Bone Growth and Structure

Study Notes

Bone growth begins as cartilage in babies and transforms into bone through ossification.
Primary ossification centers form in the middle of the bone.
Secondary ossification centers develop at the ends of the bone.
The medullary cavity forms, containing bone marrow.
Ossification centers meet at the epiphyseal plate, where articular cartilage forms, eventually becoming the epiphyseal line, signaling the end of growth.

Approximately 10% of an adult's skeletal mass is remodeled annually.
Osteoclasts are bone cells responsible for resorbing bone and dissolving its minerals.
Osteoblasts are bone cells that create the new bone matrix.
Bone remodeling allows bones to adapt to stress by becoming thicker and stronger.

1. Hematoma forms: A blood clot develops at the fracture site.
2. Internal and external calli form: Cartilaginous matrix forms, creating a bulge around the fracture.
3. Calli turns into trabecular bone: Cartilage transforms into spongy bone formation.
4. Remodeling occurs: The bone reshapes and strengthens.

### Building Strong Bones

Bone density increases until around age 30.
Calcium, Vitamin D (found in dark leafy greens), Vitamin A, and Vitamin K are essential for bone health.
Physical activity plays a vital role in maintaining strong bones.
Substance use can negatively impact bone health.

A common age-related disorder characterized by the loss of bone mass, leading to weaker bones and increased fracture risk.
Osteoporosis occurs due to an imbalance between bone formation and resorption.
Potential causes include:
- Failure to achieve peak bone mass during young adulthood.
- Excessively high bone resorption rates.
- Insufficient new bone formation by osteoblasts during remodeling.

Smooth Muscles:
- Found in internal organs.
- Non-striated (lacking a striped appearance).
- Spindle-shaped cells.
- Uninucleated (one nucleus per cell).
- Involuntary control.
Skeletal Muscles:
- Attached to the skeleton.
- Striated (striped appearance).
- Tubular shaped cells.
- Multinucleated (multiple nuclei per cell).
- Voluntary control.
Cardiac Muscles:
- Found in the heart.
- Striated.
- Branched cells.
- Uninucleated.
- Involuntary control.

Sarcomere: The smallest functional unit of a skeletal muscle fiber.
Myofibril: A long cylindrical structure within a muscle fiber, composed of numerous sarcomeres.
Actin: Thin filament within myofibrils.
Myosin: Thick filament within myofibrils.

A single motor neuron and the multiple muscle fibers it innervates.
The junction between a neuron axon terminal and a muscle fiber is called a neuromuscular junction.

### Initial Muscle Contraction Process

1. Action potential generated in motor neuron: A signal travels along the nerve.
1. Action potential reaches neuromuscular junction: The signal arrives at the nerve-muscle interface.
1. Acetylcholine (ACh) released: A neurotransmitter is released from the axon terminal.
1. ACh binds to receptors on muscle cell surface: The neurotransmitter binds to receptors on the muscle fiber.
1. Sodium (Na) channels open, initiating action potential in sarcolemma: The binding of ACh opens sodium channels, allowing sodium ions into the muscle fiber and triggering an action potential.
1. Action potential travels along T-tubules: The action potential travels deeper into the muscle fiber through T-tubules.
1. Calcium channels open in sarcoplasmic reticulum (SR), releasing calcium (Ca+2) into sarcoplasm: The action potential triggers the release of calcium from the SR.
1. Calcium binds to troponin and tropomyosin: Calcium binds to these proteins, causing them to move aside.
1. Myosin heads bind to actin, initiating muscle contraction: The myosin heads can now bind to actin filaments, resulting in muscle contraction.