Muscle - Anatomy and Physiology

Skeletal 

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

Cardiac 

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

Smooth 

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

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.

• 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.

There is:

• slow oxidative
• fast oxidative-glycolytic
• fast glycolytic 

Most skeletal muscle are a mixture of all three.

• 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.

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.

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.

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.

• 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.

• 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.