Muscles

• Muscles


  Part of the muscular system, made up of muscle tissue
• Muscle tissue


  • Made up of muscle fibers
  • Three types: cardiac, smooth, skeletal
• Cardiac muscle tissue


  Found in the heart, has branched and striated fibers, each with one nucleus, has intercalated discs, involuntary control
• Smooth muscle tissue


  Doesn't have striations, spindle-shaped fibers with one nucleus, found in digestive system, arteries/veins, bladder, iris, involuntary control
• Skeletal muscle tissue


  Striated fibers, long cylinders with multiple nuclei, attaches to bone or skin, voluntary control
• Characteristics of all muscle tissue


  • Extensibility (can stretch/extend)
  • Elasticity (can retract to starting length)
  • Excitability (can be stimulated and send action potentials)
  • Contractility (can contract)
• Skeletal muscle structure


  • Insertion (attaches to bone that moves)
  • Origin (attaches to fixed part of bone)
  • Agonist (prime mover muscle)
  • Antagonist (muscle that does opposite action)
• Skeletal muscle contraction


  1. Sarcomeres shorten

  2. Actin (thin filaments) slide past myosin (thick filaments)

  3. Myosin heads bind to actin, perform power stroke, then detach with ATP

  4. Tropomyosin and troponin regulate myosin binding to actin
• ATP is needed for myosin heads to detach from actin, which prevents rigor mortis after death
• Muscle contraction


  At the basis of all skeletal movements
• Skeletal muscles


  • Composed of muscle fibers
  • Made of repetitive functional units called sarcomeres
  • Each sarcomere contains many parallel, overlapping thin (actin) and thick (myosin) filaments
• Muscle contraction


  1. Filaments slide past each other

  2. Resulting in a shortening of the sarcomere and thus the muscle
• Sliding filament theory


  Describes the mechanism of muscle contraction
• Cross-bridge cycling


  Forms the molecular basis for the sliding movement
• Muscle contraction initiation


  1. Muscle fibers are stimulated by a nerve impulse

  2. Calcium ions are released
• Calcium ion binding

  1. Troponin units on the actin myofilaments are bound by calcium ions

  2. This displaces tropomyosin along the myofilaments

  3. Exposing the myosin binding sites
• Myosin head binding


  1. Myosin heads are bound to ADP and a phosphate molecule

  2. Myosin heads release these phosphates

  3. Myosin heads bind to the actin myofilaments via the exposed myosin binding sites
• Myofilament gliding


  1. The two myofilaments glide past one another

  2. Propelled by a head-first movement of the myosin units

  3. Powered by the chemical energy stored in their heads

  4. As the units move, they release the ADP molecules bound to their heads
• Myosin head detachment


  1. Gliding motion is halted when ATP molecules bind to the myosin heads

  2. Severing the bonds between myosin and actin
• ATP decomposition


  1. The ATP molecules are decomposed into ADP and phosphate

  2. The energy released by this reaction is stored in the myosin heads, ready to be used in the next cycle of movement
• Myosin head repositioning


  1. The myosin heads resume their starting positions along the actin myofilament

  2. Can now begin a new sequence of actin binding
• Presence of further calcium ions


  Triggers a new cycle of muscle contraction
• Synergists are the muscles surrounding the joint being moved
• Fixators are the muscle or group of muscles that steady joints closer to the body axis so that the desired action can occur.
• Stabiliser muscles (fixator muscles) are needed to ensure the stabilisation of other parts of the body or bone during movement
• The sliding filament theory

  
• Biceps and triceps in forearm

  
• Structure of a neuron

  
• Muscles, bones and nerves working together

  Skeletal muscles controlled by nerves in peripheral nervous system

  Messages transmitted along motor neurons originating in the motor area of cerebral cortex