Skeletal Muscle Physiology – Structure & Types Of Muscle Fibers

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Skeletal Muscle Physiology – Structure & Types Of Muscle Fibers

SKELETAL MUSCLE PHYSIOLOGY – STRUCTURE & TYPES OF MUSCLE FIBERS

  • Skeletal muscle constitutes nearly 40% of total body mass

COMPONENTS OF SKELETAL MUSCLE:

  • Made up of numerous muscle fibers/muscle cells.
  • “Sarcolemma”:
  • Cell membrane surrounding each muscle cell/fiber.
  • Contains thousands of myofibrils.
  • Myofibril consists of series interlocking actin & myosin myofilaments.
TYPES OF MUSCLE PROTEINS:
  • Four major muscle proteins involved: 
  • Actin.
  • Myosin.
  • Tropomyosin.
  • Troponin.

Additional structural proteins in skeletal muscles:

  • Actinin.
  • Titin.
  • Desmin.

CHARACTERISTICS OF SKELETAL MUSCLE FIBER:

  • Well-developed cross striation.
  • Produced by refractive index difference in different myofibrils parts.
  • Mixture of isotropic & anisotropic areas to polarized light.
  • Referred respectively as “I” bands & “A” bands.
  • Hence, visible as light & dark bands of myofibrils, respectively.

STRUCTURE OF SARCOMERE:

  • Z line – Divides each myofibril into compartments.
  • Sarcomere – 
  • Portion of myofibril between two Z lines.
  • Basic contractile unit of muscle

Filament types within sarcomere:

  • Thin filament (made of actin)
  • Thick filament (made of myosin).

Actin/thin filaments:

  • Attached to Z line at their periphery.
  • I & A bands are seen alternately.
  • A band – Entire length made of thick/myosin filament.

H Zone –

  • Comparatively lighter zone in middle of A band.
  • Due to non-overlapping of actin over myosin filament in central portion.
  • Hence, is light.
  • Absence of actin filament is characteristics feature.
  • H-zone disappears on muscle contraction.
  • Due to complete actin overlapping on myosin

MUSCLE DURING CONTRACTED STATE:

  • Two Z-lines come closer.
  • H-Band disappears.
  • Decreased sarcomere length.
  • I band-width decreased.
  • Unchanged A band-width. 

SARCOLEMMA: 

  • Cell membrane of muscle cell/fiber.
  • Lipid Bilayer.
Components:
  • Lipid – 
  • Phospholipids
  • Cholesterol

Proteins – 2 Types:

  • Integral/Transmembrane proteins – 
  • Dystroglycan
  • Sarcoglycans (SG)
  • Caveolin-3
  • Integrins (β, α).

Peripheral/surface proteins: 

  • Dystrophin
  • Dysferlin
  • Calpain.

Carbohydrates:

  • Associated always with lipid/protein.
  • Eg: Glycolipids & glycoproteins.

SARCOTUBULAR SYSTEM:

  • Sarcoplasm/cytoplasm of skeletal muscle contains sarcotubular system.
  • Consists of,
  • T system/T-tubules.
  • Sarcoplasmic reticulum.

T-system contains –

  • Transverse tubules. 
  • Infoldings of cell membrane/sarcolemma at each junction of A & l -bands
  • Thus two T-tubules in each sarcomere.
  • T-tubules allows penetration of electrical membrane discharge to inner core of muscle cell.

“L-tubules”:

  • Sarcoplasmic smooth surfaced endoplasmic reticulum extending between two T-tubules in longitudinal fashion.
  • Has high calcium concentration.
  • Concerned with sarcomeric calcium transport.

Two Portions: –

“Terminal cistern” –

  • Dilated portions at both ends close to T-tubules.
  • Stores maximum calcium – Hence, “Storehouse of calcium ions”

“Triad” – 

  • Central longitudinal part with 2 cisterns on both ends.

 

“Foot processes”:
  • Connects T-tubules with terminal cisterns.
  • Mainly, are Dihydropyridine receptors (DHPR).
  • Interact with Ryanodine receptors (RyR) of terminal cisterns.
  • Ryanodine receptors function as calcium channels.
  • DHPRs-RyR interaction leads to calcium ions release from cisternae.

Conditions associated:

  • Gene mutation on Ryanodine receptor causes malignant hyperthermia.
  • Due to excessive uncontrolled calcuim release.

TYPES OF MUSCLE FIBERS:

  • Two fibers types:
  • Type I/red fibers
  • Type II/white fibers.

Motor unit contains only one muscle fibers type:

  • I.e., All motor unit fibers are either type-I or type II.
  • Hence, named accordingly.
  • CHARACTERISTICS OF MUSCLE FIBER TYPES:
Parameters/Characteristics  Type I muscle fiber Type II muscle fiber
Difference according to fiber type
  • Red – 
    • Due to ↑ed myoglobin & mitochondrial content.
  • White – 
    • Lack myoglobin & have few mitochondria.
Subtypes

– Nil – 

Type IIA & Type IIB

  • IIA – Oxidative & glycolytic.
  • IIB – Glycolytic.
  • Usually type IIB fibers refer type II fibers.
Other names

1. “Slow-twitch” fibers:

  • Due to small fiber length & diameter.
  • Maintain relatively sustained contraction.

2. “Slow motor units”

  • Have early recruitment.

1. “Fast-twitch” fibers – 

  • Short duration of twitch.
  • Due to bigger diameter & ↑ed myosin & actin filaments.
  • Generate greater tension comparatively.
2. Type IIa/”Fast Fatigue Resistant” (FR) units:
  • Intermediate fatigability.
3. Type IIb/“Fast fatigable units”:
  • Late recruitment threshold
Metabolism
  • Aerobic.
  • Slow oxidative.
  • Anaerobic.
  • Fast glycolytic.
Glycolytic capacity &glycogen content

Low

High
Sarcoplasmic reticulum Normal Extensive
Blood supply High  Normal 
Myosin ATPase activity Low  High 
Conduction velocity Slow Fast
Contractile properties Slow time to peak tension
  • Type IIa – Fast time to peak tension.
  • Type IIb – Fastest time to peak tension.
Type of muscle 

Red muscles – 

  • In muscles required for prolonged contraction.
  • Also with muscles maintaining posture.
    • Back muscles (At back of hip)
    • Gluteus muscles .
    • Calf muscles.

White/Pale muscles – 

  • Muscles helpful in rapid contractions & finer movements.
  • Hand & extraocular muscles.

Exam Question

SKELETAL MUSCLE PHYSIOLOGY – STRUCTURE & TYPES OF MUSCLE FIBERS
  • Skeletal muscle constitutes nearly 40% of total body mass.
  • “Sarcolemma” – Cell membrane surrounding each muscle cell/fiber.
  • Myofibril consists series of interlocking actin & myosin myofilaments.
  • Presence of well-developed cross striation – Characteristics of skeletal muscle fiber.

Sarcomere – 

  • Portion of myofibril between two Z lines.
  • Basic contractile unit of muscle

Two types of sarcomeric filaments – 

  • Thin filament (made up of actin)
  • Thick filament (made up of myosin).

H Zone – 

  • Comparatively lighter zone in middle of A band.
  • Due to non-overlapping of actin over myosin filament in central portion.
  • Absence of actin filament.
  • Disappears on muscle contraction.

Changes in muscle during contraction:

  • Two Z-lines come closer 
  • H-Band disappears.
  • I bandwidth decreased.
SARCOLEMMA: 
Has two protein types:
  • Integral/Transmembrane proteins – 
  • Dystroglycan.
  • Sarcoglycans (SG).

Peripheral/surface proteins: 

  • Dystrophin.
  • Sarcotubular T-system contains “Transverse tubules”
  • Infoldings of cell membrane/sarcolemma.
  • Allows penetration of electrical membrane discharge to inner core of muscle cell.
  • Sarcoplasmic reticulum contains high calcium concentration.
  • Is concerned with sarcomeric calcium transport.
  • “Triad” – Central longitudinal part with two cisterns at both ends.
  • Terminal cistern is where maximum calcium is stored. Hence, “Storehouse of calcium ions”.
  • Foot processes are mainly of Dihydropyridine receptors (DHPR).
  • Ryanodine receptors function as calcium channels.
  • Gene mutation on Ryanodine receptor encoding causes malignant hyperthermia.
  • Due to excessive uncontrolled calcuim release.

Characteristics of:

  • Type I/red fibers
  • Type II/white fibers.
Parameters/Characteristics  Type I muscle fiber Type II muscle fiber
Difference according to fiber type
  • Red – 
    • Due to ↑ed myoglobin & mitochondrial content.
  • White – 
  • Have few mitochondria.
Subtypes

– Nil – 

Type IIA & Type IIB

  • Oxidative & glycolytic – IIA.
  • Glycolytic – IIB.
  • Type II fibers usually refer Type IIB fibers.
Other names

1. “Slow-twitch” fibers:

  • Due to small fiber length & diameter.
  • Maintain relatively sustained contraction.

 
  • “Fast-twitch” fibers 
    • Due to bigger diameter & ↑ed myosin & actin filaments.
  • “Fast fatigable units”/Type IIb
    • Late recruitment threshold
Glycolytic capacity & glycogen content

Low

High
Sarcoplasmic reticulum Normal Extensive
Blood supply High  Normal 
Myosin ATPase activity Low  High 
Type of muscle 
  • Red muscles – 
  • Seen in,
    • Back muscles.
    • Gluteus muscles
    • Calf muscles.
  • White/pale muscles – 
  • Seen in hand muscles & extraocular muscles.
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