Ch 9 - Muscles and Muscle Tissue (1)

nearly half of body's mass; Transforms chemical energy (ATP) to directed mechanical energy --> exerts force

• 3 types:
• skeletal; cardiac; smooth

• prefixes for muscle tissue and muscle cells:
• myo; mys; sarco

• organs attached to bones and skin
• elongated cells called muscle fibers (single cell within muscle tissue)
• striated
• vountary
• contract rapidly; tire easily; powerful
• require nervous system stimulation for contraction

• only in heart; bulk of heart walls
• striated
• con contract without nervous system stimulation
• involuntary

• in walls of hollow organs (stomach, urinary bladder, airways)
• not striated
• con contract without nervous system stimulation
• involuntary

• Excitability (responsiveness or irritability): ability to receive and respond to stimuli
• contractility: ability to shorten forcibly when stimulated
• extensibility: ability to be stretched
• elasticity: ability to recoil to resting length

• 1) movement of bones or fluids (blood)
• 2) maintaining posture and body position
• 3) stabilizing joints
• 4) heat generation (especially skeletal muscles)

- additional functions= protects organs, forms valves, controls pupil size, causes goosebumps

• each muscle is served by one artery, one nerve, and one or more veins
• (enter/exit near central part and branch through connective tissue sheaths; every skeletal muscle fiber supplied by nerve ending that controls its activity; huge nutrient and oxygen need; generates large amount of waste)

• connective tissue sheaths contain support cells and reinforce the skeletal muscle
• From external to internal:
• Epimysium = dense irregular connective tissue surrounding entire muscle; may blend with fascia (sheet of fibrous connective tissue surrounding muscle)
• Perimysium = fibrous connective tissue surrounding fascicles (groups of muscle fibers)
• Endomysium = fine areolar connective tissue surrounding each muscle fiber (muscle cell)

• attach in at least 2 places:
• -insertion = movable bone
• -origin = immovable (less movable) bone

• attachments direct or indirect:
• - direct = epimysium fused to periosteum of bone or perichondrium of cartilage
• - indirect = connective tissue wrappings extend beyond muscle as ropelike tendon (biceps brachii muscle) or sheet-like aponeurosis (palmaris longus muscle)

• densely packed, rodlike elements of skeletal and cardiac muscle cells
• ~80% of cell volume
• contain sarcomeres - contractile units (sarcomeres contain myofilaments)
• exhibit striations - perfectly aligned repeating series of: dark A bands and light I bands

• H zone: lighter region in midsection of dark A band where filaments do not overlap
• M line: line of protein myomesin bisects H zone
• Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects  to one another
• Thick filaments: run entire length of an A band
• Thin filaments: run length of I band and partway into A band
• Sarcomere: region between two successive Z discs

• smallest contractile unit (functional unit) of muscle fiber
• align along myofibril like boxcars of a train
• contains A band with 1/2 I band at each end
• composed of thick and thin myofilaments made of contractile proteins

orderly arrangement of actin and myosin myofilaments within sarcomere

• - actin myofilaments = thin filaments
• extend across I band and partway in A band; anchored to Z discs
• - myosin myofilaments = thick filaments
• extend length of A band; connected at M line

• composed of protein myosin
• each composed of 2 heavy and 4 light polypeptide chains
• -- myosin tails contain 2 interwoven, heavy long polypeptide chains
• -- myosin heads contain 2 smaller, light polypeptide chains that act as cross bridges during contraction (binding sites for actin of thin filaments; binding sites for ATP; ATPase enzymes)

• twisted double strand of fibrous protein F actin
• F actin (polypeptide chain) consists of G (globular) actin subunits
• G actin (monomer) bears active sites for myosin head attachment during contraction
• Tropomyosin and Troponin are regulatory proteins bound to actin

• elastic filament: composed of protein titin; holds thick filaments in place; helps recoil after stretch; resists excessive stretching
• dystophin: links thin filaments to proteins of sarcolemma
• nebulin, myomesin, C proteins bind filaments of sarcomeres together; maintain alignment

• network of smooth endoplasmic reticulum surrounding each myofibril (most run longitudinally)
• pairs of terminal cisternae form perpendicular cross channels
• functions in regulation of intracellular Ca²⁺ levels (stores and releases Ca²⁺)

• continuations of sarcolemma
• lumen continuous with extracellular space
• increase muscle fiber's surface area
• penetrate cell's interior at each A band - I band junction
• associate with paired terminal cisterns to form triads that encircle each sarcomere

• T tubules conduct impulses deep into muscle fiber; every sarcomere
• integral proteins protrude into intermembrane space from T tubule and SR cistern membranes - act as voltage sensors
• SR foot proteins: gated channels that regulate Ca2+ release from SR cisterns

• in relaxed state, thin and thick filaments overlap only at ends of A band
• myosin heads bind to actin; sliding begins
• cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere (causes shortening of muscle fiber; pulls Z discs toward M line -- I bands shorten; Z discs closer; H zones disappear; A bands move closer- length stays same)
• generation of force
• does not necessarily cause shortening of fiber
• shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

• sliding filament model of contraction:
• - during contraction, thin filaments slide past thick filaments --> actin and myosin overlap more
• - occurs when myosin heads bind to actin --> cross bridges

• Activation (at neuromuscular junction)
• must be nervous system stimulation -- voluntary
• must generate action potential in sarcolemma

• Excitation-contraction coupling
• action potential propogated along sarcolemma
• intracellular Ca2+ levels must rise briefly

• situated midway along length of muscle fiber
• axon terminal and muscle fiber separated by gel-filled space called synaptic cleft
• synaptic vesicles of axon terminal contain neurotransmitter acetylcholine (ACh)
• junctional folds of sarcolemma contain ACh receptors
• NMJ includes axon terminals, synaptic cleft, junctional folds

• skeletal muscles are stimulated by somatic motor neurons
• axons of motor neurons travel from central nervous system via nerves to skeletal muscle
• each axon (nerve fiber) forms several branches as it enters muscle
• each axon ending forms neuromuscular junction with single muscle fiber

• 1) AP arrives at axon terminal
• 2) voltage-gated Ca2+ channels in axon terminal open. Ca2+ moves down electochemical gradient
• 3)Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis
• 4) ACh diffuses across synaptic cleft to muscle -- binds to its receptors on the sarcolemma
• 5) ACh binding to ion channels opens them (allows simultaneous passage of Na+ into muscle fiber and K+ out of muscle fiber. More Na+ ions enter than K+ ions exit, which produces local change in membrane potential called the end plate potential)
• 6) Extracellular ACh is broken down (ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from junction)