Musculoskeletal System: Structure, Function, and Pathophysiology

Musculoskeletal System

Introduction

The musculoskeletal system is essential for movement, support, and protection of the human body. It consists of bones, muscles, tendons, ligaments, and associated connective tissues. This guide covers the microscopic and macroscopic structure of skeletal muscle, mechanisms of muscle contraction, fracture repair, and the pathophysiology of common joint disorders.

Muscle Tissue: Macro to Micro Structure

Organizational Levels of Skeletal Muscle

Skeletal muscle is organized from large groups down to microscopic components, each surrounded by connective tissue layers.

• Muscle: Composed of bundles of fascicles, covered externally by the epimysium.

• Fascicle: A bundle of muscle fibers, surrounded by perimysium.

• Muscle Fiber (Cell): An elongated, multinucleate cell, surrounded by endomysium.

• Myofibril: Rod-like structures within muscle fibers, made up of myofilaments.

• Myofilaments: The contractile proteins (actin and myosin) within myofibrils.

Table: Structure and Organizational Levels of Skeletal Muscle

Level	Description	Connective Tissue Wrapping
Muscle	Hundreds to thousands of muscle cells, blood vessels, nerve fibers	Epimysium
Fascicle	Discrete bundle of muscle cells	Perimysium
Muscle Fiber	Elongated, multinucleate cell (striated)	Endomysium
Myofibril	Contractile organelle within muscle fiber	None
Myofilament	Actin and myosin filaments	None

Microscopic Structure of Muscle Tissue

Muscle Fiber Components

• Muscle fibers: Long, cylindrical cells with multiple nuclei.

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber, containing organelles and myofibrils.

• Myofibrils: Made up of myofilaments (actin and myosin).

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions (Ca2+).

• T tubules: Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

Myofilaments and Sarcomere

• Sarcomere: The smallest contractile unit of muscle fiber; functional unit for contraction.

• Thick filaments: Composed of the protein myosin.

• Thin filaments: Composed of actin, with regulatory proteins tropomyosin and troponin bound to actin.

Muscle Contraction Mechanism

Excitation-Contraction Coupling

Muscle contraction is initiated by signals from the nervous system and involves several steps:

1. Excitation of motor cortex and motor neurons in the spinal cord.

2. Action potential arrivesat the neuromuscular junction and spreads along the muscle fiber.

3. Action potential triggers the release of Ca2+ from the sarcoplasmic reticulum.

4. Ca2+ enables myofilaments to slide past each other (cross-bridge cycle), resulting in contraction.

5. Relaxation occurs when Ca2+ is reabsorbed and myofilaments return to resting position.

Neuromuscular Junction

• Action potential arrives at the axon terminal of the motor neuron.

• Voltage-gated calcium channels open, allowing Ca2+ entry.

• Ca2+ triggers release of acetylcholine (ACh) into the synaptic cleft.

• ACh binds to receptors on the sarcolemma, opening Na+ channels and generating an end plate potential.

• ACh is degraded by acetylcholinesterase, terminating the signal.

Fracture Repair

Steps of Fracture Repair

1. Formation of hematoma at the fracture site.

2. Fibrocartilaginous callus formation.

3. Bony callus formation.

4. Bone remodeling restores the bone's original shape and structure.

Bone Fractures

Manifestations and Treatment

• Manifestations: Unnatural alignment, swelling, muscle spasm, tenderness, pain, impaired sensation, possible muscle spasms.

• Treatment: Closed manipulation, traction, open reduction, internal and external fixation.

Factors Delaying Bone Healing

• Patient's age

• Current medications

• Debilitating diseases

• Local stress around the fracture site

• Circulatory problems

• Coagulation disorders

• Poor nutrition

Support Structure Injuries

Types of Injuries

• Strain: Tear or injury to a tendon.

• Sprain: Tear or injury to a ligament.

• Avulsion: Complete separation of a tendon or ligament from its bony attachment site.

Osteoarthritis (OA)

Pathophysiology and Evaluation

Osteoarthritis is a degenerative joint disease that increases with age and joint stress (e.g., obesity). It is characterized by:

• Local areas of damage

• Loss of articular cartilage

• New bone formation at joint margins (osteophytes/bone spurs)

• Subchondral bone changes

• Variable degrees of synovitis and joint capsule thickening

Evaluation:

• Pain and stiffness in unilateral joints

• Enlargement, tenderness, limited motion, joint instability, deformity

• Clinical assessment, X-ray, CT, arthroscopy, MRI

Rheumatoid Arthritis (RA)

Pathophysiology and Evaluation

Rheumatoid arthritis is an inflammatory joint disease caused by systemic autoimmune damage to connective tissue, primarily affecting the synovial membrane.

• Presence of rheumatoid factors (RF): antibodies (IgG and IgM) against antibodies

• Inflammatory exudate in joint fluid

Evaluation: Diagnosis requires four or more of the following:

• Morning joint stiffness lasting at least 1 hour

• Arthritis of 3 or more joint areas

• Arthritis of hand joints

• Symmetric arthritis

• Rheumatoid nodules

• Abnormal serum rheumatoid factor

• Radiographic changes

Comparison: Osteoarthritis vs. Rheumatoid Arthritis

Key Differences

Feature	Osteoarthritis (OA)	Rheumatoid Arthritis (RA)
Nature	Degenerative	Autoimmune, inflammatory
Joint Involvement	Unilateral, localized	Symmetric, multiple joints
Cartilage	Loss of articular cartilage	Inflammation of synovial membrane
Bone Changes	Osteophytes, subchondral changes	Erosions, joint deformity
Systemic Symptoms	Rare	Common

References

• Marieb, E.N. & Hoehn, K. (2023). Human Anatomy & Physiology, Global Edition (12th ed.). Pearson Education Inc.

• Huether, S.E., McCance, K.L., Brashers, V.L., & List, S.L. (2022). Understanding Pathophysiology (4th ANZ ed.). Elsevier Australia.

• Potter, J., Douglas, C., Rebeiro, G., & Waters, D. (2020). Potter and Perry's Fundamentals of Nursing (6th ANZ ed.). Elsevier Australia.