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Muscular System

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Muscular System

	Includes organs comprised mainly of skeletal muscle tissue and connective tissue
	General functions Movement Heat production Posture Body shape

Skeletal muscle organs

	Made up of muscle fibers, nerves, blood vessels and connective tissues Muscle cells are usually called muscle fibers
	Functional characteristics of muscle fibers   pp  Excitable - respond to stimuli by fluctuation of voltage in membrane Contractile - can shorten in length "Contraction" can also refer to steadily resisting a load--not always shortening in length Extensible - can stretch (compliance) Muscles can exert force while extending, as in slowly lowering a heavy weight in your hand Elastic - can recoil to starting length after being stretched
	General principles of functional anatomy of the muscle organ Skeletal muscle organs contract (not expand) with force Muscles pull on bones Bones are levers, joints are the fulcrums   pp  Muscles that move a part are usually proximal to that part  pp  Skeletal muscle organs act in teams   pp  Antagonistic (opposing) pairs Prime mover and antagonist must coordinate contraction/relaxation Synergists (synergy = when a combined effect is greater than the expected sum of effects) Agonists (same/similar action as prime mover) Origin - attached to "stationary" bone Insertion - attached to "mobile" bone Fibrous connective tissue wraps and compartmentalizes muscle fibers   pp  Endomysium wraps single fibers Perimysium wraps bundles or fascicles "Fascicle" from fascis = gang (of fibers) and -iculus = small Epimysium wraps whole organ Combined fibrous tissue extends beyond muscle tissue on each end, forming tendons that connect to bones Aponeurosis - a tendon in the form of a broad, flat sheet Aponeuroses (plural) Tendon sheath - synovial membrane that reduces friction Required: read carpal tunnel syndrome box in Chapter 10 (p. 374) Loose fibrous connective tissue between and around muscle organs is deep fascia Hernia - "rupture" or protrusion (sticking out) through a wall Required: read section on hernias at end of Chapter 11 (p.333) Inguinal hernia - protrusion through inguinal canal     GA Femoral hernia - protrusion through femoral ring Umbilical hernia - protrusion through umbilicus (navel) Hiatal hernia - protrusion through hiatus (opening) of diaphragm     ACT

Muscle fiber structure

	Sarcolemma = plasma membrane T (transverse) tubules are inward extensions of the sarcolemma, like tunnels through a mountain  pp
	Sarcoplasm = intracellular material
	Sarcoplasmic reticulum (SR) = special form of smooth ER with Ca++ ion pumps that allow SR to store Ca++ Important principle you will use throughout life (and into the next): "cells HATE calcium" SR is up against both sides of each T tubule
	Myofibrils Cylindrical units of the cytoskeleton made up of microfilaments (= myofilaments) Sarcomeres: repeating, overlapping pattern of thin and thick [myo]filaments in the myofibril Each sarcomere extends from one Z disk to the next A Z disk (old name: Z line) is a network of fibers formed where the thin filaments are anchored together (looks like a zigzag line from the side) Thick filaments Mostly myosin, a golf-club-like protein molecule with a hinged "head" Myosin loves actin (has a binding site that "fits" actin's binding site) Myosin head has a "battery holder" for ATP, which energizes the head --allowing the head to "cock back" like a sling shot Thin filaments Mostly actin, bead-like protein molecules strung together like a string of pearls (actin returns myosin's affections) Tropomyosin is a strand of protein that blocks actin's active sites, preventing an actin-myosin rendezvous Troponin is a "glob" of protein that holds tropomyosin in its blocking position (otherwise, myosin's hinged head would simply knock tropomyosin out of the way to get to its true love, actin) Troponin has a crush on Ca++, which will "never" be fulfilled because all the Ca++ is in the SR or outside the muscle fiber (that is, troponin has an open binding site for Ca++)

Muscle fiber contraction (a love story)

	Myosin and actin want each other (can't get together because of tropomyosin and troponin)
	Troponin wants Ca++ but Ca++ is unavailable
	Scenes of the unfolding love story: Motor neuron releases acetylcholine (Ach; a neurotransmitter) at neuromuscular junction Ach induces an impulse (voltage fluctuation) in the sarcolemma Impulse travels along the sarcolemma and through the T tubules Impulse in T tubules "zaps" the SR, which suddenly releases its Ca++ Ca++ diffuses throughout sarcoplasm Troponin's dreams are realized when Ca++ binds to troponin In the heat of passion, troponin now twists around --pulling tropomyosin out of its blocking position Myosin heads can now reach actin's binding sites and, well, you know what happens then Myosin's head pulls actin toward the center of the sarcomere (then grabs another and another actin)  This causes the thin filaments to slide past the thick filaments, shortening the sarcomere "sliding filament model" of muscle fiber contraction    ANIM In the mean time, SR has recovered its wits and is again pumping Ca++ into storage Ca++ is thus pulled away from troponin, which flips back to its position and pulls tropomyosin back into its blocking position Myosin is now blocked from reaching another actin; the contraction stops
	Myosin and ATP    ANIM ATP is the energy source  pp  ATP binds to a myosin head and breaks into ADP + P Released energy is used to "cock back" the myosin head If actin is available, the myosin head will bind to actin and pull If actin is unavailable, the myosin head will wait (cocked back and ready to go) until actin becomes available A new ATP is now needed to get myosin to release from actin and cock back again At death, no ATP is available and the myosin & actin are "stuck" in place (rigor mortis = stiffness of death)
	Summary of the story    ANIM Excitation Contraction Relaxation (Excitation-contraction coupling refers to the fact that these two processed are linked [coupled])

Not required  Click here to see animations of some of these muscle concepts. 

Not required  See p. 166 in Survival Guide for Anatomy & Physiology for a summary of the "muscle love story"

Energy for contraction

Energy sources ATP - from aerobic (slow) and anaerobic (fast) respiration  pp  Myoglobin (red pigment like hemoglobin) stores extra O2 for aerobic respiration  pp   pp  Creatine phosphate (CP) - "backup battery" for quick recharging of ATP More on this (a little) later

Fiber types Slow fibers specialized for endurance; rely mainly on aerobic respiration Fast fibers specialized for quick, strong contractions; rely mainly on anaerobic respiration Specific types: Type I: slow oxidative   [red muscle]  e.g. soleus muscle Postural muscles -- slow but efficient;  fatigue resistant   Abundant capillaries, high myoglobin content Type IIa: fast oxidative  [red muscle] e.g. gastrocnemius muscle   Intermediate characteristics: moderately fast and moderately fatigue resistant   moderate capillary supply and myoglobin content Type IIx: fast glycolytic   [white muscle] e.g., eyes and fingers [also called Type IIb] Rely heavily on glycolysis; high glycogen content   Fast but fatigue easily; brief, powerful contractions Muscle organs are mix of types     FIG  pp

Muscle organ contraction

	Motor unit - group of muscle fibers all connected to the same motor neuron, thus acting as a unit   pp  Different levels of contraction in a muscle organ can result from recruitment of different numbers of motor units
	Myography produces a wave-like picture of muscle organ contraction called a myogram
	Twitch contraction = single, brief contraction in response to a single stimulus One or more motor units contracting in unison Three phases: latent, contraction, relaxation phase
	Treppe (staircase effect) Increase in the strength of contraction in the first few of a series of contractions Due to increasing warmth, more diffusion of Ca++ and other factors
	Wave summation (tetanus) = sustained contraction Relays of motor units (or groups of motor units) result in a sustained contraction Complete tetanus = no relaxation between relays Incomplete tetanus = some relaxation between relays (usually, fatigue causes some relays to "drop out" because they are tired) Most normal contractions are tetanic (not single twitches)
	Muscle tone Continuous, low-level sustained (tetanic) contraction of any (or all) muscle organ(s) --the starting point for stronger contractions Flaccidity - abnormally low tone, as in paralysis or immobility (cast) Spasticity - abnormally high tone, as in CP or Parkinsonism
	Fibrillation  Abnormal; occurs with fatigue, chemical imbalance Asynchronous, uncoordinated contractions within a muscle organ Produces no effective movement
	Spectrum of muscle organ contractions Isotonic contractions = same tension; shorter length Mobilize body parts Two types: Concentric - muscle shortens (as in lifting a load) Eccentric - muscle lengthens while contracting (as in setting down a load without dropping it) Isometric contractions = increased tension; same length Stabilize body parts (as in maintaining posture) Most muscle contractions are somewhere between two ends of spectrum
	Graded strength principle Recruitment of more or less motor units Length (generally, the greater the starting length, the greater the strength)  Metabolic condition Stretch reflexes
	Skeletal muscles contract only if stimulated
	Muscle size Atrophy - disuse atrophy is a reduction in muscle size resulting from lack of use  Hypertrophy - increase in muscle size resulting from maximal use, especially heavy-load-bearing contractions Stretching muscles can cause a rapid increase in the length of myofibrils

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