Edexcel International A Level Biology

Revision Notes

7.10 The Muscle Fibre

Muscle Fibre: Structure

  • Skeletal muscle makes up the muscles in the body that are attached to the skeleton
  • Skeletal muscle is made up of muscle fibres
  • A muscle fibre is a highly specialised cell-like unit:
    • Each muscle fibre:
      • Contains an organised arrangement of contractile proteins in the cytoplasm
      • Is surrounded by a cell surface membrane
      • Contains many nuclei (multi-nucleated– this is why muscle fibres are not usually referred to as cells
  • The different parts of a muscle fibre have different names to the equivalent parts of a normal cell:
    • Cell surface membrane = sarcolemma
    • Cytoplasm = sarcoplasm
    • Endoplasmic reticulum = sarcoplasmic reticulum (SR)
  • The sarcolemma has many deep tube-like projections that fold in from its outer surface:
    • These are known as transverse system tubules or T-tubules
    • These run close to the SR and help spread electrical impulses throughout muscle fibre
  • The sarcoplasm contains mitochondria and myofibrils
    • The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
    • Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction
  • The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR
    • Calcium ions are needed for the contraction of muscle

Ultrastructure of striated muscle (1)_1Ultrastructure of striated muscle (2)_1

The ultrastructure of skeletal muscle and of a section of muscle fibre


  • Myofibrils are located in the sarcoplasm
  • Each myofibril is made up of two types of protein filament:
    • Thick filaments made of myosin
    • Thin filaments made of actin
  • These two types of filaments are arranged in a particular order, creating different types of bands and lines

Myofibrils Parts & Descriptions Table

Myofibrils parts & descriptions table

Structure of a myofibril (1)Structure of a myofibril (2)

The structure of a myofibril

Fast & Slow Twitch Muscle Fibres

  • There are two types of muscle fibres found in muscles
    • Fast twitch fibres
    • Slow twitch fibres
  • Human muscles are made up of both types of muscle fibres
  • Some muscles have higher proportions of a particular fibre type due to their different properties

Fast twitch muscle fibres

  • Fast twitch muscle fibres contract rapidly
    • The myosin heads bind and unbind from the actin-binding sites five times faster than slow twitch muscle fibres
    • Their rapid contraction-relaxation cycle means they need large amounts of calcium ions present to stimulate contraction
  • They rely on anaerobic respiration for ATP supply
  • They are suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration
  • These muscle fibres are often found in high proportions in the limbs of animals that flee a predator or hunt prey at high speeds
    • For example, the wings of a robin and legs of a cheetah
  • There are high proportions of fast twitch muscle fibres in human eyelids
    • They contract in short bursts and do not need to sustain the rapid movement
  • Fast twitch muscle fibres have fewer capillaries
    • Blood containing glucose and oxygen flow through the capillaries
    • This means they have quite a slow supply of oxygen and glucose for aerobic respiration
  • Low amounts of myoglobin are present in fast twitch muscle fibres
    • Myoglobin is a red pigment molecule that is similar to haemoglobin
    • Myoglobin functions as a store of oxygen in muscles and increases the rate of oxygen absorption from the capillaries
  • Due to this fast twitch muscle fibres appear paler in colour than slow muscle fibres

Slow twitch muscle fibres

  • Slow twitch muscle fibres contract more slowly and are suited to sustained activities like walking and perching
  • They rely on aerobic respiration for ATP
  • They fatigue less quickly due to less lactate production, making them ideal for endurance
  • These muscle fibres are often found in high proportions in the limbs of animals that migrate or stalk prey over long distances
    • For example, the wings of geese and legs of wolves
  • Human back muscles have a high proportion of slow twitch muscle fibres
    • These muscles have to contract for long periods of time in order to keep the skeleton erect when standing or sitting
  • Slow twitch muscle fibres have a denser network of capillaries
    • Blood containing glucose and oxygen flows through the capillaries
    • This means they have a short diffusion distance and a good supply of oxygen and glucose for aerobic respiration
  • High amounts of myoglobin, haemoglobin and mitochondria are present in slow twitch muscle fibres
    • This increases the rate of oxygen supply, oxygen absorption and aerobic respiration
  • Due to the high amounts of red pigment, slow twitch muscle fibres appear a dark red

Fast Twitch & Slow Twitch Muscle Fibres Table
Fast and Slow Muscle Fibres

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.