Bone and Muscle Biology

Periosteum

thin tissue covering the outer surface of bones during life; covers all bone surfaces not covered by cartilage

Endosteum

poorly defined membrane that lines the inner surface of bones

Osteoblasts

Bone-forming cells
§ Deposit osteoid (unmineralized bone) that forms the bone
bone forming cells

• Deposit osteoid (unmineralized bone) that forms the bone matrix

• Blast = build

Osteoclasts

bone reabsorbing cells

Osteocytes

living bone cells that no longer secrete osteoid but maintain bone tissue

Osteoprogenitor cell

• genitor = producing

Lacunae

Osteocytes live within cavities called lacunae within the lamellae

canaliculi

Osteocytes are connected to each other via channels called canaliculi

= bone is increased where needed and resorbed where not
Wolff’s Law

bone is increased where needed and reabsorbed where not needed

• bones are remodeled during life to meet mechanical demands

Bone Biomechanics

§ Compression

§ Tension

§ Shear

§ Torsion

§ Response depends on:

• § Duration

• § Frequency

• § Number of repetitions

• § Age

• § Gender

• § Use/disuse

Trabecular Bone and Loading

Trabecular Bone : spongy, porous bone found inside bones, such as the vertebrae

• (loading = force applied to bone)

• Slow loading: The bone has more time to deform under pressure.

• Fast loading: The bone behaves more stiffly, meaning it resists deformation more strongly under compression.

• Viscoelastic: ~ the response to force: deformation curve ,is dependent on the rate of loading

• Trabecular bone becomes stiffer in compression the faster it is loaded

Bone Remodeling/Trauma

Woven

• found during growth and repair (e.g. from a fracture or disease)

• Disorganized

• Not a lot of structure or stability

Primary

• laid down on woven bone

Secondary (Lamellar bone):

• Formed during bone remodeling

• Most resistant to fractures

Bone development (ossification) occurs in 2 general sites

• Intramembranous ossification

• Endochondral ossification

Intramembranous Ossification

bones ossify by apposition on tissue within an embryonic connective tissue membrane (e.g., skull bones)

Endochondral Ossification

bones preceded by cartilage which ossifies over time (e.g., most bones in the body)

Intramembranous Ossification

• Flat bones

• From mesenchymal (embryonic) tissue

Center of ossification

location in the membrane where ossification begins

Fontanels

large membrane covered spaces between developing skull bones (unossified)

Endochondral Ossification

• Long bones

• Cartilage model precursor

Osteogenesis

Bone Growth/Ossification

• Thin membrane called the perichondrium surrounds the cartilage model of the long bone.

• Osteoblasts just beneath the perichondrium deposit bone

• Once bone is deposited, the membrane becomes the periosteum

Bone Growth/Ossification

• More bone is deposited layer by layer, increasing the diameter of the shaft

• Appositional growth = growth in diameter

• Second center of ossification is located between the metaphysis and epiphysis

• Allows bones to lengthen = growth plate

Bone Growth/Ossification

• Usually around 450 ossification centers at birth

• “Primary” ossification centers usually appear before birth

• “Secondary” ossification centers usually appear after birth

• 206 bones in an adult skeleton

Bone Growth/Ossification

• Usually around 450 ossification centers at birth

• Some major bones fuse at predictable ages through growth

• Ages of epiphyseal fusion are used to provide age estimates on immature skeletons

Long bones

• Epiphysis: end part of long bone (pl: epiphyses)

• Diaphysis: shaft of long bone

• Metaphysis: where the epiphysis meets the diaphysis (pl: metaphyses)

Musculoskeletal system

system of bony levers operated by muscles

Joint

connection between different bony elements

Articulation

• Where different bones join or touch is called an articulation

• Ligaments and cartilage are often found at articulation points

Tendons

closely packed parallel bundles of collagen fibers; attach muscles to bones

• Movement at joints is controlled by the shapes of the articular surfaces and the ligaments that bind the joints together and prevent dislocation

3 major types of Joints

§ Synovial § Cartilaginous § Fibrous

Synovial joint

freely moving joints composed of a joint capsule containing a joint cavity lined with hyaline cartilage and synovial fluid

• Very moveable

Joint capsule

• Articular capsule

• Surrounds the synovial cavity (joint cavity)

• Synovial fluid

Cartilaginous joint

articulating bones are united by cartilage which allows very little movement

• 2 types of cartilaginous joints depending on type of cartilage between the bones :

  • Synchondrosis ⟶ hyaline cartilage EX. EPIPHYSEAL PLATE

  • Symphysis ⟶ fibrocartilage EX : PUBIC SYMPHYSIS

Fibrous joint

joint between bones filled with fibrous tissue only and allowing no movement

3 Types of Fibrous joints:

• Suture

• Syndesmosis

• Gomphosis

Cranial sutures ( Types of Fibrous Joints)

fibrous joints of the skull bones; united by fibrous tissue

Syndesmosis

tight, inflexible fibrous joint between bones and united by dense fibrous tissue such as membranes ligaments (e.g., tibia-fibula)

Gomphosis

joint between the roots of the teeth and the jaw bones

Fibrous joints

• immovable

Skeletal movement

• Movement of the skeleton generally takes place at synovial joint

• Movement generally caused by muscles working across joints

• Muscles connected to bones by tendons

• Ligaments

Ligaments

connective tissue extending between adjoining bones

3 types of muscle tissue:

• Striated (skeletal) muscle

• Smooth muscle

• Cardiac muscle

Skeletal Muscle

• Attached to bone

• Allows movement (voluntary)

• Striated

Cardiac Muscle

• Heart muscle

• Involuntary

• Striated

Smooth Muscle

• Non-striated

• Found in walls of hollow internal structures

• Involuntary

Basic Muscle Biology

• Muscles are composed of fiber bundles

• Fibers made up of fibrils (myofibrils)

• Fibrils (myofibrils) made up of myofilaments

• Myofilaments are banded

Basic Muscle Biology

• Bands within each myofilament are known as A bands (darker) and I bands (lighter in color)

• Z-lines ⟶ thin dark strips within the I bands)

• Unit between two Z-lines called a sarcomere ⟶ smallest functional unit of the myofilament

Basic Muscle Biology

• Structure within a sarcomere

• Central section of thick myosin positioned between thin filaments of actin

• When muscle contracts, thick section slides between the thin filaments and reduces the distance between Z-lines

Basic Muscle Biology

• Muscles are innervated by nerves

• Nerve impulse depolarizes muscle fibers and results in an interaction of the actin and myosin molecules which results in the muscle contracting

• Nerve and the group of muscle fibers that it innervates ⟶ motor unit

Work Formula

• Work performed by a muscle = F*d

• Work ⟶ Force x distance

• Therefore, length of the muscle is related to how much work can be performed

Two muscles exerting the same force but of different length….which can perform more work?

The longer muscle, because it can contract over a greater distance. Work = Force × Distance.

Origin

in general, site that stays stable during contraction; usually closest to the trunk (proximal)

Insertion

in general, the site that is moved during contraction; usually farthest from the trunk (distal)

Antagonists

Many muscles perform opposite motions

These muscle pairs are called antagonists

• E.g., flexors and extensors