Musculoskeletal System Notes

Axial vs Appendicular Skeleton

  • Axial skeleton
    • Provides central attachment sites for large muscle groups that control the trunk, proximal extremities, neck, and head
    • Includes the vertebral column, ribs, sternum, and skull
  • Appendicular skeleton
    • Any bone outside of the axial skeleton
    • Includes bones of the upper and lower limbs
  • Overall division
    • The skeleton can be divided into two subgroups: the axial skeleton and the appendicular skeleton
    • Axial skeleton comprises skull (cranium), vertebral column, ribs, and sternum
    • Appendicular skeleton comprises the bones of the upper and lower limbs

Components of Bone

  • The skeletal system functions as the rigid structural framework of the body
    • Supports soft tissue structures (e.g., muscles, blood vessels)
    • Protects vital organs (e.g., brain, lungs)
  • Bone tissue types
    • Compact bone (also called cortical bone)
    • Found in the shaft of long bones (e.g., humerus, femur)
    • Provides rigid support to sustain forces
    • Cancellous bone (also called spongy or trabecular bone)
    • Found within the marrow cavity and at the ends of long bones (e.g., femoral head)
    • Composed of trabeculae that increase ability to absorb compressive loads
  • Articular (hyaline) cartilage
    • Covers the articulating ends of long bones
    • Dense connective tissue that cushions and absorbs repetitive compressive forces between bones
  • Composition note
    • Bone is primarily made of collagen and a calcium mineral base called hydroxyapatite
  • Additional terms (from bone cross-section)
    • Osteon (Haversian system): structural unit of cortical bone
    • Medullary cavity: central cavity within the bone shaft that contains bone marrow
    • Periosteum: dense membrane covering the outer surface of bone

Ligaments vs Tendons

  • Ligaments
    • Connect bones to bone
    • Composed of dense connective tissue (mainly collagen fibers)
    • Resists tensile forces and contributes stability to joints
    • Can be found surrounding or within joints
    • Often limit movement in specific planes
  • Tendons
    • Connect muscle to bone
    • Transfer the force of muscle contraction to bone to enable joint movement
    • Do not primarily stabilize joints in the same way ligaments do

Types of Muscle and Locations

  • Three types of muscle
    • Skeletal (striated) muscle
    • Moves the bones of the skeleton; provides force for movement
    • Connects to bone by tendon (exceptions include muscles of the face and eye)
    • Voluntary control (somatic nervous system)
    • Smooth (visceral) muscle
    • Found within internal organs (e.g., intestines, vessels)
    • Nonstriated; contracts slowly and automatically
    • Involuntary control (autonomic nervous system)
    • Cardiac muscle
    • Forms the myocardium of the heart
    • Striated and organized into segments; fibers interconnected to contract in a wavelike pattern
    • Involuntary control
  • Summary of control and organization
    • Skeletal: voluntary control; moves skeleton
    • Smooth: involuntary control; internal organ/vessel movement
    • Cardiac: involuntary control; heart pumping function

Basic Structural Joint Types

  • Fibrous joints
    • Have little to no movement; designed for stability
    • Example: sutures of the skull
  • Cartilaginous joints
    • Have little to no movement; designed for stability
    • Example: pubic symphysis of the pelvis
  • Synovial joints
    • Mobile joints that allow purposeful movement
    • Most common type in the body
  • Functional note
    • Synovial joints are diarthrotic (highly movable)
    • Fibrous and cartilaginous joints tend to be amphiarthrotic or synarthrotic (limited or no movement)

Classification of Synovial Joints (Structural)

  • Six common structural classifications:
    • Ball-and-socket
    • Ellipsoid
    • Hinge
    • Saddle
    • Gliding
    • Pivot
  • Functional aspect (how many axes):
    • Many synovial joints are multiaxial or biaxial, depending on the joint

Components of a Typical Synovial Joint

  • Articular cartilage
    • Hyaline cartilage covering the ends of the articulating bones
    • Reduces friction and absorbs shock during movement
  • Joint capsule
    • Outer fibrous layer
    • Inner synovial membrane (synovium)
  • Joint cavity
    • Narrow space between articulating surfaces
    • Filled with synovial fluid for lubrication and shock absorption
  • Synovial fluid
    • Lubricates the joint; nourishes the articular cartilage
  • Other capsule features (not exhaustive in transcript but commonly related)
    • Ligaments and sometimes bursae around the joint contribute to stability and movement control
  • Functional outcome
    • These joints are diarthroses (free-moving joints) with a large range of motion

Synovial Joint Capsule, Blood Supply, and Innervation

  • Articular capsule composition
    • Outer fibrous layer provides structural integrity
    • Inner synovial membrane secretes synovial fluid
  • Blood supply
    • Arteries surrounding the joint form an anastomoses network to nourish the joint
  • Innervation
    • Articular nerves in the capsule provide proprioceptive information about movement and position of the bones
  • Additional note
    • Articular cartilage covers the ends of the bones to enable smooth sliding with less friction and to absorb shock

Classification and Examples of Synovial Joints

  • Structural classifications (repeated for clarity): Ball-and-Socket, Ellipsoid, Hinge, Saddle, Gliding, Pivot
  • Functional examples by type
    • Ball-and-Socket: shoulder joint (glenohumeral) – spherical surface fits into a concave cup; allows rotation around at least three axes
    • Ellipsoid: radiocarpal (wrist) joint – oval convex into elliptical concave; permits flexion/extension and abduction/adduction (radial/ulnar deviation) around two axes
    • Hinge: humeroulnar (elbow) joint – flexion and extension around a single axis; uniaxial; collateral ligaments limit medial/lateral movement
    • Saddle: carpometacarpal (CMC) joint of the thumb – convex and concave surfaces like two saddles; two-axis movement
    • Gliding: carpal joints of the wrist – flat surfaces; least movement; translation (sliding) movements
    • Pivot: atlantoaxial joint between the first and second cervical vertebrae – rotation around a single axis

Cartilage and Cartilage Types

  • Types of cartilage in the body:
    • Hyaline (articular) cartilage
    • Fibrocartilage
    • Elastic cartilage
  • Articular cartilage specifics
    • Covers articulating ends of long bones
    • Dense connective tissue that cushions and absorbs repetitive compressive forces between bones
  • Maturation and healing
    • Mature cartilage is avascular (lacks blood supply) and aneural (lacks innervation)
    • This limits healing after injury
  • Pathology
    • Osteoarthritis involves degeneration of cartilage within a joint

Osteology: Common Skeletal Landmarks and Features

  • Descriptive terms for non-articular bone features
    • Process: an expansion of bone that usually functions as a site for muscle attachment (e.g., transverse process of vertebrae)
    • Tuberosity: a large, rounded prominence for muscle attachment (e.g., deltoid tuberosity)
    • Tubercle: bumps on the bone surface that vary in size (e.g., greater and lesser tubercles of the humerus)
    • Fossa: a smooth depression that serves as a site for muscle attachment (e.g., coronoid fossa of the humerus)
    • Sulcus: a groove or depression between two prominences (e.g., intertubercular sulcus)
    • Foramen: an opening in a bone that does not serve as a muscle attachment (e.g., vertebral foramen)
    • Facet: an articular surface; usually associated with rib or vertebral joints (e.g., superior articular facet of vertebra)
    • Intertubercular sulcus: groove between tubercles (example of anatomy on the humerus)
    • Transverse process: lateral projection on a vertebra
    • Superior articular facet: articular surface on vertebrae for articulation with the superior vertebra
    • Vertebral foramen: opening through which the spinal cord passes
  • Illustration reference (common plane views)
    • Commonly taught as a set of labeled features on a vertebra and long bone

Origin and Insertion of Muscles

  • Definitions in relation to muscle function
    • Origin: the attachment site that moves least during contraction; typically proximal to the trunk
    • Insertion: the attachment site that moves the most during contraction; typically distal from the trunk
  • Practical implication
    • Muscles shorten during contraction, pulling on the insertion to move a bone relative to its origin

Surface Anatomy and Palpation

  • Palpation
    • The use of physical touch to identify musculoskeletal structures beneath the skin
    • Examples of tissue responses
    • Impaired circulation or innervation can cause skin to feel cold and lifeless
    • Acute soft tissue injury may cause local skin temperature increase due to inflammation
  • Surface anatomy
    • Describes features of anatomy that are palpable or visible on the surface of the skin
  • Bony landmarks
    • A specific, palpable component of a bone that protrudes beneath the skin (e.g., ulnar head on the dorsal/ulnar aspect of the wrist)
  • Practical application
    • Palpation helps estimate underlying bone location, joint positions, and targeted assessments during exams or manual therapies

Connections and Practical Implications

  • Structural relationships
    • The axial skeleton houses the central organs and supports head/neck/trunk; the appendicular skeleton enables limb movement and locomotion
  • Healing considerations
    • Avascular cartilage explains why articular cartilage has limited healing capacity after injury; thus joint degeneration (e.g., osteoarthritis) has substantial functional impact
  • Joint stability and movement
    • Ligaments provide joint stability and guide motion; tendons translate muscle force into bone movement; muscles themselves produce movement through their contractile property
  • Proprioception and input
    • Joint innervation provides proprioceptive feedback, essential for coordinated movement and balance