The document summarizes the structure and function of the musculoskeletal system. It describes how the musculoskeletal system provides support, protection, and motion through connective tissues like tendons, ligaments, and cartilage. It also discusses bones, muscles, and joints and how they work together through connective tissues to enable movement and support for the body.

1. 1 © Alan Hedge, Cornell University 11/1999
Structure and Function of the
Musculoskeletal System
Professor Alan Hedge
DEA 325
Functions of the Musculoskeletal System
è Support and protect the body and its organs.
è Provide motion.
Musculoskeletal System:
Major substructures
è Tendons.
è Ligaments.
è Fascia.
è Cartilage.
è Bone.
è Muscle.
è Joints - allow motion between body segments.
Connective Tissues
è Cells.
è Extracellular matrix:
l Fibers.
l Ground substance.
Connective Tissue Cells
è Fibroblasts- cells producing the matrix for skin, tendons
and ligaments.
è Chrondrocytes - cells producing the matrix for cartilage.
è Osteocytes - cells producing the matrix for bone.
Connective Tissue Matrix
è Collagen fibers:
l Type I - tendons, ligaments, bone, skin.

2. 2 © Alan Hedge, Cornell University 11/1999
l Type II - cartilage.
l Type III - blood vessel walls.
è Mechanicalproperties depend on fiber types and fiber
arrangements.
Tendons
Ligaments
Fascia
Mechanical Properties of Fibers
è Stress - force on a fiber e.g. weight.
è Strain - % stretching of fibers.
è Elastic limit - point at which the elasticity of the fiber is lost.
è Failure - point at which the fiber breaks.
Collagen fibers
è Strengthof collagen is 50% strength of bone.
è Under tension, collagen fibers first elongate slightly and
then become increasing stiff until failure.
Elastic fibers
è Weak and brittle fibers.
è At low loads they strain greatly and can increase ~200% in
length before failure.
Tendon structure
è Tendon
è Tendon sheath:
l Synovium - lubricant producing tissue.
l Synovial fluid - lubricant fluid.
Tendons and Ligaments
è Atcertain points, ligaments surround parts of the tendon
sheath to act as:
l pulleys.

3. 3 © Alan Hedge, Cornell University 11/1999
l tendon guides.
è Ligament attachments allow tendons to work around
corners, as in the fingers and toes.
Cartilage: 3 types
è Hyaline cartilage - present in the growth plates at the end
of bones and on the articular surfaces of joints. Also
present in the respiratory tract (e.g. trachea).
è Fibrocartilage - present in intervertebral discs, which
consist of islands of hyaline cartilage in collagen fibers.
è Elastic cartilage - present in ears, nose, epiglottis etc.
Consists mainly of elastic fibers.
Bone
è 99% of bodies calcium is in bone.
è 80% of bone tissue by weight is minerals.
Bone groups
è Axial (appendicular) bones - flat bones, such as the skull,
pelvis, ribs, vertebrae
è Long bones - bones of the extremities. Round bones
comprising:
l diaphysis - shaft
l epiphyses - 2 expanded ends
Types of Bone
è Cortical (compact) bone - tissue with high proportion of
bone. 5-30% porosity.
è Cancellous (spongy) bone - less dense bone tissue found
at the epiphyses of the long bones and in axial bones,
such as the skull. 30-90% porosity.
Cortical (Compact) Bone
Cancellous (Spongy) Bone

4. 4 © Alan Hedge, Cornell University 11/1999
Anisotropy
è All bone is anisotropic (i.e. its mechanical property
changes when loads are applied in different directions).
Anisotropic behavior of cortical bone.
Bone: Mechanical properties
è Most important properties are strength and stiffness of
bone.
è Fractures occur when the bone is loaded to failure.
è Compression fractures are commonest in cancellous bone
(e.g. fractured skull)
è Bending and torsional fractures are commonest in cortical
bone (e.g. broken tibia).
Speed of loading to fracture.
Bone Structure and Growth
è Ossification - processes of bone formation.
è Osteoblasts - cells that form the bone matrix. They
transform into:
l Osteocytes - cells isolated inside the mineralized bone matrix.
è Osteoclasts - cells important to bone remodeling.
Skeletal muscle
è Skeletal muscle is ~50% body weight.
è Skeletal muscle is striated (striped), and excludes cardiac and
smooth muscle.
è Several hundred muscles in the body.
è Skeletal muscle is under voluntary control.
è Each muscle is a separate organ.
è Each muscle is attached to bone by tendons that cross one or
more joints.
è Generally, skeletal muscles generate moments about joints.

5. 5 © Alan Hedge, Cornell University 11/1999
Properties of Skeletal Muscle
è Isometric (static) contraction - muscles contract to change
muscle tone without changing length. This increases
postural stability.
è Static contraction is prone to rapid fatigue.
è Isotonic (dynamic) contraction - muscles contract to
change muscle length. This produces movement at joints.
è Dynamic contraction increases blood flow through muscles
which slows fatigue.
Muscle Organization at Joints
è Agonist muscle - the muscle directly engaged in
contraction (e.g. in flexing the elbow the biceps brachii is
the agonist).
è Antagonist - the opposing muscle in relaxation (e.g. in
flexing the elbow the triceps is the antagonist).
Structure of Skeletal Muscle
è Skeletal muscle is covered by a fascia called the epimysium.
è The epimysium penetrates and subdivides the muscle into muscle
fiber bundles called the fascicles (fasciculi).
è Each fascicle is covered by connective tissue called the
perimysium.
è Each individual muscle fiber is covered by connective tissue called
the endomysium.
è Connective tissue provides pathways for nerves and blood vessels
+ contributes to the mechanical properties of the muscle.
Skeletal Muscle Innervation
è Nerves (neurons) connect from the brain and spinal cord to
muscles.
è Efferent nerves carry motor instructions for contraction to
the muscles.
è Afferent nerves carry sensory information to the brain.
l Proprioceptive feedback - muscle tone.
l Kinesthetic feedback - muscle + joint status.

6. 6 © Alan Hedge, Cornell University 11/1999
è Mixed nerves carry a mix of both sensory and motor
neurons (e.g. median nerve).
Neuromotor Junction
Motor Units
è Motor unit - group of muscle fibers innervated by branches of the
same efferent neuron.
è Functional unit of the muscle.
è Motor units are small in muscles requiring precise control (e.g. eye
muscles), and large in coarse acting muscles (e.g. gastrocnemius
muscle).
è Motor units work in an “all-or-none” way.
è Progressive contraction occurs because of recruitment of motor
units.
Muscle Junctions
è Myotendinal junction - area where tendon fibers insert into
muscle connective tissue and also into bone matrix
(Sharpey’s fibers).
è Neuromuscular junction - motor endplate where nerve
fibers connect with myofibrils.
Muscle fibers
è Long, cylindrical cells containing several nuclei.
è Most of the fiber volume is occupied by the contractile
elements - myofibrils.
è Myofibrils show a banding pattern (transverse striation) of
thick myofilaments (myosin) and thin ones (actin).
è The contractile unit of the myofibril is called the sarcomere.
Sarcomere
Sarcomere and Contraction
Joints
èA joint is the union of 2 or more bones. 3 types of joint are
found in the body.

7. 7 © Alan Hedge, Cornell University 11/1999
è Synovial joint (diarthrodial joint) - no tissue between the
articular surfaces. Most of the body joints are this type.
è Fibrous joint - fibrous tissue bridge between bones (e.g.
skull).
è Cartilagenous joint - cartilage bridge between bones (e.g.
intervertebral disc in spine).
Structure of a Synovial Joint
è Joint capsule - fibrous capsule around the joint.
è Synovial membrane - membrane lining the joint capsule and
surrounding the synovial cavity. This tissue secretes the lubricating
synovial fluid.
è Synovial cavity (joint cavity) - cavity containing synovial fluid + bone
ends
è Articular cartilage (hyaline cartilage) - cartilage covering articular
surfaces of the bone ends.
Joint degeneration
è Cartilage does not have a capillary blood supply.
è Cartilage has a poor ability to repair and regenerate itself.
è With age, overuse and/or disease the articular cartilage
degenerates and bone ends make more direct contact,
causing pain.
Intervertebral Disc Structure
è Nucleus pulposus - incompressible watery gel contained
within an elastic sac.
è Annulus fibrosis - fibrocartilage lamellae arranged in layers
around the nucleus. Fiber orientation varies from layer to
layer.
è End plates - hyaline cartilage end plates protect each end
of the disc.
è Motion segment - 2 vertebrae either side + intervertebral
disc.
Fibrocartilage: Intervertebral Discs