• 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