Muscles

• Locomotion


  Moving part of an organism
• Basic Principles of Locomotion

  • Must be propelled in right direction
  • Support against medium
  • Stable at end of movement
• Basic Principles of Movement

  • Overcome friction + resistance of medium
  • Requires energy expenditure
  • Chemical kinetic energy
  • Contractile tissue associated with moving parts
  • Co-ordinated by control mechanisms
  • Exerts downwards + backwards force on substratum against friction, resistance & gravity
• Muscle Tissue


  • Composed of elongated, contractile cells
  • Elastic, stretch + regain original size
  • Well-supplied with blood, O2 + nutrients
  • Amount of blood reaching muscles adjustable
• Smooth Muscle


  • Unstriated/unstriped
  • Involuntary; controlled by autonomic nervous system
  • Spindle shaped, unbranched
  • 1 nucleus per muscle fibre cell
  • Contracts and fatigues slowly
  • Found in walls of organs (e.g. intestines) + arteries & aids passage of material through them
• Cardiac Muscle


  • Has striations due to actin/myosin arrangement
  • Myogenic / self-stimulating
  • Rate controlled by autonomic (sympathetic & parasympathetic) nervous system
  • Does not fatigue
  • Fast + coordinated contractions due to electrical synapses at intercalated discs
  • Intercalated discs have gap junctions + desmosomes
  • Branched structure reinforcement
  • 1 nucleus per cell
  • Found only in heart to pump blood
• Skeletal / Striated Muscle

  • Striated
  • Unbranched; elongated cylindrical shape
  • Multinucleated fibres
  • Contract + fatigue quickly
  • Voluntary; controlled + innervated by somatic nervous system
  • Found attached to bone
  • Mainly concerned with locomotion + voluntary movement; sometimes have role in involuntary movement e.g. shivering
• Ligaments


  Band/chord of strong, fibrous connective tissue which attach 2 movable bones
• Tendons


  Chord of dense, fibrous connective tissue connecting muscle to bone
• Point of Origin


  Fixed, relatively non-movable part of skeleton
• Insertion


  Movable part of skeleton, moves during muscular contraction
• Antagonistic Muscles


  • Bring about opposite effect/movement in bone/joint
  • When one contracts, the other relaxes, except to maintain posture (both contract)
  • Flexor muscles contract & bend/flex the joint, making angle between bones smaller
  • Extensor muscles contract & extend the joint, making angle between bones larger
• Flexion makes bones come closer together
• Extension separates/moves bones apart
• Muscle Fibre


  • Surrounded by sarcolemma, contains many multinucleated myofibrils
  • Myofibrils contain series of sarcomeres with specific arrangement of actin & myosin
  • Arranged parallel to each other and have cylindrical shape
  • Surrounded by sarcolemma; sarcoplasm contains many nuclei, sarcoplasmic reticulum + myofibrils
  • Myofibrils run parallel to each other within sarcoplasm of muscle fibre
  • Myofibrils create striated effect due to arrangement of actin + myosin
  • Each muscle has its own abundant blood + nerve supply to produce ATP
• Sarcoplasmic Reticulum


  • Specialised as SER, surrounds each myofibril and stores Ca2+
  • Near Z-lines, there are infoldings of sarcolemma = T-tubules
• Sarcomere


  • Repeating units of myofibrils
  • Consists of overlapping filaments of actin & myosin which create cross-striated appearance when aligned with adjacent sarcomeres
  • Largest protein in body = titin, which runs from Z-line to Z-line and holds myosin bundles in centred position + prevent stretching of sarcomere when muscle is relaxed
• Z-lines

  Anchor actin filaments in place
• I-band

  Light band, only thin actin filaments present
• A-band

  Dark band, appear due to overlapping myosin filaments + actin
• H-zone

  Slightly lighter region in centre of dark A-band since no actin, no overlap, darker than I-band since myosin = thicker
• M-line

  Centre of A-band, contains proteins which maintain regular arrangement of myosin
• Myosin


  • 2 polypeptide chains spirally arranged around each other
  • 2 globular heads with actin binding site + ATP binding site
  • Myosin filaments made of many myosin molecules parallel to each other, with heads projecting out at intervals
• Actin


  • 2 helical strands of globular actin molecules twisted around each other, forming fibrous strand (F-actin)
  • 2 tropomyosin strands longitudinally wrapped around, cover actin-myosin binding sites
  • Troponin bound to tropomyosin at intervals (globular)
• Excitation + Action Potential

  1. Wave of depolarisation/AP conducted along axon

  2. On reaching synaptic knob, AP causes voltage-gated Ca2+ channels to open, allowing entry of Ca2+ into pre-synaptic cell

  3. Ca2+ helps ACh-containing vessicles fuse with pre-synaptic membrane, causing release of ACh into synaptic cleft by exocytosis

  4. Binding of ACh to receptors on motor end-plate causes ligand-gated Na+ channels to open

  5. This causes depolarisation/EPP in muscle fibre which is propagated along sarcolemma to T-tubules
• Transverse tubules

  • Run vertically through muscle fibres and are in close association with sarcoplasmic reticulum
  • When AP conducted along T-tubule, causes depolarisation of S.P.R. membrane, opening voltage-gated Ca2+ channels on S.P.R. membrane, allowing Ca2+ to leak out of S.P.R. into sarcoplasm
• Sarcoplasmic Reticulum


  • Stores Ca2+
  • At rest, Ca2+ is actively (using ATP) pumped into sarcoplasmic reticulum, while voltage-gated Ca2+ channels on SPR membrane are closed, [Ca2+] in S.P.R. = ↑↑
• Excitation-Contraction Coupling


  Release of Ca2+ into sarcoplasm brings about muscular contraction
• Role of Ca2+


  • Ca2+, once released into sarcoplasm, can bind to troponin
  • At rest, tropomyosin molecules on actin filaments cover the myosin-binding sites on actin
  • When Ca2+ binds to troponin, change in 3° structure occurs which moves troponin + tropomyosin, exposing the binding sites for myosin on actin
  • Now that binding sites are uncovered, activated myosin heads can bind to actin forming cross bridges
• Activation of Myosin


  • For myosin to be activated, ATP must be available to bind to myosin head
  • When ATP binds to myosin head, ATPases hydrolyse it into ADP + Pi = activation of myosin
• Cross-Bridge Formation


  1. Activated myosin head fits into exposed binding site on actin (thanks to Ca2+) to form cross-bridge

  2. Cross-bridge strengthened by release of Pi, initiating power stroke
• Power Stroke


  1. ADP is released

  2. Causes myosin head to bend (power stroke)

  3. Causes actin filaments to slide inwards towards M-band (centre of sarcomere) - sliding filament
• Cross-Bridge Detachment


  1. Another ATP molecule binds to myosin head

  2. Causes conformational change which causes actin-myosin cross-bridge to detach
• As muscle contracts, filaments slide, sarcomere shortens + band pattern changes
• • A-band stays same length/size (myosin does not get shorter due to more overlap)
  • I-band get shorter (slide + more overlap)
  • Z-lines move closer to each other
• Reactivation of Myosin


  To reactivate itself, myosin head ATPases hydrolyse ATP to ADP + Pi, an exergonic process which releases energy for return myosin to its original position where it can again form a new cross-bridge, so long as Ca2+ is still present in sarcoplasm
• Cycle can repeat itself over + over again so long as Ca2+ + ATP available
• Once excitation stops, [Ca2+] in sarcoplasm decreases since voltage-gated Ca2+ channels close again, causing tropomyosin to cover binding sites again, stopping contractions
• Energy Supply for Skeletal Muscles

  • ATP is essential for muscle contractions
  • Main source of energy for muscles = glucose from blood, glycogen stores, fatty acids
  • O2 generally supplied by Hb, muscles also have their own store of O2 in myoglobin
  • Resting muscle = ↓ ATP levels quickly used up when active + must be restored until rate of aerobic resp. ↑
  • Phosphocreatine used - donates P, to ADP forming ATP, only for short-term (=1min) + eventually must be replenished
  • When activity level ↑, O₂ supply becomes insufficient, switch to anaerobic respiration + create O2 debt, anaerobic resp. causes lactic acid accumulation = muscle fatigue + cramps
  • Tolerance + endurance built up by training