7.1.2 Muscle Fibres

• 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
  • Each muscle fibre is surrounded by a cell surface membrane
  • Each muscle fibre 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

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

Myofibrils

• 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 line

Myofibrils Parts & Descriptions Table

The structure of a myofibril

• 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 flow 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