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HBIO4 > Muscles
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Structure of Skeletal Muscle

  • Skeletal muscle is joined to bone by inelastic tendons
    • Muscle contraction / pulls on tendons / bone moves
  • Each muscle is made of bundles of muscle fibres surrounded by connective tissue
  • An individual muscle fibre
    • Has many nuclei → muscle fibre develops from fusion of many cells
    • Sarcoplasm (cytoplasm) filled by parallel myofibrils
    • Sarcolemma (surface membrane) forms deep tubes (T tubules) into the sarcoplasm along its length
    • Network of membranes called sarcoplasmic reticulum (ER)

The Sliding-Filament Theory

Striations in Skeletal Muscle

  • Actin filament / thinner than myosin → lighter striations
  • Myosin filament / thicker than actin filament → darker striation
  • Distance between 2 adjacent Z lines: sarcomere / actin filament is attached to Z lines and extended into sarcomeres on either side
  • Striation of actin alone → I band
  • Striation of myosin alone → H zone
  • Length of myosin → A band
  • Central thickening of each myosin filament → M line

Actin and Myosin Filaments

  • Actin filament: 2 actin strands twisted around each other
    • Troponin-tropomyosin-actin complex blocks binding site for myosin
  • Myosin filament: bundles of myosin molecules
    • Bundle of myosin tails form a central stalk
    • Globular heads attach to specific sites on actin filaments
    • Myosin heads contain ATPase that hydrolyses ATP

Neuromuscular Junction

  • Synapse between motor neurone and muscle fibre
  • \ skeletal muscle fibres are stimulated by motor neurones
  1. Influx of Ca2+ / synaptic vesicles fuse with presynaptic membrane
  2. Release of acetylcholine (ACh) into synaptic cleft by exocytosis
  3. Neurotransmitter diffuses across cleft
  4. Binds with receptors on motor end plate (→postsynaptic membrane of muscle fibre)
  5. Depolarises sarcolemma
  6. Threshold stimulates wave of depolarisation along muscle fibre
  7. Changes permeability of sarcoplasmic reticulum to Ca2+
  8. Ca2+ move into sarcoplasm / causes contraction of myofibril

Role of ATP and phosphocreatine

Stimulation Of Muscle Fibres By The Nervous System

  • CONTRACTION → myosin heads attach to actin binding sites / form temporary cross bridges / bridges rapidly break and reform / new cross bridges form further along actin filament / causing shortening of each sarcomere
  • WHEN STIMULATION STOPS → Ca2+actively taken up by sarcoplasmic reticulum / myosin head detaches from actin / cross bridges reform / muscle relaxes
  • NO ATP AVAILABLE → cross bridges cannot detach / muscle becomes stiff / unable to relax / extreme form: rigor mortis / occurs after death

Cycle Of Events During Contraction Of A Myofibril

  • Ca2+ ions enter sarcoplasm during wave of depolarisation
  • Bind to troponin / changes shape of protein / removes block of tropomyosin / exposes actin binding sites
  • ATP binds to myosin / stimulates ATPase / RELEASES ENERGY
  • Allows myosin heads to form cross bridges with actin
  • Allows POWER STROKE: myosin head changes angle / pulls on actin filaments
    • Width of I band, H zone decrease → filaments overlap increases
    • Z lines move closer together → length of sarcomere decreases
    • No change to A band → lengths of filaments stay constant
  • Allows Ca2+ ions to be pumped back in by active transport
  • New ATP binds to myosin / allows detachment from actin
  • Myosin head changes to original position (cross bridges reform)
  • Next attachment to actin filament and power stroke can occur
    • Ca2+ and ATP required for cycle to continue

Energy In Active Muscle Cells

  • Breakdown of phosphocreatine / releases PI + energy / attach to ADP / forms ATP
    • ATP is used faster than it can be supplied by respiration
    • Phosphocreatine allows regeneration of ATP without respiration
  • Thus, Muscle cells continue exercise until slower pathways synthesis ATP
    • Breakdown of glycogen in muscle cells / aerobic respiration of glucose
    • Aerobic respiration of glucose, fatty acids from bloodstream / fatty acids last longer
  • Prolonged exercise / not enough O2 for aerobic respiration
    • Anaerobic respiration continues
    • Lactate may cause cramps

Muscles As Effectors

  • Motor neurones stimulate glands and muscles into action
  • Respond to a stimulus → are effectors

Structure, location and general properties of slow and fast skeletal muscle fibres


Fast muscle

Slow muscle

- Role in body

- Rapid, powerful movements
- Short-lasting

- Slow movement
- Long-lasting

- Diameter of fibres
- Capillaries
- Sarcoplasmic reticulum
- Mitochondria

- Large
- Few
- High
- Few

- Small
- Many
- Low
- Many (ETC, Krebs cycle)

- Speed of contraction
- Rate of pumping Ca2+

- Fast
- High

- Slow
- Slower

- ATPase activity
- Respiration
- Glycogen content
- Myoglobin content
- Resistance to fatigue

- High, split ATP quickly
- Anaerobic
- High
- Low
- Low

- Low, split ATP slowly
- Aerobic
- Low
- High
- High


Arms and legs
(running and throwing)

Back and neck
(postural muscles)

Slow muscles contain myoglobin in sarcoplasm → appears bright red