3.6.3 SKELETAL MUSCLES

• Muscles are effectors — contract in response to nervous impulses.

  3 types:

  • smooth (walls of organs)
  • cardiac (heart)
  • skeletal (movement). 

  Smooth and cardiac muscle contract without conscious control. 
• • Skeletal muscles are attached to bones by tendons. 
  • Ligaments attach bones to other bones.
• Pairs of skeletal muscles contract and relax to move bones at a joint — rigid bones act as levers, for muscles to pull against.

  Antagonistic Pairs = muscles that work together to move a bone. 

  • Agonist - contracting muscle, antagonist - relaxing muscle.
  • Bicep contracts, triceps relax - pulls bone so arm bends. Tricep contracts, bicep relaxes - pulls bone so arm straightens. 
• Skeletal Muscles

  Made up of bundles of muscle fibres (long cells). 

  • Sarcolemma = cell membrane of muscle cells
  • Sarcoplasm = cytoplasm of muscle cell
  • Transverse (t) tubules - folds of sarcolemma sticking into sarcoplasm. Help spread electrical impulses throughout sarcoplasm.
  • Sarcoplasmic reticulum - Network of internal membranes throughout sarcoplasm. Stores and releases calcium ions for muscle contraction. 
• Muscle fibres (muscle cells) have lots of:

  • Mitochondria - to provide the ATP needed for muscle contraction
  • Nuclei (multinucleate) 
  • Myofibrils - Long, cylindrical organelles made up of proteins and specialised for contraction
• Microscopic Structure -

  Blue = nuclei (multinucleate) 

  Cross-striations (alternating darker and lighter pink stripes) are the A-bands and I-bands of the myofibrils.
• Myofibrils (organelles in muscle fibres)

  Contain bundles of thick and thin myofilaments that move past each other to make muscles contract. 

  • Thick myofilaments made of the protein myosin
  • Thin myofilaments made of the protein actin

  A myofibril under an electron microscope has alternating dark and light bands. Dark bands contain the thick myosin filaments and some overlapping thin actin filaments — A-bands. Light bands contain thin actin filaments only — I-bands.
• A myofibril is made up of many short units called sarcomeres.

  • Ends of each marked with Z-line. 
  • M-line in the middle of each. Is the middle of the myosin filaments. 
  • Around M-line is H-zone - contains only myosin filaments.
• The Sliding Filament Theory of Muscle Contraction

  • Myosin and actin filaments slide over each another to make the sarcomeres contract.
  • Simultaneous contraction of many sarcomeres means myofibrils and muscle fibres contract.
  • Sarcomeres return to original length as muscle relaxes.
• Myosin filaments - have globular heads that are hinged, so they can move back and forth. Each myosin head has a binding site for actin and a binding site for ATP.
• Actin filaments - have binding sites for myosin heads, called actin-myosin binding sites. Another protein called tropomyosin is found between actin filaments. It helps myofilaments move past each other.
• Resting Muscles

  • For myosin and actin filaments to slide past each other, myosin head needs to bind to actin-myosin binding site on actin filament. 
  • In a resting (unstimulated) muscle actin-myosin binding site blocked by tropomyosin. (So myosin heads can't bind).
• Muscle Contraction 1:

  • Action potential depolarises sarcolemma.
  • Spreads down T-tubules to sarcoplasmic reticulum - releases Ca ions into sarcoplasm. 
  • Ions bind to protein attached to tropomyosin, making it change shape. 
  • Pulls tropomyosin out of actin-myosin binding site on actin filament allowing myosin head to bind.
  • Actin-myosin cross bridge formed.
• Actin-myosin cross bridge = the bond formed when a myosin head binds to an actin filament.
• Muscle Contraction 2:

  • Ca ions activate ATP hydrolase (hydrolyses ATP)
  • Energy causes myosin head to bend, pulling actin filament along.
  • Another ATP molecule provides energy to break the actin-myosin cross bridge, so myosin head detaches from actin filament.
  • Myosin head returns to starting position and reattaches to different binding site further along actin filament.
  • Cycle repeated and many actin-myosin cross-bridges form and break rapidly, pulling actin filament along — shortens sarcomere, causing muscle to contract. Continues as long as Ca ions present.
• Muscle no longer stimulated:

  • Ca ions leave binding sites and moved by active transport back into sarcoplasmic reticulum (needs ATP too).
  • Causes tropomyosin molecules to move back to block actin-myosin binding sites. (No muscle contraction as no actin-myosin cross bridges can be formed). 
  • Actin filaments slide back to relaxed position, lengthening sarcomere.
• Energy for Muscle Contraction -

  Lots of energy needed for muscle contraction - ATP used up quickly. Continually generated so exercise can continue — in 3 main ways:

  • Aerobic respiration - Only works when there’s oxygen so good for long periods of low-intensity exercise.
  • Anaerobic respiration - Lactate can quickly build up in the muscles and cause muscle fatigue. Good for short periods of hard exercise.
  • ATP-phosphocreatine (PCr) system
• ATP-phosphocreatine (PCr) system:

  • ATP is made by phosphorylating ADP — adding a phosphate group taken from phosphocreatine (PCr). 
  • ADP + PCr --> ATP + Cr
  • System generates ATP very quickly. Anaerobic + doesn't form lactate.
  • PCr stored inside cells and runs out after a few seconds so it’s used during short bursts of vigorous exercise.
  • Some of the Cr gets broken down into creatinine - removed from the body via the kidneys. Creatinine levels can be higher in people who exercise regularly and those with a high muscle mass. High creatinine levels may also indicate kidney damage.
• Skeletal muscles are made up of two types of muscle fibres — slow twitch and fast twitch. 

  Different muscles have different proportions of slow and fast twitch fibres. 
• Slow twitch muscle fibres

  • Contract slowly and can work for a long time without getting tired. 
  • Good for endurance activities eg. long distance run, maintaining posture. 
  • High proportions found in muscles used for posture eg. back.

  Energy released slowly through aerobic respiration. 

  Have lots of mitochondria and blood vessels to supply the muscles with oxygen. Mitochondria mainly found near edge of muscle fibres - short diffusion pathway for oxygen from the blood vessels to the mitochondria. 

  Reddish in colour - rich in myoglobin (red-coloured protein that stores oxygen)
• Fast twitch muscle fibres

  • Contract very quickly but tire very quickly.
  • Good for short bursts of speed and power, e.g. sprinting and eye movement. 
  • High proportions found in muscles used for fast movement eg. legs, arms, eyes.

  Energy is released quickly through anaerobic respiration using glycogen. 

  Have stores of PCr so energy can be generated very quickly when needed.

  Have few mitochondria or blood vessels. 

  Whitish in colour - don’t have much myoglobin, so can’t store much oxygen.