Anatomy: The Skeletal System

• An adult skeleton is composed of 206 bones.
• Joints give the body flexibility and allow movement.
• Bone ends are covered with cartilage.
• Articulation or joint or fibro-cartilage intervertebral disc is where two bones meet.

Skeletal Divisions

• Axial skeleton includes the skull, vertebrae, sacrum, rib cage, and hyoid bone.
• Appendicular skeleton includes the shoulder girdle, arms, hands, pelvic girdle, leg, and foot.

Function of Bone

• Bones support the body by providing an internal framework that anchors soft organs.
• Bones protect soft organs; for example, the skull protects the brain, vertebrae protect the spinal cord, and the rib cage protects the thoracic cavity.
• Bones store fats and minerals; yellow marrow stores fat as a fuel source, and bones store minerals like calcium and phosphorus.

Intramembranous Ossification

• Some flat bones, such as cranial bones, the scapula, breast bones, and coxal bones, form from loose fibrous membranes rather than cartilage.
• Osteoblasts are bone-forming cells.
• Osteocytes are mature bone cells.
• Osteoclasts are bone-destroying cells that break down the bone matrix for remodeling and release calcium ions (Ca+) into the bloodstream.

Endochondral Ossification

• Most bones form from hyaline cartilage.

Primary and Secondary Ossification Centers

• The primary ossification center is in the diaphysis, where bone formation occurs.
• Secondary ossification centers move into the epiphyses, with the proximal and distal ends ossifying together.
• The epiphyseal plate allows for growth in the length of long bones during childhood, with new cartilage continuously formed.
• Older cartilage becomes ossified, and bone replaces the cartilage in longitudinal growth or interstitial cartilage.

Blood Cell Formation and Movement

• Blood cell formation, called hematopoiesis, takes place in red bone marrow.
• All blood cells are synthesized in bone.
• Muscle contraction causes bone movement, and bones act as levers.

Bone Structure

• Compact bone is mainly in the diaphysis or shaft of long bones.
• Spongy bone is mainly in the epiphysis or ends of long bones and in flat bones, and it is hard.
• The osteon, or Haversian system, is the functional unit of compact bone.

Endochondral Ossification Phases

• Hyaline cartilage is covered with bone matrix and bone-forming cells called osteoclasts around the diaphysis. Hyaline cartilage is digested away with osteoclasts, opening up a medullary cavity within the newly formed bone.

Remodeling and Growth

• Remodeling, a process using osteoblasts and osteoclasts, occurs at puberty or until the early 20s.
• Appositional growth involves bones changing shape and growing in width/diameter under the periosteum.
• Remodeling involves renewing and reshaping bone, with new bone added to the periosteum and old bone destroyed and remodeled under the endosteum.
• Mechanical stress makes bone stronger, and weight-bearing exercise helps in the process.

Hormones

• Hormones release chemicals/molecules into the bloodstream and affect cells far away, altering the activity of chondrocytes, osteoblasts, and osteoclasts.
• Growth hormone, or somatotropin, is released by the anterior pituitary gland and stimulates the liver to release somatomedin, which directly stimulates cartilage growth in the epiphyseal plate.
• Thyroid hormone stimulates bone growth by influencing the basal metabolic rate of bone cells.
• Estrogen and testosterone are female and male hormones that accelerate cartilage growth and bone formation, often stopping during puberty.
• Serotonin, a neurotransmitter and hormone, plays a role in bone remodeling by affecting osteoblast differentiation.
• Parathyroid hormone is produced by the parathyroid gland when blood calcium drops below 8.9 mg/dL.
• Calcitriol is the active form of vitamin D.
• Calcitonin is produced by parafollicular cells of the thyroid gland when blood levels rise above 10.1 mg/dL.
• Regular blood calcium level is 8.9mg/dl to 10.1 mg/dl.

Bone Conditions and Fractures

• Osteopenia is the demineralization that makes bone thinner and weaker and is a natural part of aging.
• Osteoporosis, or "porous bone," involves decreased bone mass and weak bone, posing a risk for the elderly, white women, smokers, and those post-menopause.
• Estrogen is part of maintaining bone health and mass.
• In a closed, simple fracture, the broken bone stays inside the skin.
• In an open compound fracture, the broken bone penetrates through the skin, which is prone to infection and tissue damage.
• A pathologic fracture occurs when bone is weakened by disease.
• Reduction is the realignment of the ends of the broken bone.
• Immobilization involves using a splint or a hardened cast to stabilize the broken bone.

Bone Repair Stages

• Formation of fracture hematoma.
• Formation of fibro-cartilage (soft) callus or mass.
• Formation of bony (hard) callus or mass.
• Remodeling of the newly repaired bone.

Bone Scans and Spinal Conditions

• Bone scans use radioactive tracers absorbed by tissues to measure metabolic activity, with "hot spots" indicating increased activity and "cold spots" indicating decreased activity.
• A fontanel is a soft spot in the skull of infants.
• Kyphosis is a humpback.
• Lordosis is a swayback condition.
• Scoliosis is an abnormal lateral curvature of the spine.
• A herniated disc can pinch and doesn't heal.
• Sciatica is pain affecting the back and outer side of the leg caused by compression of a spinal nerve root.
• Pes planus, or flat foot condition.
• Thalidomide, a drug to stop morning sickness in pregnant women, affected the growth of limbs in infants.

Joint Classification

• Structurally, joints are classified as fibrous, cartilaginous, or synovial based on the material that comprises the joint and whether a space occurs between the articulating joints.
• Fibrous joints have no joint cavity, and bones are held together by dense regular (fibrous) connective tissue, making them non-moveable.
• Cartilaginous joints have no joint cavity, and bones are held together by cartilage, making them partially moveable.
• Synovial joints have a filled joint cavity that separates the articulating surfaces of the bones and ligaments.
• Synarthroses are immovable fibrous joints, such as the suture of the skull.
• Amphiarthroses are slightly moveable cartilaginous joints.
• Diathroses are freely moveable synovial joints.

Joint Components

• Articular cartilage is a thin layer of cartilage on the articulating end of each bone in the articulation.
• Articular cartilage reduces friction in joints, acts as a spongy cushion to absorb compression, and prevents damage to the ends of articulating bones.
• Synovial fluid lubricates articular cartilage, nourishes the chondrocytes of articular cartilage, and acts as a shock absorber for the joints.
• Ligaments are dense regular connective tissue that connects bone to bone, providing strength and flexibility to stabilize and reinforce most synovial joints.
• Tendons are dense regular connective tissue that connects bone to muscle, alleviating friction where they overly and rub together.
• Bursae are fibrous sacs found where there is excessive friction, such as in the wrist and ankle.
• Tendon sheaths are elongated bursae found in areas of excessive friction, like the wrist or ankle.

Joint Types

• Plane joint: uniaxial with flat surfaces.
• Hinge joint: uniaxial with movement around one axis only, the elbow joint.
• Pivot joint: uniaxial with rotation, such as between vertebrae and the radius and ulna.
• Condylar, ellipsoid joint: biaxial and found in phalanges.
• Saddle joint: biaxial and found in the thumb.
• Ball and socket joint: multiaxial and the most freely moveable synovial joint, such as the shoulder and hip.

Joint Injuries and Conditions

• Dislocation of the Glenohumeral Joint: the head of the humerus dislocates out of the glenoid cavity due to a wide range of motion.
• Hip and coxal joints are very strong but have a limited range of motion.
• Knee injuries:Unhappy Triad, can occur when the knee is impacted from the lateral aspect and is driven medially.
• Knee injuries involve: Tearing of the tibial collateral ligaments, tearing of the medial meniscus, and tearing of the anterior cruciate ligament (ACL).
• Arthritis: inflammation or degenerative disease of joints.
• Gouty arthritis/gout: a metabolic disorder where uric acid in the blood forms crystals, mostly in the big toe.
• Osteoarthritis: most common and related to aging processes, involving wear and tear on joints.
• Rheumatoid arthritis: an autoimmune disease that destroys synovial joints, articular cartilage, and sometimes bone, potentially leading to ankylosis, where bone ends fuse.

Muscle Types

• Skeletal muscle is voluntary, striated, multinucleated, and attached to tendons.
• Cardiac muscle is involuntary, striated, has one nucleus, and is connected by intercalated discs.
• Smooth muscle is visceral, non-striated, spindle-shaped, and involuntary.
• Muscle cells are also known as muscle fibers.
• Actin is the thin filament in muscle cells.
• Myosin is the thick filament in muscle cells.
• Contraction is due to the movement of myofilaments.

Muscle and Tissue Structure

• Skeletal mass accounts for 40% of body mass.
• Endomysium surrounds each single muscle fiber.
• Perimysium surrounds each fascicle, or bundle of muscle fibers.
• Epimysium covers the entire skeletal muscle tendon or aponeuroses.
• Fascia is outside of the epimysium.
• Deep fascia is dense fibrous connective tissue that surrounds blood vessels, muscles, and nerves.
• Superficial fascia, or reticular layer of the dermis, shapes the body.

Skeletal Muscle Characteristics

• Excitability: the ability to respond to stimuli.
• Conductivity: the ability to propagate electrical signals.
• Contractility: the ability to shorten and generate force.
• Elasticity: the ability to return to original length after stretching.
• Extensibility: the ability to be stretched.
• Myoblasts, embryonic skeletal muscle stem cells, fuse together during development to form multinucleated skeletal muscle cells/fibers.
• Sarcolemma: the plasma membrane of muscle fibers
• Sarcoplasmic reticulum: a specialized smooth endoplasmic reticulum that stores Ca++ ions, which are required for muscle contraction.
• Myofibril: bundles of protein myofilaments; myofibrils are aligned to make distinct bands of striations.
• Sarcomere: a contractile unit of muscle fibers; the structural and functional unit of skeletal muscle; from Z disc to Z disc.
• Alternating I and A bands cause muscle fibers to appear striated or "banded".
• Connectin, also called titin, is a protein that contributes to muscle elasticity.
• Nebulin acts as a "ruler" to set the length of the thin filament when the sarcomere is being assembled.
• Dystrophin: genetic disorder "muscle dystrophy" caused by abnormal structure or amount of the dystrophin protein.

Neural Control

• Motor unit: one motor neuron and all skeletal cells it stimulates.
• Neuromuscular junction: a gap or space between the axon terminal end bulb/knob, nerve ending, of a motor neuron and a muscle cell, also known as the synaptic cleft.
• Synaptic cleft: gap between axon terminal end bulb/knob of the motor neuron and muscle cell membrane.
• Motor end plate: where the terminal end bulb knob connects.
• Motor end plate: contains a vast number of Acetylcholine Receptors.
• Neurotransmitters: chemicals released by a neuron upon arrival of electrical nerve impulses at the terminal end bulb/knob where the nerve synapses with muscle fibers, and in skeletal muscle, the neurotransmitter is ACh.
• Acetylcholine (ACh) always stimulates skeletal muscle to contract.
• Isotonic contraction: myofilaments are able to slide past each other during contraction and muscle shorten, for example, lifting a light weight.
• Isometric contraction: tension in the muscle increases, but the muscle is unable to shorten, for example, when pushing against bricks or trying to move immovable objects.