Study Guide: Bones, Joints, and Muscles in Human Anatomy & Physiology

Chapter 6: Bones and Skeletal Tissue

Main Cells of Bone Tissue

The bone tissue contains several specialized cell types, each with distinct functions in bone formation, maintenance, and remodeling.

• Osteoblasts: Cells responsible for bone formation; they synthesize and secrete the bone matrix.

• Osteocytes: Mature bone cells derived from osteoblasts; they maintain bone tissue.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone tissue.

• Bone Lining Cells: Flat cells found on bone surfaces where bone remodeling is not occurring.

Bone Remodeling: Osteoclasts resorb bone, while osteoblasts form new bone, maintaining bone strength and mineral homeostasis.

Hormones in Blood Calcium Regulation

Blood calcium levels are tightly regulated by several hormones:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclast activity and calcium release from bones.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and promoting calcium deposition in bones.

• Vitamin D (Calcitriol): Enhances calcium absorption from the intestine.

Mechanism: When blood calcium is low, PTH is released; when high, calcitonin is secreted.

Types of Fractures

Fractures are classified based on their characteristics and the pattern of the break:

• Closed (Simple) Fracture: Bone breaks but does not penetrate the skin.

• Open (Compound) Fracture: Bone breaks and pierces the skin, increasing infection risk.

• Displaced Fracture: Bone fragments are out of alignment.

• Compression Fracture: Bone is crushed, often seen in vertebrae.

• Spiral Fracture: Caused by twisting forces.

• Comminuted Fracture: Bone breaks into several pieces.

• Greenstick Fracture: Incomplete break, common in children.

Steps in Repair: Hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Chapter 8: Joints

Types of Joints

Joints are classified by their structure and function:

• Synarthroses: Immovable joints (e.g., sutures in the skull).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., shoulder, knee).

Examples: Synarthroses - cranial sutures; Amphiarthroses - pubic symphysis; Diarthroses - elbow joint.

Types of Synovial Joints

Synovial joints are characterized by a fluid-filled joint cavity and allow various movements:

• Pivot Joint: Allows rotation (e.g., atlas and axis in the neck).

• Hinge Joint: Allows flexion and extension (e.g., elbow, knee).

• Condyloid Joint: Allows movement in two planes (e.g., wrist).

• Ball-and-Socket Joint: Allows movement in all directions (e.g., shoulder, hip).

Structure: Synovial joints have articular cartilage, synovial membrane, joint cavity, and supporting ligaments.

Chapter 9: Muscles and Muscle Tissue

Structure of Thick and Thin Filaments

Muscle contraction is driven by the interaction of thick and thin filaments within the sarcomere.

• Thick Filaments: Composed mainly of myosin proteins.

• Thin Filaments: Composed of actin, troponin, and tropomyosin.

• Arrangement: Filaments are organized in repeating units called sarcomeres.

Cross Bridge Cycle: Myosin heads bind to actin, pull, release, and reset, causing muscle contraction.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber, enabling muscle activation.

• Events: Action potential arrives at axon terminal, acetylcholine (ACh) is released, ACh binds to receptors on muscle fiber, leading to depolarization and muscle contraction.

• Structure: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

ATP Replenishment in Muscle Cells

Muscle cells require ATP for contraction and relaxation. ATP is replenished by several mechanisms:

• Creatine Phosphate: Transfers phosphate to ADP to form ATP rapidly.

• Anaerobic Glycolysis: Produces ATP without oxygen, yielding lactic acid.

• Aerobic Respiration: Produces ATP using oxygen in mitochondria.

Equation:

$\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

Differences Between Skeletal and Smooth Muscle

Skeletal and smooth muscles differ in structure, function, and control.

Feature	Skeletal Muscle	Smooth Muscle
Location	Attached to bones	Walls of hollow organs
Control	Voluntary	Involuntary
Cell Shape	Long, cylindrical, multinucleated	Spindle-shaped, single nucleus
Striations	Present	Absent
Contraction Speed	Fast	Slow

Example: Skeletal muscle - biceps brachii; Smooth muscle - intestinal wall.