forceful contraction longer than skeletal muscle • Smooth muscle can stretch as organs fill, yet maintain a constant pressure inside the organs

40. Cardiac Muscle • Cardiac muscle is found only in the heart • Contraction is the same as skeletal and smooth • There are some important differences

41. Cardiac Muscle • Cardiac muscle is composed of branching, striated cells • Has many filaments of actin and myosin • Each end of a cardiac muscle cell is connected to an intercalated disc • These allow impulses to pass freely so they travel rapidly from cell to cell • When one part of the network is stimulated, the impulse passes to the rest of the network • The whole structure contracts as a unit

42. Cardiac Muscle • Cardiac muscle is also self-exciting and rhythmic • A pattern of contraction and relaxation repeats, causing the pattern of the heartbeat

43. Muscles of Mastication:Masseter • Origin • Zygomatic Arch • Insertion • Posterior lateral surface of the mandible • Action • Elevates and protracts the mandible

44. Masseter

45. Muscles of Mastication:Temporalis • Origin • Temporal bone • Insertion • Mandible • Action • Elevates and retracts the mandible

46. Temporalis

47. Muscles the Move the Head:Sternocleidomastoid • Origin • Manubrium of the sternum & clavicle • Insertion • Temporal bone • Action • Individual: Laterally flexes head and neck to the same side, rotates head to the opposite side • Together: pull the head forward and down

48. Sternocleidomastoid

49. Muscle that Move the Head:Splenius capitis • Origin • Spinous process of C7 (7th cervical vertebra) and upper thoracic vertebrae • Insertion • Occipital bone and temporal bone • Action • Individual: rotates head to the same side • Together: bring head to upright position

50. Splenius capitis