The Muscular System (Ch. 10)



(terminology: myo, mys = muscle)

  1. Characteristics of muscle tissue
    1. Excitability - can receive and respond to a stimulus (electrical)
    2. Contractility - individual cells contract (shorten) when stimulus is received; convert chemical energy (ATP) to mechanical energy (movement)
    3. Extensibility - can be stretched
    4. Elasticity - return to original shape after contraction or extension
    1. Review 3 types of muscle tissue from Muscle Table (in tissue notes): including shape of fibers (cells), locations, etc.
  1. Skeletal muscles - attach to skeleton; tissue making them up is skeletal muscle tissue, which is striated & voluntary
    1. Functions of (skeletal) muscular system
      1. Motion
      2. Posture
      3. Support
      4. Guards openings
      5. Heat production
      6. Stability of joints
      7. Keeps fluids (blood & lymph) moving
    2. Organization of a skeletal muscle (fig. 10. __)
      1. Connective tissue component - binds muscle fibers together into a bundle called a muscle, and anchors muscle in place
        1. Whole muscle wrapped in epimysium - sheet of dense irregular collagenous connective tissue
        2. Perimysium (extensions of epimysium) - penetrates muscle and divides it into fascicles (visible muscle 'strings')
        3. Endomysium (extensions of perimysium) - penetrates fascicle and separates it into muscle fibers (cells)
        4. Epi-, peri-, and, endo-mysium are continuous w/ one another
        5. Epimysium extends beyond muscle to become - tendons or aponeuroses
        6. Tendons & aponeuroses are continuous w/ periosteum of bone or epimysium of another muscle
      2. Nerve and blood supply
        1. Skeletal muscle fiber contracts only when - stimulated by a neuron from the somatic (voluntary) division of the nervous system
        2. Muscle fibers need good blood supply - demand lots of nutrients and oxygen for contraction
        3. Nerves & blood vessels travel through the connective tissue to deep in the interior of muscle
          1. each muscle fiber makes contact with one neuron in one spot
          2. each muscle fiber is surrounded by blood capillaries
    3. Histology (fig. 10.2, 10.3, 10.6)
      1. Muscle fibers (recall muscle cell = muscle fiber)
        1. A muscle fiber is microscopic (10-100 micrometers) in diameter; but up to 12 inches in length

Components of a muscle fiber:

        1. Sarcolemma = cell membrane in skeletal muscle cells

T tubules - invaginations of sarcolemma that extend into muscle fiber

        1. Sarcoplasm = cytoplasm in skeletal muscle cells
          1. single fiber has many nuclei in sarcoplasm
          2. stores glycogen (how animals store glucose)
          3. has many mitochondria (why?)
          4. has myoglobin (protein similar to hemoglobin) - binds to and stores a small amount of oxygen; makes muscle red
          5. sarcoplasmic reticulum = smooth ER in skeletal muscle cells; system of membrane tubes that wrap internal cellular structures; packed with calcium
          6. myofibrils - rod- shaped submicroscopic (1-2 micrometers in diameter) protein structures; run throughout the long axis of a muscle fiber
      1. Organization of a skeletal muscle (recap; fig 10.6)
        1. Skeletal muscle - surrounded by epimysium; contains many muscle fascicles
        2. Muscle fascicle - surrounded by perimysium; contains many muscle fibers (cells)
        3. Muscle fiber - surrounded by endomysium; contains many myofibrils
        4. Myofibrils - surrounded by sarcoplasmic reticulum; contain two kinds of myofilaments (short, thread-like protein structures) organized into sarcomeres (fig. 10.5, 10.7)

Components of a sarcomere:

          1. thin myofilaments
            1. made mostly of actin protein
            2. shaped like a thin filament with loops (handles) sticking out
            3. also have two other associated proteins: troponin, tropomyosin
          2. thick myofilaments
            1. made of myosin protein
            2. shaped like a rod w/ heads (hands) sticking out
            3. hands sometimes grab actin handles
          3. the arrangement of these proteins (sarcomere) gives skeletal muscle its striated appearance: thin filaments on the outside, thick filaments on the inside


      1. Sarcomere structure (fig. 10.4, 10.5)
        1. Ends of a single sarcomere are Z-lines
        2. Actin (thin) filaments attach to Z-lines and extend toward center of sarcomere, but do not touch one another
        3. Myosin (thick) filaments 'float' in middle of sarcomere
        4. Pattern:
          1. A-band = region in which actin and myosin overlap (dark)
          2. I-band = region which has actin thin filaments only (light)
          3. H-zone = region which has myosin thick filaments only (in center of each sarcomere)
          4. NOTE: A-bands and I-bands (respectively) are the visible dark and light striations
        5. A chain of sarcomeres makes up a myofibril (think 'boxcars on a train')


    1. Skeletal muscle contraction - the sliding filament theory
      1. Recall
        1. Sarcomere is the functional (contractile) unit of muscle tissue
        2. Thin (actin) filaments run from the middle of the sarcomere, and are attached to the Z-lines; each actin filament has active sites ("handles")
        3. In between actin filaments are myosin (thick) filaments, which don't touch the Z-line; each thick filament has heads ("hands")
      2. How a sarcomere contracts:
        1. To contract (shorten) sarcomere (and therefore shorten muscle fiber), myosin "hands" must grab actin "handles", and pull
        2. This will shorten the whole sarcomere
        3. When an active site "handle" and a head "hand" are attached, it is called a crossbridge
        4. ATP is required to for the "hand" to let go of the "handle"
        5. Hand grabs the next handle, etc.
        6. Notes
          1. Length of sarcomere shortens, but
          2. Length of myofilaments does not change
          3. H-zone and I-band may disappear during contraction (fig. 10.8)
      3. Many sarcomeres contracting together shortens an entire myofibril, which will shorten the whole muscle fiber, which will shorten the whole muscle
    2. Control of skeletal muscle activity; How is muscle contraction triggered?
      1. Motor unit = one neuron + the skeletal muscle fiber(s) it innervates (fig. 10.9)
      2. Muscle fiber must have stimulus to contract
      3. Nerves (specifically, axons of motor neurons) attach directly to surface (sarcolemma) of muscle at neuromuscular junction
      4. Excitation-contraction coupling (table 10.1):

Action potential (AP) on axon Ca+2 channels open acetylcholine secreted from axon (via exocytosis) acetylcholine hits receptors on sarcolemma local AP generated on sarcolemma AP transmitted through T tubules AP reaches sarcoplasmic reticulum

      1. Contraction cycle begins: (fig. 10.10)
        1. Release of calcium from SR: AP causes calcium to be released from the ER of the muscle fiber (sarcoplasmic reticulum) into the sarcoplasm (cytoplasm)
        2. Exposure of active sites: calcium allows muscle contraction to occur by exposing the actin active sites "handles"
          1. NOTE: Under normal circumstances, the actin active sites "handles" are covered with a protein called tropomyosin (barricade), which prevents crossbridges from forming
          2. When calcium is present, it "turns off" the tropomyosin by binding to it at small fibers called troponin (on/off switch) fibers
        3. Crossbridge attachment: now the actin "handles" are available for the myosin "hands" to form a crossbridge
        4. Pivoting of myosin head: hands pull (powerstroke)
        5. Crossbridge detachment and myosin reactivation: hands release and recock for the next powerstroke (requires ATP)

      1. NOTES:
        1. Attachment, movement, and release happens many times during a single contraction
        2. When calcium in cytoplasm returns to normal levels, contraction stops
      2. Relaxation:
          1. NOTE: acetylcholine is released and is almost simultaneously destroyed by an enzyme present at the neuromuscular junction (acetylcholinesterase), so that the fiber will not be continuously stimulated
          2. AP on axon ends
          3. acetylcholine destroyed
          4. AP on sarcolemma stops
          5. Ca+2 is actively transported into SR for storage
          6. with Ca+2 removed, barrier is up; (tropomyosin) prohibits crossbridge formation
          7. actin (thin) filaments slip back into resting position
          8. stays relaxed until another AP arrives
      3. This process produces lots of heat! *shiver
      4. Clinical notes
        1. Rigor mortis
          1. No ATP being made after death, so ...
          2. Calcium can't get back into SR
          3. Myosin "hand" can't detach from actin "handle"
          4. Myosin hand also can't re-cock
          5. Causes sustained muscle contraction - stiff as a board
          6. Stiffness leaves body 12-15 hours after death, as cells begin to decompose
        2. Myasthenia gravis - autoimmune disease that causes destruction of motor end plates; lose Ach receptors
        3. Botulism - often contracted from improperly canned foods; toxin produced by bacterium prevents release of Ach; paralysis
        4. Chemicals which affect neuromuscular junction
          1. curare - blocks receptors on sarcolemma; causes asphyxiation b/c of paralysis of diaphragm
          2. some nerve gases and pesticides - inhibit acetylcholinesterase; effect?
    1. Energy for contraction (table 10.2 and fig. 10.18)
      1. ATP is source
      2. Obtaining ATP from food is slow; several reserves exist
        1. ATP stored in thick myofilaments (2 seconds)
        2. Creatine phosphate + ADP creatine + ATP (15 seconds)

Enzyme needed is CPK (creatine phosphokinase); CPK spills out of damaged muscle cells; can be used diagnostically

        1. Break down glycogen


glycogen (hydrolysis) glucose (glycolysis) pyruvate (fermentation) lactic acid + ATP

Krebs cycle and electron transport system

(requires O2)

CO2 + H2O + lots of ATP



        1. Finally, fatigue due to lack of ATP and buildup of lactic acid
        2. 'Oxygen debt' built up; heavy breathing reverses fermentation and sends it down Krebs cycle and ETS; ATP replaced
        3. Of energy released as glucose is broken down, only about 40% is used for muscle contraction; rest released as heat


    1. All-or-none law of skeletal muscle contraction
      1. A muscle fiber either contracts to its fullest extent or does not contract at all

"Fullest extent" may be modified by fatigue (lack of nutrients or accumulation of wastes)

      1. How is amount of tension in a muscle controlled?
        1. Motor unit summation
          1. Recruitment = how many fibers contract; determines the degree of muscle contraction
          2. The # of muscle fibers contracting depends on the number of motor neurons firing
          3. The number of motor neurons firing is determined by the central nervous system
          4. Muscles are divided into motor units (groups of muscle fibers working together)
            1. Precise movements - 10 muscle fibers innervated by one neuron
            2. Gross movements - 500 muscle fibers innervated by one neuron
          5. In order to increase the flex of a muscle, more motor units are recruited = motor unit summation
        2. Temporal summation
          1. a muscle is not allowed time to completely relax between contractions
          2. the first contraction and the second contraction are added to one another
    1. Kinds of contraction (depends upon kind of stimulus) REVIEW ON YOUR OWN
      1. Twitch (fig. 10.10)
        1. Rapid, jerky response to a single stimulus
        2. Does not normally occur in body
        3. Spasmodic twitching made involuntarily by muscles usually under voluntary control = tic
      2. Incomplete tetanus (fig. 10.13b)
        1. Stimuli come so rapidly that the muscle can only partially relax b/w stimuli
        2. Results in sustained contraction
      3. Complete tetanus (fig. 10.13c)
        1. Stimuli come faster still
        2. No relaxation at all
      4. Other terms
        1. Tetanus
          1. an infectious disease with intermittent spasms and convulsions
          2. toxin from bacteria growing in dirty wound alters the transmission of the nerve impulse to the muscle fibers
          3. sometimes called 'lockjaw'
        2. Spasm - sudden, involuntary contraction of short duration
        3. Cramp - painful, involuntary, complete tetanic contractions of a group of muscles
        4. Charleyhorse
          1. result of injury; inflammation of muscles and connective tissue in thigh resulting in pain & muscle spasms
          2. same condition in lower back = lumbago
      5. Tonic contraction (tonus or tone)
        1. Sustained, partial contraction of portions of muscle in response to stretch receptors
        2. At any given time, some fibers are contracting, while others are relaxed. Tightens muscle, but doesn't produce movement. Even relaxed muscles have tone.
        3. Motor units alternate firing, and relieve each other. Tonus can be maintained for a long time
        4. Essential for maintaining posture
        5. A muscle w/ less than normal tone = flaccid
          1. damage to nerve - muscle eventually replaced w/ connective tissue; irreversible
          2. disuse atrophy - bedridden or cast; can be reversed after short durations (4 months); lose muscle after 2 years
    2. Muscle growth
      1. In embryo, mitosis occurs forms many small (uninucleated) muscle cells cells fuse to form long, multinucleated muscle fibers (syncytium) (fig. 10.2)
      2. Once the cells have fused, fibers cannot undergo mitosis
      3. At birth, child has all the muscle fibers he will ever have
      4. Increase in size of muscle results from increasing length and diameter, not from more fibers
        1. In adult, muscles hypertrophy only if contracted to 75% maximum tension
          1. only a few contractions at a time are necessary
          2. work out every other day; muscles grow during rest and recovery
          3. increases # of myofibrils and myofilaments, not # of fibers
          4. mostly white fibers (fast-twitch) that enlarge (little myoglobin, make ATP anaerobically, but large and strong) fig. 10.19b
        2. Weak activity (<75% max tension), even if prolonged, will not result in hypertrophy
          1. red fibers (slow-twitch) respond (make ATP aerobically) fig. 10.19a
          2. develop more capillaries, more myoglobin, more mitochondria
          3. results in greater endurance; fatigue resistant, if oxygen available
      5. Repair of skeletal muscle (and cardiac muscle) is usually with scar tissue (process = ___________________)
      6. Satellite cells
  1. Cardiac muscle tissue (fig. 10.21)
    1. Main component of heart wall
    2. Striated but involuntary
    3. Similarities to skeletal muscle tissue
      1. Arrangement of actin and myosin
      2. Sarcoplasmic reticulum
      3. T-tubules
    4. Differences
      1. Shorter fibers w/ one nucleus per fiber
      2. Branched fibers which join w/ one another to form network
      3. Thickening of sarcolemma b/w ends of fibers (intercalated disks). Disks have:
        1. Desmosomes which add strength to heart muscle
        2. Gap junctions through which ions pass from cell to cell; allows direct transmission of AP through entire network of muscle fibers
        3. Energy for contraction is produced almost entirely aerobically (needs constant supply of oxygen)
      4. Two networks
        1. Upper chambers (_____________) form a network
        2. Lower chambers (_____________) form a network
        3. One fiber in one network stimulated, stimulus spreads, and whole network contracts together
      5. Source of stimulation
        1. Each skeletal muscle fiber must receive an impulse to contract
        2. Cardiac muscle fibers can contract w/o a nerve impulse; depolarize spontaneously
        3. Lump of specialized muscle tissue in heart wall (sinoatrial - SA - node)
        4. 72X/minute, SA node sends out an electrical impulse that spreads over the network and stimulates contraction
        5. Autonomic (involuntary) nerves to heart can speed up this rate of contraction or decrease it
  2. Smooth muscle tissue (fig. 10.22)
    1. Nonstriated and involuntary
    2. Cells small, spindle-shaped, uninucleated
    3. Contain actin and myosin, but filaments are not so orderly (no sarcomeres, no striations)
    4. Can contract well even when stretched (plasticity)
    5. Types
      1. Visceral (single-unit)
        1. Walls of hollow visceral organs (stomach, intestines, bladder, uterus)
        2. Cells bound together in sheets; contract as a unit
          1. tends to depolarize spontaneously; affected by hormones, stretching, etc.
          2. gap junctions allow AP to move from cell to cell
          3. AP moves slowly; contraction occurs in waves (peristalsis)
      2. Multiunit
        1. Walls of blood vessels, walls of airways, around pupil, arrector pili
        2. Each cell has its own nerve supply, from the autonomic (involuntary) division of the nervous system; impulse does not spread from cell to cell