Muscular System

• Involuntary muscle

  • Smooth muscle - unstriated (no visible bands), makes up walls if organs e.g. stomach, digestive system.
  • Cardiac Muscle - striated (visable bands), makes up the heart
• Skeletal Muscle

  • Enables us to carry out voluntary physical activity
  • Under conscious control
  • Attached to bones
  • Contractions bring about movements at joints
  • "Elasticity and extensibility"
• Muscle Shape

  • Tendons - Fibrous, inelastic connective tissue. Connects muscle to bone.
  • Belly - fleshy portion of the muscle between the tendons.
• Attachment

  • Origin - end of the muscle fixed to the stationary bone
  • Insertion - end of the muscle fixed to the moveable bone
• Antagonists

  • Muscles that move parts of the skeleton are always grouped in pairs, producing movements in the opposite directions.
  • When one muscle contracts (i.e. biceps) the opposing muscle relaxes to allow movement (i.e triceps).
  • Such pairs are known as antagonists.
  • Examples: biceps and triceps
  • Agonist- a muscle that causes a desired action (also known as prime mover).
  • Antagonist- the muscle that moves in a way opposite of the agonist.
• Synergists

  • Synergists- muscles that help the prime mover.
  • They may produce the same movement as the prime mover OR they may immobilise the joint.
  • When they act to immobilise a joint they are known as fixators.
• Muscle tone and posture

  • Muscle tone- partial contraction of skeletal muscles.
  • Some muscle fibers are relaxed while others are contracting.
  • This tightens a muscle but does not produce movement.
  • This holds many of our body parts in position.
  • Posture- the way a person holds their body when standing or sitting. Depends on muscle tone. Ballerina vs. soldier.
• Structure

  • Muscle fibers (cells) held together as bundles.
  • Connective tissue sheaths surround bundles.
  • Each sheath combines together to form a tendon.
• • Sarcolemma- plasma membrane containing cytoplasm (sarcoplasm) surrounding each muscle fiber.
  • Within the sarcoplasm of each muscle fiber lie thousands of thread like myofibrils.
• Myofibril structure

  • Many myofilaments make up one myofibril.
  • Two types of myofilaments;
  • ’Thick’ myofilaments are made up of the protein called myosin.
  • ‘Thin’ myofilaments are made up of the protein called actin.
  • The arrangement of myosin and actin gives a banded appearance to the muscle.
  • Myofibrils can be divided into units called sarcomeres.
• Striations

  • Patterns formed by repeating sarcomeres.
  • Cardiac and skeletal muscle is striated. 
• Sliding Filament Model

  • Explains how muscle contraction works.
  • When muscles contract, the sarcomeres shorten.
  • This occurs because the actin and myosin filaments slide over one another.
• Energy

  • Energy is required for the shortening of muscle fibers.
  • This energy comes from the breakdown of ATP into ADP and a phosphate group.
  • When energy is again available from cellular respiration, ATP is re-formed.
• Muscle contraction


  1. Nerve impulse causes calcium ions to be released within the muscle

  2. Calcium ions connect to actin producing a 'myosin binding site'

  3. Myosin attaches to actin producing a cross bridge

  4. Myosin heads bend, dragging actin filaments towards the middle of the sarcomere

  5. Actin and myosin slide over one another
• Muscle contraction


  • Shortens the sarcomere and therefore the muscle, which causes movement
  • Z lines move closer together