Chapter 6: Skeletal System

Main Divisions of the Skeletal System

The skeletal system is divided into two main parts:

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Includes the bones of the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton.

Number of Bones

• The adult human skeleton typically consists of 206 bones.

Functions of Bone

Bones perform several vital functions in the body:

• Support: Provides structural support for the body and soft tissues.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Attachment site: Serves as points of attachment for muscles, enabling movement.

• Storage: Stores minerals such as calcium and phosphate.

• Blood cell production: Occurs in red bone marrow (hematopoiesis).

• Energy storage: Yellow bone marrow stores fat.

Types of Cartilage

There are three main types of cartilage:

• Hyaline cartilage

• Elastic cartilage

• Fibrocartilage

Bone Markings and Features

Bones have various surface features that serve as attachment points or passageways:

• Tuberosity: Rounded projection

• Crest: Narrow, prominent ridge

• Trochanter: Large, blunt, irregular surface (found only on femur)

• Line: Narrow ridge of bone

• Tubercle: Small rounded projection

• Epicondyle: Raised area above a condyle

• Spine: Sharp, slender projection

• Process: Any bony prominence

• Meatus: Canal-like passageway

• Sinus: Cavity within a bone

• Fossa: Shallow, basinlike depression

• Groove: Narrow, slitlike opening

• Foramen: Round or oval opening through a bone

• Head: Bony expansion carried on a narrow neck

• Facet: Smooth, nearly flat articular surface

• Condyle: Rounded articular projection

• Ramus: Armlike bar

Anatomy of a Long Bone

• Diaphysis: Shaft of the bone

• Epiphysis: Ends of the bone

• Epiphyseal plate: Area of growth between diaphysis and epiphysis

• Medullary cavity: Central cavity containing bone marrow

Bone Cells

Bone tissue is maintained by four main types of cells:

• Osteogenic cells: Stem cells that differentiate into osteoblasts

• Osteoblasts: Bone-forming cells

• Osteocytes: Mature bone cells

• Osteoclasts: Bone-resorbing cells

Osteoclasts are responsible for breaking down bone tissue. Osteoblasts build bone tissue.

Blood Cell Production

• The process of blood cell production in bone marrow is called hematopoiesis.

Bone Membranes

• Periosteum: Outer fibrous layer covering the bone; contains osteogenic cells, osteoblasts, and is secured to bone by Sharpey's fibers.

• Endosteum: Lines internal bone surfaces; contains osteoblasts and osteoclasts.

Bone Matrix and Minerals

• Bones are a major reservoir for minerals, especially calcium phosphate and calcium carbonate.

Importance of Calcium

• Muscle contraction

• Transmission of nerve impulses

• Enzyme activity

• Blood clotting

• Secretion by glands and nerve cells

• Cell division

Hormonal Regulation of Calcium

• Parathyroid hormone (PTH): Increases blood calcium levels

• Calcitonin: Lowers blood calcium levels

Fracture Classification

• Nondisplaced: Bone ends retain normal position

• Displaced: Bone ends out of alignment

• Complete: Bone broken all the way through

• Incomplete: Not broken all the way through

• Linear: Parallel to long axis of bone

• Transverse: Perpendicular to long axis of bone

• Compound (open): Bone ends penetrate the skin

• Simple (closed): Bone ends do not penetrate the skin

• Comminuted: Bone breaks into three or more pieces

• Compression: Bone is crushed

• Greenstick: Bone breaks incompletely (common in children)

Ossification (Bone Formation)

There are two main processes of bone formation:

1. Intramembranous ossification: Bone develops from fibrous membrane (forms flat bones, e.g., cranial bones).

2. Endochondral ossification: Bone forms by replacing hyaline cartilage (forms most of the rest of the skeleton).

Bone formation begins in the second month of development and continues until early adulthood. Bone remodeling and repair are lifelong processes.

Epiphyseal Plate and Growth

• Epiphyseal plates close between ages 18-25.

• Growth hormone, thyroid hormone, testosterone, and estrogen regulate bone growth and epiphyseal plate closure.

Bone Repair

1. Hematoma formation: Blood vessels break, forming a clot.

2. Fibrocartilaginous callus formation: Soft callus forms as new blood vessels and fibroblasts invade.

3. Bony callus formation: New trabeculae form a hard callus.

4. Bone remodeling: Bone is remodeled to original shape.

Axial and Appendicular Skeleton

• Axial skeleton: Skull, vertebral column, thoracic cage

• Appendicular skeleton: Pectoral girdle, pelvic girdle, bones of the limbs

Vertebral Column

• Cervical vertebrae: Smallest, most delicate, have transverse foramina

• Thoracic vertebrae: Articulate with ribs, have long spinous processes

• Lumbar vertebrae: Largest, bear most weight

The 12 thoracic vertebrae correspond to the 12 pairs of ribs.

Pelvic Girdle

• Ilium: Largest part of the hip bone

• Ischium: Posterior, inferior part

• Pubis: Anterior part

The femur is the largest and longest bone in the body.

Bone Diseases

• Osteoporosis: Bone resorption outpaces bone deposit, leading to fragile bones.

• Paget's disease: Excessive and haphazard bone deposit and resorption.

• Rickets: Vitamin D deficiency in children, leading to bowed legs and deformities.

• Osteomalacia: Softening of bones in adults due to vitamin D deficiency.

Chapter 9/10: Muscular System

Types of Muscle Tissue

• Skeletal muscle: Striated, voluntary

• Cardiac muscle: Striated, involuntary, found in the heart

• Smooth muscle: Non-striated, involuntary, found in walls of hollow organs

Functions of Muscle

• Movement

• Posture

• Stabilization of joints

• Heat production

• Regulation of organ volume

Muscle Structure

• Muscle fiber: Muscle cell

• Myofibril: Rod-like unit within muscle fiber

• Fascicle: Bundle of muscle fibers

• Perimysium: Connective tissue surrounding fascicles

• Endomysium: Connective tissue surrounding each muscle fiber

Contractile and Regulatory Proteins

• Actin and myosin: Main contractile proteins

• Troponin and tropomyosin: Regulatory proteins

Sarcomere

• The sarcomere is the smallest contractile unit of muscle, extending from one Z disc to the next.

Neuromuscular Junction (NMJ)

• The NMJ is the site where a motor neuron communicates with a skeletal muscle fiber.

• Neurotransmitter released: Acetylcholine (ACh)

• Binding of ACh to receptors on the sarcolemma opens Na+ channels, initiating muscle contraction.

Muscle Fiber Types

• Slow-twitch fibers: Aerobic, fatigue-resistant

• Fast-twitch fibers: Anaerobic, fatigue quickly

Muscle Contraction

• Sarcoplasmic reticulum (SR): Stores and releases calcium ions needed for contraction

• Sliding filament theory: Explains how actin and myosin filaments slide past each other to shorten the muscle

Summary Table: Types of Muscle Tissue

Key Equations

• Calcium homeostasis:

• Sliding filament:

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.