Joints and Muscles: Structure, Function, and Disorders

Joints (Articulations)

Structure of Diarthrosis Joints

Diarthrosis joints, also known as synovial joints, are the most movable type of joint in the human body. Their structure allows for a wide range of motion and is essential for locomotion and manipulation.

• Articular Cartilage: Hyaline cartilage covering the ends of bones, reducing friction and absorbing shock.

• Joint (Synovial) Cavity: A space filled with synovial fluid, allowing free movement.

• Articular Capsule: Double-layered structure surrounding the joint; consists of an outer fibrous layer and an inner synovial membrane.

• Synovial Fluid: Viscous fluid secreted by the synovial membrane, lubricating and nourishing the joint.

• Ligaments: Bands of dense connective tissue that reinforce the joint and limit movement to prevent injury.

• Nerves and Blood Vessels: Supply the joint, providing nutrients and sensory feedback.

Example: The knee, shoulder, and elbow are classic examples of diarthrosis joints.

Joint Classification

Joints are classified both structurally and functionally:

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue; little to no movement (e.g., sutures of the skull).

  • Cartilaginous Joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

  • Synovial Joints: Bones separated by a fluid-filled cavity; freely movable (e.g., shoulder, hip).

• Functional Classification:

  • Synarthrosis: Immovable joints.

  • Amphiarthrosis: Slightly movable joints.

  • Diarthrosis: Freely movable joints.

Movement Terms

Movements at synovial joints are described by specific terms:

• Flexion/Extension: Decreasing/increasing the angle between bones (e.g., bending and straightening the elbow).

• Abduction/Adduction: Moving a limb away from/toward the midline.

• Rotation: Turning a bone around its own axis (e.g., turning the head).

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Other terms: Pronation, supination, inversion, eversion, elevation, depression, etc.

Bursae and Meniscus

Accessory structures that aid joint function:

• Bursae: Small fluid-filled sacs that reduce friction between moving structures (e.g., tendons and bones).

• Meniscus (plural: menisci): Crescent-shaped fibrocartilage pads that improve fit and stability of the joint (e.g., knee menisci).

Joint Disorders

• Arthritis: Inflammation of joints, including osteoarthritis (degenerative) and rheumatoid arthritis (autoimmune).

• Dislocation: Displacement of bones at a joint.

• Sprain: Stretching or tearing of ligaments.

Muscles: Naming, Structure, and Function

Naming Muscles

Muscles are named based on several criteria:

• Location: e.g., brachialis (arm).

• Shape: e.g., deltoid (triangle-shaped).

• Size: e.g., gluteus maximus (largest).

• Direction of fibers: e.g., rectus abdominis (straight).

• Number of origins: e.g., biceps brachii (two heads).

• Location of attachments: e.g., sternocleidomastoid (sternum, clavicle, mastoid process).

• Action: e.g., flexor carpi radialis (flexes wrist).

Muscle Roles: Prime Mover, Antagonist, Synergist, Fixator

• Prime Mover (Agonist): Main muscle responsible for a movement.

• Antagonist: Muscle that opposes the action of the prime mover.

• Synergist: Assists the prime mover by adding force or reducing unwanted movement.

• Fixator: Stabilizes the origin of the prime mover.

Example: In elbow flexion, the biceps brachii is the prime mover, triceps brachii is the antagonist, and brachialis acts as a synergist.

Fascicle Arrangement and Examples

The arrangement of muscle fibers (fascicles) affects muscle power and range of motion:

• Parallel: Fibers run parallel to the long axis (e.g., sartorius).

• Pennate: Short fibers attach obliquely to a central tendon (e.g., rectus femoris).

• Convergent: Broad origin, fibers converge toward a single tendon (e.g., pectoralis major).

• Circular: Fibers arranged in concentric rings (e.g., orbicularis oris).

Muscle Identification and Injection Sites

• Common Muscles for Intramuscular Injections:

  • Deltoid (shoulder)

  • Gluteus medius (hip)

  • Vastus lateralis (thigh)

Muscle Disorders

• Muscular Dystrophy: Genetic disorder causing progressive muscle weakness.

• Myasthenia Gravis: Autoimmune disease affecting neuromuscular transmission.

• Strain: Overstretching or tearing of muscle fibers.

Muscle Organization and Physiology

Muscle Organization

Muscles are organized in hierarchical structures:

• Muscle (organ) → Fascicle → Muscle fiber (cell) → Myofibril → Myofilaments (actin and myosin)

Sarcomere Structure

The sarcomere is the basic contractile unit of muscle fiber:

• Z lines: Define the boundaries of a sarcomere.

• A band: Dark area; length of thick filaments (myosin).

• I band: Light area; thin filaments (actin) only.

• H zone: Central region of A band with only thick filaments.

• M line: Center of the sarcomere, supporting proteins.

Neuromuscular Junction

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber:

• Motor neuron releases acetylcholine (ACh) into the synaptic cleft.

• ACh binds to receptors on the muscle fiber, triggering an action potential.

• Action potential leads to muscle contraction.

Muscle Contraction

Muscle contraction is explained by the sliding filament theory:

• Calcium ions bind to troponin, exposing binding sites on actin.

• Myosin heads attach to actin, forming cross-bridges.

• ATP is hydrolyzed, providing energy for the power stroke.

• Result: Sarcomere shortens, causing muscle contraction.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Myoglobin

Myoglobin is an oxygen-binding protein in muscle fibers, facilitating oxygen storage and transport within muscle tissue.

General Characteristics of Muscle Tissue

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original length.

Types of Muscle Tissue: Comparison

Type	Location	Control	Striations	Function
Skeletal	Attached to bones	Voluntary	Yes	Movement of body
Cardiac	Heart	Involuntary	Yes	Pumping blood
Smooth	Walls of hollow organs	Involuntary	No	Movement of substances

Isotonic vs. Isometric Contraction

• Isotonic Contraction: Muscle changes length (shortens or lengthens) while tension remains constant; produces movement.

• Isometric Contraction: Muscle tension increases but length does not change; no movement occurs.

Example: Lifting a weight (isotonic); holding a weight steady (isometric).

Muscle Fatigue

Muscle fatigue is the decline in ability of a muscle to generate force, often due to:

• Depletion of ATP

• Accumulation of lactic acid

• Impaired calcium regulation

Subtypes of Skeletal Muscle

Subtype	Contraction Speed	Fatigue Resistance	Color	Example
Type I (Slow-twitch)	Slow	High	Red	Postural muscles
Type IIa (Fast-twitch, oxidative)	Fast	Intermediate	Red-pink	Leg muscles
Type IIb (Fast-twitch, glycolytic)	Fastest	Low	White	Arm muscles

Muscle Disorders

• Muscle cramps: Sudden, involuntary contractions.

• Fibromyalgia: Chronic widespread pain and tenderness.