Voluntary and involuntary muscles

Key terms

Myofibrils: long cylindrical organelles found in muscle which are made of protein and specialised for contraction

Sarcomere: functional unit of myofibril

Types of muscles

• Skeletal muscle

  • Bulk of body muscle tissue

  • Responsible for movement

• Cardiac muscle

  • Found in heart

  • Myogenic

    • Contract without nervous stimulus

  • Causes heart to beat in regular rhythm

• Involuntary muscle

  • Found in walls of hollow organs

  • Found in walls of blood vessels and digestive tract

Important differences in structure of different muscles

• Skeletal

  • Striated fibres

    • Show cross striations

    • Tubular and multinucleated

  • Conscious control

  • Regularly arranged arrangement

    • Muscle contracts in one direction

  • Rapid, short contractions

• Cardiac

  • Specialised striated fibres

    • Striations

    • Branched and uninucleated

  • Involuntary control

  • Cells branch and interconnect

    • Simultaneous contraction

  • Intermediate, medium length contractions

• Involuntary

  • Non-striated fibres

    • No cross striations

    • Fibres spindle shaped and uninucleated

  • Involuntary control

  • No regular arrangement

    • Contract in different directions

    • Slow, long contractions

Skeletal muscle fibres

• Bundles of muscle fibres

• Enclosed in plasma membrane

  • Sarcolemma

• Contain nuclei

• Longer than normal cells

  • Formed from many individual embryonic muscle cells fusing

  • Makes muscle stronger

• Shared cytoplasm within muscle fibre

  • Sarcoplasm

• Parts of sarcolemma fold inwards

• Help spread electrical impulses

  • Throughout sarcoplasm

• Ensures whole of fibre receives impulse

  • Contract at same time

• Lots of mitochondria

  • Provide ATP needed for muscle contraction

• Modified version of endoplasmic reticulum

  • Sarcoplasmic reticulum

  • Extends throughout muscle fibres

  • Contains calcium ions required for muscle contraction

Myofibrils

• Long cylindrical organelles

• Made of protein

  • Specialised for contraction

• Collectively powerful

• Lined up parallel

  • Provide maximum force when contracted together

• Two types of protein filament

  • Actin

    • Thinner filament

    • Consists two strands twisted around each other

  • Myosin

    • Thicker filament

    • Consists long rod-shaped fibres

    • Bulbous heads project to one side

• Dark bands

  • Appear dark

    • Presence of thick myosin filaments

  • Edges particularly dark

    • Myosin overlapped with actin

• Light bands

  • Appear light

    • Region where actin and myosin filaments don’t overlap

• Z-line

  • Centre of each light band

  • Distance between adjacent Z-lines

    • Sarcomere

  • Sarcomere is functional unit of myofibril

    • Muscle contracts and it shortens

• H-zone

  • Lighter coloured region in centre of each dark band

  • Myosin filaments present

  • Muscle contracts and it shortens

Drawing sarcomere

• Short 2 Z-lines

• Heads on myosin filaments

• Connect actin filaments to Z-line

• Label light and dark bands

• Show H-zone

Histology of skeletal muscle

• Individual muscle fibres

  • Long and thin

  • Multinucleated fibres

  • Crossed with regular pattern of red and white lines

• Highly structured sarcomeres

  • Dark (A-bands) and light bands (A-bands)

• Streaks of connective and adipose tissue

• Capillaries running in between fibres

Slow twitch fibres

• Fibres contract slowly

• Provide less powerful contractions

  • Over longer period

• Used endurance activities

  • Don’t tire easily

• Gain energy from aerobic respiration

• Rich in myoglobin

  • Red protein

  • Stores oxygen

• Rich supply of blood vessels and mitochondria

• Found in large proportions in muscles

  • Help maintain posture

Fast twitch fibres

• Fibres contract quickly

• Produce powerful contractions

  • Over short periods

• Used for short bursts of speed and power

  • Tire easily

• Gain energy from anaerobic respiration

• Pale coloured

  • Low levels of myoglobin and blood vessels

• Contain more, thicker myosin filaments

• Store creatine phosphate

  • Rapidly generates ATP from ADP in aerobic conditions

• Found in high proportions in muscles

  • Need short bursts of intense activity