Musculoskeletal System

Protection of Vital Organs:
- Protects the heart, lungs, brain, and spinal cord.
Movement Support:
- Provides anchor points for muscles.
- Transmits forces generated by muscular contraction similar to a system of pulleys and levers.
Body Support:
- Provides support for the entire body.
Mineral Storage and Release:
- Stores essential minerals such as calcium and phosphorus.
- Hematopoiesis: Formation of blood cells occurs in the marrow of some bones, with some white blood cells maturing outside of the bone.

Bone is a living tissue made up of:
- Cells
- Fibres embedded in a hard matrix consisting of calcium phosphate and calcium hydroxide, collectively known as hydroxyapatite.

Cancellous (Trabecular or Spongy Bone)
Compact Bone
Classification by Shape:
- Irregular Bone (e.g., vertebrae)
- Long Bone (e.g., femur, humerus)
- Sesamoid Bone (e.g., patella)
- Short Bone (e.g., carpal and tarsal bones)
- Flat Bone (e.g., sternum, skull bones)

Short Bones:
- Cube-shaped, nearly equal in length and width.
- Mostly consist of spongy bone with a compact bone surface.
- Examples include carpal and tarsal bones (excluding phalanges).
Flat Bones:
- Thin and protective with two parallel plates of compact bone enclosing spongy bone.
- Examples include cranial bones, sternum, ribs, scapulae, and ilium.
Long Bones:
- Central shaft (diaphysis) is thick compact bone.
- Ends (epiphyses) consist of spongy bone covered by compact bone.
- Examples include humerus and femur.
Irregular Bones:
- Variable amounts of spongy and compact bone.
- Examples include vertebrae, ischium, pubis, and sacrum.
Sesamoid Bones:
- Develop in tendons under stress to protect tendons from wear.
- Change the direction of tendon pulls and improve mechanical advantage.
- Examples are patellae and sesamoid bones in palms and soles of feet.

The outer surface of bone is covered by the periosteum, a specialized connective tissue layer.
Functions of the periosteum:
- Connector for tendons and ligaments to bone.
- Enables new bone formation and repair.

Forms a protective outer shell around every bone.
Constitutes roughly 80% of skeletal mass, providing strength against bending and torsion.
Structure: Comprised of osteons (Haversian systems) containing:
- A central vascular channel (Haversian canal) that houses capillaries, venules, and nerves.
- Surrounding concentric layers termed lamellae, which contain osteocytes embedded in fluid-filled lacunae, connected by canaliculi.
Osteocytes are the most abundant bone cell type, approximately 42 billion, with a half-life of about 25 years.

Comprises 20% of skeletal mass yet occupies 80% of bone surface area.
Less dense and more elastic than compact bone
Contains a network of trabecular structures that maintain shape under compressive forces.
Predominantly found in the ends of long bones and throughout the interior of short bones.

Different bones form through various mechanisms, largely dependent on their type and function.
Timeline: Long bones begin developing approximately 2 months into fetal development.

Cartilage Model Formation:
- Formation begins with mesenchymal cells creating a cartilaginous matrix, which later becomes calcified.
- Establishment of perichondrium around the cartilage structure.
Osteoblast Formation:
- Some mesenchymal cells differentiate into osteoblasts that surround the cartilage model with a bony collar.
- The periosteum contributes to bone formation via osteoblast generation.
Center of Ossification:
- The middle cartilage breaks down and is invaded by blood vessels and osteoblasts leading to bone tissue formation.
- This progresses longitudinally toward the epiphyses, replacing cartilage with spongy bone.
Secondary Ossification Centers:
- These form in the epiphyses, eventually leading to a marrow cavity formation as spongy bone is resorbed.
Epiphyseal Plate: Maintains cartilage for lengthening until adulthood (~20 years old), after which further growth stops.

Process: Formation of flat bones via ossification of soft sheets of mesenchymal cells (membrane bones), featuring:
- Mesenchymal cell condensation and blood capillary permeation.
- Osteoid tissue deposition by osteoblasts, resulting in spongy bone formation before converting to compact bone.

Bone strength is maintained by continuous remodeling involving:
- Resorption: Removal of old bone by osteoclasts.
- Formation: Creation of new bone by osteoblasts.
As individuals age, resorption may outweigh formation, leading to conditions such as osteopenia or osteoporosis.

Bone resorption is regulated by parathyroid hormone (PTH):
- Released in response to low calcium and phosphorus levels, stimulating osteoclasts for increased bone resorption and mineral release.
- It also converts vitamin D, affecting intestinal calcium absorption.
- Calcitonin acts oppositely, decreasing osteoclast activity and promoting osteoblast function while managing blood calcium levels.

A flexible tissue comprised of glycoprotein (chondroitin), collagen fibers, and cartilage cells.
Locations include:
- Trachea and larynx
- Nose and ear lobes
- Between ribs and breastbone
- Articular surfaces in joints.

Types of Joints According to Mobility:
- Fibrous Joints: No movement (e.g., bones of the skull).
- Cartilaginous Joints: Limited movement (e.g., between vertebrae).
- Synovial Joints: Increased movement with lubricated cavities, allowing for flexibility at joints.

Ligaments: Tough bands of fibrous tissue linking bones at joints, limiting joint mobility.
Tendons: Connect muscle to bone, facilitating movement.

Cardiac Muscle= smooth, involuntary, heart
Skeletal Muscle= striated, voluntary, and responsible for movements of the skeleton. Composed of bundles of muscle cells (fascicles) with connective tissue envelops (epimysium and perimysium).
Smooth Muscle= smooth, involuntary and found in the walls of hollow organs such as the intestines, bladder, and blood vessels.

Thick Filaments: Composed of myosin.
Thin Filaments: Comprised of actin along with troponin and tropomyosin.
Sliding Filament Model:
- Triggered by nervous action potentials causing Ca2+ release from the sarcoplasmic reticulum.
- Calcium binding to troponin moves tropomyosin, exposing myosin binding sites on actin.
- Myosin heads attach to actin and pivot to pull actin filaments, shortening the sarcomere.
- ATP replaces ADP to detach myosin from actin, with the process repeating as long as calcium is present.

Related to muscle cross-sectional area, with thicker muscles exerting more force but longer muscles allowing for more considerable movement.
Muscle fibers are fixed in number after early life; further development results from hypertrophy (thickening).

Often illegally used in sports to boost muscle mass; can cause severe health issues, including liver damage, kidney issues, and increased heart disease risk, mimicking testosterone effects.