Human Physiology Engineering Unit 11 ReviewMusculoskeletal System

Key Anatomical Structures

• Skeletal system provides structural support, protection for vital organs, and attachment points for muscles
  • Consists of 206 bones in the adult human body
  • Divided into axial skeleton (skull, vertebral column, ribs, sternum) and appendicular skeleton (limbs, pelvis, shoulder girdle)
• Muscular system enables movement, maintains posture, and generates heat
  • Includes over 600 skeletal muscles, as well as smooth and cardiac muscle tissues
  • Skeletal muscles attach to bones via tendons and work in pairs (agonist and antagonist) to facilitate movement
• Joints are the points of articulation between bones, allowing for various degrees of movement
  • Classified as fibrous (immovable), cartilaginous (slightly movable), or synovial (freely movable)
  • Examples of synovial joints include ball-and-socket (hip, shoulder), hinge (elbow, knee), and pivot (atlantoaxial joint in the neck)
• Ligaments are dense connective tissue bands that connect bones to other bones, providing stability to joints
• Tendons are connective tissue structures that attach muscles to bones, transmitting the force generated by muscle contraction

Bone Composition and Function

• Bones are composed of a dense outer layer called cortical bone and a spongy inner layer called trabecular bone
  • Cortical bone provides strength and rigidity, while trabecular bone offers lightweight support and flexibility
• Bone matrix consists of organic components (primarily collagen) and inorganic minerals (mainly hydroxyapatite)
  • Collagen fibers provide tensile strength and flexibility
  • Hydroxyapatite, a calcium phosphate mineral, provides compressive strength and hardness
• Osteoblasts are bone-forming cells that secrete the organic matrix and initiate mineralization
• Osteoclasts are bone-resorbing cells that break down and remodel bone tissue
• Osteocytes are mature bone cells embedded within the bone matrix, responsible for maintaining bone health and responding to mechanical stimuli
• Bones serve multiple functions, including structural support, protection of vital organs, mineral storage (calcium and phosphorus), and hematopoiesis (blood cell production in the bone marrow)
• Bone remodeling is a continuous process involving the coordinated actions of osteoblasts and osteoclasts, allowing bones to adapt to mechanical stresses and repair microdamage

Muscle Types and Mechanics

• Skeletal muscle is voluntary, striated, and attached to bones via tendons
  • Consists of bundles of muscle fibers (cells) containing myofibrils, which are composed of sarcomeres (the basic contractile units)
  • Sarcomeres contain thick filaments (myosin) and thin filaments (actin) that slide past each other during muscle contraction
• Smooth muscle is involuntary, non-striated, and found in the walls of hollow organs (blood vessels, digestive tract, uterus)
  • Contracts slowly and maintains tension for extended periods
• Cardiac muscle is involuntary, striated, and found only in the heart
  • Exhibits rhythmic contractions and has a high resistance to fatigue
• Muscle contraction occurs when the myosin heads bind to actin filaments, forming cross-bridges and pulling the filaments towards the center of the sarcomere
  • The sliding filament mechanism is powered by the hydrolysis of ATP (adenosine triphosphate)
• The force of muscle contraction depends on the number of cross-bridges formed, which is influenced by the level of muscle activation and the length-tension relationship
• Muscles work in pairs, with the agonist (prime mover) causing the desired motion and the antagonist opposing or controlling the movement
• Muscle fiber types include slow-twitch (Type I) fibers, which are fatigue-resistant and suited for endurance activities, and fast-twitch (Type II) fibers, which generate high forces but fatigue quickly

Joint Classification and Movement

• Fibrous joints are immovable and connected by dense connective tissue
  • Examples include the sutures between skull bones and the gomphosis between teeth and their sockets
• Cartilaginous joints are slightly movable and connected by cartilage
  • Examples include the pubic symphysis and the intervertebral discs between vertebrae
• Synovial joints are freely movable and characterized by a joint cavity containing synovial fluid, which lubricates the joint and reduces friction
  • The ends of the bones are covered with articular cartilage, which provides a smooth, low-friction surface
  • The joint capsule, lined with the synovial membrane, encloses the joint cavity and provides stability
• Types of synovial joints include:
  • Ball-and-socket joints (hip, shoulder) allow for flexion, extension, abduction, adduction, and rotation
  • Hinge joints (elbow, knee) allow for flexion and extension
  • Pivot joints (atlantoaxial joint) allow for rotation
  • Condyloid joints (wrist, metacarpophalangeal joints) allow for flexion, extension, abduction, adduction, and circumduction
  • Saddle joints (carpometacarpal joint of the thumb) allow for flexion, extension, abduction, adduction, and opposition
  • Plane joints (intercarpal joints) allow for limited gliding movements
• Joint movement is described using anatomical planes and axes:
  • Sagittal plane divides the body into right and left halves (flexion, extension)
  • Frontal plane divides the body into anterior and posterior portions (abduction, adduction)
  • Transverse plane divides the body into superior and inferior portions (rotation)

Neuromuscular Integration

• Motor neurons in the spinal cord and brain stem control skeletal muscle contraction
  • Upper motor neurons originate in the motor cortex and descend through the spinal cord
  • Lower motor neurons originate in the spinal cord and directly innervate skeletal muscle fibers
• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber
  • Acetylcholine, released by the motor neuron, binds to receptors on the muscle fiber membrane (sarcolemma), causing depolarization and muscle contraction
• Motor units consist of a single motor neuron and all the muscle fibers it innervates
  • Smaller motor units (few muscle fibers) allow for fine motor control, while larger motor units (many muscle fibers) generate greater force
• Muscle spindles are sensory receptors embedded within muscles that detect changes in muscle length
  • Provide feedback to the central nervous system to maintain proper muscle tone and coordinate reflexes (e.g., stretch reflex)
• Golgi tendon organs are sensory receptors located in tendons that detect changes in muscle tension
  • Provide feedback to the central nervous system to prevent excessive muscle force and protect tendons from injury
• Proprioception is the sense of body position and movement, mediated by muscle spindles, Golgi tendon organs, and joint receptors
  • Essential for maintaining balance, coordinating movement, and learning new motor skills

Common Injuries and Disorders

• Sprains are injuries to ligaments caused by overstretching or tearing
  • Common examples include ankle sprains and wrist sprains
  • Graded as mild (Grade 1), moderate (Grade 2), or severe (Grade 3) based on the extent of ligament damage
• Strains are injuries to muscles or tendons caused by overstretching or tearing
  • Common examples include hamstring strains and rotator cuff strains
  • Graded similarly to sprains based on the extent of tissue damage
• Fractures are breaks in bone tissue, classified as open (compound) or closed (simple)
  • Stress fractures are tiny cracks in bone caused by repetitive stress
  • Compression fractures occur when vertebrae collapse due to osteoporosis or trauma
• Osteoarthritis is a degenerative joint disorder characterized by the breakdown of articular cartilage, leading to pain, stiffness, and reduced mobility
  • Commonly affects weight-bearing joints such as the knees, hips, and spine
• Rheumatoid arthritis is an autoimmune disorder that causes chronic inflammation of the synovial membrane, leading to joint damage and deformity
• Osteoporosis is a condition characterized by low bone mass and deterioration of bone tissue, increasing the risk of fractures
  • More common in postmenopausal women due to decreased estrogen levels
• Muscular dystrophy is a group of inherited disorders characterized by progressive muscle weakness and wasting
  • Duchenne muscular dystrophy is the most common form, primarily affecting boys and causing difficulty walking and breathing

Biomechanical Principles

• Lever systems in the body consist of a fulcrum (joint), effort (muscle force), and load (resistance)
  • First-class levers (fulcrum between effort and load) are rare in the body, but an example is the atlantooccipital joint during head extension
  • Second-class levers (load between fulcrum and effort) are also rare, but an example is the ankle joint during standing on tiptoes
  • Third-class levers (effort between fulcrum and load) are the most common in the body, such as the elbow joint during biceps curl
• Mechanical advantage is the ratio of the load arm to the effort arm in a lever system
  • A mechanical advantage greater than 1 indicates that less effort is required to move the load, but the trade-off is a smaller range of motion
• Force-velocity relationship describes how the force generated by a muscle decreases as the velocity of contraction increases
  • Muscles generate the greatest force during isometric contractions (no change in muscle length) and the least force during high-velocity concentric contractions
• Length-tension relationship describes how the force generated by a muscle varies with its length
  • Muscles generate the greatest force when sarcomeres are at their optimal length, allowing for maximum overlap of actin and myosin filaments
• Stretch-shortening cycle is a muscle action that involves an eccentric contraction followed immediately by a concentric contraction
  • Enhances muscle performance by storing elastic energy in the muscle-tendon unit during the eccentric phase and releasing it during the concentric phase
  • Examples include jumping, running, and throwing

Diagnostic Techniques and Treatments

• X-rays use electromagnetic radiation to create images of dense tissues like bones and teeth
  • Useful for diagnosing fractures, joint abnormalities, and changes in bone density
• Computed tomography (CT) scans use X-rays and computer processing to create detailed cross-sectional images of the body
  • Provides better visualization of soft tissues and complex fractures compared to plain X-rays
• Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to create detailed images of soft tissues, such as muscles, ligaments, and tendons
  • Does not involve ionizing radiation and is particularly useful for diagnosing soft tissue injuries and disorders
• Electromyography (EMG) measures the electrical activity of muscles using surface or needle electrodes
  • Helps diagnose neuromuscular disorders, such as muscular dystrophy and peripheral neuropathy
• Bone mineral density (BMD) tests, such as dual-energy X-ray absorptiometry (DXA), measure bone density and assess the risk of osteoporosis and fractures
• Physical therapy involves exercises, stretches, and manual techniques to improve strength, flexibility, and function after an injury or surgery
  • Modalities such as heat, cold, ultrasound, and electrical stimulation may be used to reduce pain and promote healing
• Occupational therapy focuses on helping individuals perform daily activities and adapt to physical limitations using assistive devices and modifications to the environment
• Surgery may be necessary to repair severe fractures, torn ligaments or tendons, or to replace damaged joints with prosthetic implants (e.g., total hip or knee replacement)
• Medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and muscle relaxants, can help manage pain, inflammation, and muscle spasms associated with musculoskeletal conditions