Joints and Muscles

Question 1

Types of Joints

Answer 1

• Fibrous
  
  • Connected by fibrous connective tissue
  • Limited/no movement
    
    • Skull
• Cartilaginous
  
  • Connected by fibrous tissue
  • Cartilage between bones
  • Limited movement
    
    • Symphysis pubic, intervertebral disk
• Synovial Joints
  
  • Synovial cavity
  • Synovial membrane secretes synovial fluid
    
    • Lubricant (low coefficient of friction)
  • Synovial membrane covers capsule and bone, NOT cartilage
  • Fibrous joint capsule surrounds joint
  • Hyaline cartilage covers articular surfaces
  • Most movement
    
    • Gleno-humeral joint (shoulder), hip joint
  • Arthritis
    
    • Osteoarthritis (degenerative arthritis)
      
      • Bone against bone due to degeneration of articular cartilage in synovial joint
    • Inflammatory arthritis (i.e. rheumatoid arthritis)
      
      • Inflammation of the synovium and surrounding connective tissue

Question 2

Motor Unit

Answer 2

• Functional unit in a muscle
• Motor neuron + all skeletal muscle cells innervated by the neuron
• Contraction movement:
  
  • Concentric contraction (muscle shortens) lift arm
  • Isometric contraction (muscle length is constant) hold arm elevated
  • Eccentric contraction (muscle lengths) slowly lower arm
• Sustained muscle contraction
  
  • Asynchronous firing of motor units (some rest while others active, and switch) to avoid fatigue
• All or nothing
  
  • Neuron depolarization -> all muscle cells contract fully (or none at all)
  • NO partial contraction o individual muscle fibers
    
    • Gradation of contraction for different amounts of force = alter number of motor units contracting at one time
    • Double number of motor fibers = double the force
• Size
  
  • Small amount of muscle fibers (<20) / motor unit = fine movement, weaker (ie hand)
  • Many muscle fibers (>500) / motor unit = gross movement, strong (ie pelvic girdle)

Question 3

Muscle Action at Joints

Answer 3

• Determine muscle action
  
  • Bones attached to
  • Joint(s) crossed
  • Axis of rotation for the joints
  • Muscle vector crosses the axis? Which side (flexor/extensor)?
  • Angle formed between vector and axis?
    
    • 90º is best
    • 0º or 180º is ineffective
• Torque = (force) (distance) (sin θ)
• Examples:
  
  • Trapezius
    
    • Origin: vertebrate
    • Insertion: scapula, clavicle
    • Action: upper/lower fibers cause medial rotation of the scapula. Middle fibers cause retraction of scapula