Musculoskeletal System Overview

Introduction

The musculoskeletal system comprises bones, joints, muscles, and connective tissues, providing structural support, movement, and protection for the body. Understanding its anatomy and physiology is essential for grasping how the body moves and maintains stability.

Bone Growth and Structure

Appositional and Longitudinal Growth

• Appositional growth refers to the increase in bone diameter by adding new layers to the surface.

• Long bones also grow longitudinally at the epiphyseal plates during development.

• Epiphyseal line forms when growth ceases, replacing the plate with bone tissue.

Bone Tissue Types

• Compact bone is homoeotic, containing red marrow and sites for blood cell synthesis.

• Spongy bone contains trabeculae and is found at the ends of long bones.

Endochondral Ossification Steps

• I. Bone collar forms around diaphysis

• II. Cartilage in diaphysis calcifies and cavities form

• III. Periosteal bud invades and spongy bone forms

• IV. Diaphysis elongates and medullary cavity forms

• V. Epiphyses ossify

Functions of the Skeletal System

• Support, protection, movement, hematopoiesis (blood cell formation), lipid storage, and hormone production.

Joints and Articulations

Types of Joints

• Synovial joints are freely movable and contain synovial fluid within a capsule.

• Fibrocartilage structures, such as the pubic symphysis, provide limited movement and shock absorption.

• Menisci are fluid-filled packets that reduce friction in joints.

Joint Stability

• Stability is influenced by articular surfaces, muscle tone, and ligaments.

Common Joint Damage

• Intercalated disks are unique to cardiac muscle, not joints.

• Most common form of joint damage is osteoarthritis, involving cartilage breakdown.

Muscle Structure and Physiology

Muscle Types

• Skeletal muscle: Voluntary, striated, responsible for movement.

• Cardiac muscle: Involuntary, striated, found in the heart.

• Smooth muscle: Involuntary, non-striated, found in organs and vessels.

Muscle Contraction Mechanism

• During contraction, the H zone decreases in length until disappearing.

• Ca2+ signals bind to troponin, exposing myosin binding sites on actin.

• F-actin refers to the whole strand of actin filaments; G-actin is the individual actin monomer.

• Isometric contraction: muscle length remains constant while tension increases.

Motor Units

• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.

• Smaller motor units are found in muscles requiring fine control (e.g., fingers).

Muscle Fiber Types

• Slow twitch fibers: Fatigue-resistant, high endurance, more mitochondria.

• Fast twitch fibers: Fatigue quickly, generate rapid force, fewer mitochondria.

Sarcomere Structure

• Sarcomeres are the functional units of muscle, spanning from Z-line to Z-line.

• Arrangement of thick (myosin) and thin (actin) filaments determines striation.

Muscle Contraction Steps

• Action potential arrives at neuromuscular junction (NMJ).

• Ca2+ released from sarcoplasmic reticulum.

• Troponin binds Ca2+, exposing actin binding sites.

• Myosin heads bind actin, causing contraction.

Rigor Mortis

• Occurs when ATP is depleted post-mortem, preventing detachment of myosin from actin.

• Ca2+ cannot be pumped out, leading to sustained contraction.

Bone and Joint Anatomy (Image-Based)

Key Structures in Synovial Joints

Bone Fracture and Repair

Steps in Fracture Repair

• I. Hematoma formation

• II. Fibrocartilaginous callus formation

• III. Bony callus formation

• IV. Bone remodeling

Calcium Homeostasis

Regulation of Blood Ca2+ Levels

• Calcitriol levels increase absorption.

• PTH (parathyroid hormone) increases blood Ca2+ by stimulating bone resorption.

• Calcitonin decreases blood Ca2+ by inhibiting bone resorption.

Wolff's Law

Bone Remodeling in Response to Stress

• Bones adapt to the loads under which they are placed.

• Weight training increases bone density and strength.

• Loss of stress (e.g., inactivity) leads to bone resorption and weakening.

Muscle Tissue Features

Articulations and Movement

Types of Joint Movement

• Nonaxial: No axis of movement (e.g., gliding joints).

• Uniaxial: Movement in one plane (e.g., hinge joints).

• Biaxial: Movement in two planes (e.g., saddle joints).

• Multiaxial: Movement in multiple planes (e.g., ball-and-socket joints).

Examples of Articulations

• Head of femur and acetabulum: Ball-and-socket joint (multiaxial).

• Flat bones of the skull: Sutures (nonaxial).

• Carpal bones: Gliding joints (nonaxial).

• Ulna and humerus: Hinge joint (uniaxial).

• Metacarpal and trapezium: Saddle joint (biaxial).

Energy Pathways in Muscle Activity

ATP Production

• During initial physical activity, ATP is obtained via the anaerobic pathway (glycolysis).

• Direct phosphorylation of creatine phosphate is used after stored ATP is exhausted.

• During recovery, oxygen is added back, converting lactic acid into pyruvate for the Krebs cycle.

Phases of Muscle Contraction

• Latent period: Time between stimulus and contraction.

• Contraction period: Muscle fibers shorten.

• Relaxation period: Muscle fibers return to resting length.

Key Terms and Definitions

• Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells.

• Osteocytes: Mature bone cells maintaining bone matrix.

• Perimysium: Connective tissue sheath encapsulating muscle fibers.

• Endosteum: Membrane lining the inner surface of bone.

Important Equations

• ATP Hydrolysis:

• Calcium Homeostasis:

• Muscle Force:

Additional info:

• Some context and definitions have been expanded for clarity and completeness.

• Tables have been reconstructed based on the image and question content.