The musculoskeletal system

• muscle coverings

  • Epimysium - covers whole muscle
  • Perimysium - surrounds muscle fascicles
  • Endomysium - surrounds muscle fiber

  All fibrous connective tissue

  All contribute to tendons: allows transmission of force to bone when muscle contracts
• muscle fiber

  • Muscle fiber = muscle cell
  • 30cm long and 10x wider than most cells
  • Many mitochondria (energy)
  • Sarcoplasmic reticulum (Ca2+ store)
  • Glycogen (glucose store)
• muscle filaments

  • myosin = thick
  • actin = thin
• synapses

  • Region where a nerve fiber makes a functional connection with its target cell
  • Presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic receptors
  • Neurotransmitter (acetylcholine/ACh) release causes stimulation of muscle cell
• muscle contraction and relaxation

  • Four actions involved in this process…
  • excitation - action potentials in the nerve lead to formation of action potentials in muscle fiber
  • excitation-contraction coupling - action potentials on the sarcolemma activates microfilaments
  • contraction - tensing or shortening of muscle fiber
  • relaxation is the return of fiber to its resting length
• What initiates skeletal muscle contraction?

  Skeletal muscle contraction is initiated by an action potential (AP).
• How does an action potential reach muscle fibers?

  An action potential travels along a motor nerve to its endings on muscle fibers.
• What neurotransmitter is secreted at the motor nerve endings?

  Acetylcholine (ACh) is secreted at the motor nerve endings.
• What is the effect of acetylcholine on muscle fibers?

  ACh acts locally on the muscle fiber membrane to open ACh-gated cation channels.
• What happens when ACh-gated channels open?

  Large quantities of sodium (Na) ions diffuse to the interior of the muscle fiber membrane.
• What is the result of sodium ions diffusing into the muscle fiber?

  This action causes a local depolarization, leading to the opening of voltage-gated sodium (Na) channels.
• What initiates an action potential at the muscle membrane?

  The opening of voltage-gated sodium (Na) channels initiates an action potential at the membrane.
• What does the action potential do to the muscle membrane?

  The action potential depolarizes the muscle membrane.
• What does the depolarization of the muscle membrane cause the sarcoplasmic reticulum to do?

  It causes the sarcoplasmic reticulum (SR) to release large quantities of Ca ions.
• What role do calcium ions play in muscle contraction?

  Ca ions produce attractive forces between actin and myosin filaments, causing them to slide alongside each other.
• What is the process that leads to muscle contraction?

  The sliding of actin and myosin filaments alongside each other leads to the contractile process.
• What happens to calcium ions after a fraction of a second?

  Calcium ions are pumped back into the sarcoplasmic reticulum by a Ca-membrane pump.
• Where do calcium ions remain stored until a new muscle action potential occurs?

  Calcium ions remain stored in the sarcoplasmic reticulum (SR).
• What causes muscle contraction to cease?

  The removal of Ca ions from the myofibrils causes muscle contraction to cease.
• muscle contractions

  • Isometric
  • Wall squat
  • Concentric - most common
  • Stand up from a chair
  • Eccentric
  • Sit down on a chair
• muscle adaptatons to exercise

  Resistance training 

  • Muscle fiber hypertrophy 
  • Neural activation 
  • recruit more motor units
  • synchronous recruitment

  Aerobic training

  • Muscle fiber type (more type 1 fibers)
  • Capillary supply
  • Myoglobin levels
  • Mitochondrial function
  • Storage of more glycogen and fat
  • Adaptations of CV system
• adaptations due to immobilisation or stopping training

  • Rate of protein synthesis decreases
  • Atrophy - loss of protein 
  • Decreased neuromuscular activity
  • Decreased flexibility
  • Endurance decreases even after 2/52 in activity
  • Can recover if training resumed but the period of time is longer than that to lose adaptation