Muscle - Anatomy and Physiology


Types of muscle tissue


  • Striated 
  • Works mainly in voluntary manner 
  • Supported by connective tissue
  • Can be controlled by somatic division of nervous system


  • Striated 
  • Stimulated by instrinsic conduction system and autonomic motor neurons 
  • Involuntary 
  • Autorhythmicity 


  • Lines walls of hollow internal organs 
  • Inervated by autonomic motor neurones
  • Involuntary
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Skeletal muscle

Consists of individual muscle fibres bundle into fascicles, surrounded by extensions of fascia.

3 layers of connective tissue:

  • epimysium 
  • perimysium 
  • endomysium 

T tubules tunnel in from surface toward centre of each muscle fibre. Muscle action potentials travel along sarcolemma and through T tubules, quickly spreading.

Sarcoplasm includes glycogen for synthesis of ATP and myoglobin, which binds to oxygen.

Image result for skeletal muscle fibre (

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Excitation-Contraction Coupling

  • Action potentials are sent along motor neurons to muscle fibres. Action potentials travel along sarcolemma and down T tubules.
  • Action potentials causes Ca2+ channels to open in membrane of sarcoplasmic reticulum. Ca2+ released.
  • As SR surrounds myofibrils, a lot of Ca2+ is released throughout muscle fibre all at the same time.
  • Ca2+ combine with troponin and chane it's configuration. Causes tropomyosin to move, revealing myosin-binding sites on actin filaments.
  • Myosin heads bind to actin filaments and process of contraction can begin.
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Contraction cycle

  • ATP hydrolysis: myosin head includes ATP binding sites and an ATPase; this hydrolysis reaction energises myosin head. ADP and Pi remain attached to head.
  • Attachment of myosin to actin to form cross-bridges: energised myosin head attaches to myosin-binding site and releases phosphate group.
  • Power stroke: Site on cross-bridge, where ADP is still bound to, opens and cross-bridge rotates to release ADP. Force generated as it rotates towards centre of sarcomere, sliding thin filament past thick filament towards M-line.
  • Detachment of myosin from actin: cross-bridges remain attached to actin until it binds to another ATP. As another ATP binds, myosin head detaches from actin
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Muscle fibre types

There is:

  • slow oxidative
  • fast oxidative-glycolytic
  • fast glycolytic 

Most skeletal muscle are a mixture of all three.

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  • Sarcoplasmic reticulum encircles each myofibril. 
  • In a relaxed muscle, SR stores Ca2+; released to trigger muscle contraction.
  • Within myofibrils are smaller proteins - myofilaments/filaments.
  • Both thin and thick filaments directly involved in contractile process.
  • Arranged in compartments called sarcomeres.
  • Z-discs separate sarcomere from each other.
  • Dark middle part of sarcomere is A band, extends entire length of thick filaments.
  • I band is lighter, less dense area, containing rest of thin filaments.
  • Z discs passes through centre of I band.
  • Narrow H zone in centre of each A band contains thick filaments.
  • M line supports thick filaments at centre of H zone.
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Image result for sarcomere electron micrograph labelled (

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Contractile proteins

Myosin in thick filaments function as motor proteins.

Myosin tail points towards M line in centre of sarcomere; tails of neighbouring myosin molecules lie parallel to one another, form shaft of thick filament.

Myosin heads project outward from shaft in spiraling fashion, each extending 1 of 6 filaments surround each thick filament.

Thin filaments are anchored to Z discs; main component is actin.

Actin filament is twisted into a helix. Each actin molecule has a myosin-binding site.

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Regulatory proteins

Tropomyosin and toponin apart of thin filaments.

In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover myosin-binding sites.

Tropomyosin strands helped into place by troponin. Ca2+ bind to troponin; causes change in shape so tropomyosin can move away and allow muscle contraction to begin.

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Structural proteins

Titin molecules span half a sarcomere from Z disc to an M line; connects a Z disc to M line, helping stabilise position of thick filament. Titin accounts for much of elasticity and extensibility of myofibrils. 

Z discs contain alpha actinin; bind to actin and titin.

Myomesin form M line; bind to titin and connect to adjacent thick filaments to one another.

Nebulin is wrapped around thin filaments; helps anchor them to 2 discs and length.

Dystrophin links thin filaments to integral membrane proteins of sarcolemma.

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Cardiac muscle

  • Has same arrangmen as skeletal muscle, but also intercalated discs.
  • Allows rapid transport of ions and therefore electricitial activity between cells, resulting in almost simultaneous contraction throughout muscle.

Image result for cardiac muscle fibre (

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Smooth muscle

  • Contractions start more slowly and last longest.
  • Can shorten and stretch to greater extent.
  • Ca2+ enter smooth muscle cells slowly - as there are no T tubules.
  • Also leave cells slowly resulting in prolonged smooth contraction.

Image result for smooth muscle fibre (

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