Joints and Their Functional Classification

Overview of Joint Function

Joints, or articulations, are connections between bones that allow for varying degrees of movement. The functional classification of joints is based on the amount of movement they permit.

• Immovable Joints (Synarthroses):

  • These joints do not allow movement.

  • Bones are rigidly joined by fibrous tissue or cartilage.

  • Examples: Sutures between skull bones, manubriosternal joint.

• Slightly Movable Joints (Amphiarthroses):

  • Allow limited or slight movement.

  • Usually connected by cartilage.

  • Examples: Pubic symphysis, intervertebral joints.

• Freely Movable Joints (Diarthroses/Synovial Joints):

  • Permit a wide range of movement.

  • Characterized by a synovial cavity filled with synovial fluid.

  • Examples: Elbow, shoulder, hip joints.

Types of Joints by Structure

Major Joint Types

Joints can also be classified structurally based on the material binding the bones and the presence of a joint cavity.

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

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity (e.g., intervertebral discs, pubic symphysis).

• Synovial Joints: Bones separated by a fluid-filled joint cavity; most common and movable type (e.g., knee, elbow, shoulder).

Types of Synovial Joints

Classification by Movement

Synovial joints are further classified by the type of movement they allow:

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

• Saddle Joint: Allows movement in two planes (e.g., thumb/carpometacarpal joint).

• Plane (Gliding) Joint: Allows sliding or gliding movements (e.g., acromioclavicular joint).

• Pivot Joint: Allows rotation around a single axis (e.g., atlantoaxial joint in the neck).

• Condyloid (Ellipsoid) Joint: Allows movement but no rotation (e.g., metacarpophalangeal joints).

• Ball and Socket Joint: Allows movement in multiple axes and planes (e.g., hip, shoulder).

Types of Joint Movements

Basic Movements at Synovial Joints

Joints allow for a variety of movements, each defined by the direction and nature of the motion:

• Flexion: Decreasing the angle between two bones (e.g., bending the elbow).

• Extension: Increasing the angle between two bones (e.g., straightening the knee).

• Abduction: Moving a limb away from the midline (e.g., raising the arm sideways).

• Adduction: Moving a limb toward the midline (e.g., lowering the arm to the side).

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

• Circumduction: Moving a limb in a circular motion (e.g., arm circles).

• Dorsiflexion: Moving the top of the foot toward the shin.

• Plantarflexion: Moving the sole of the foot downward (pointing toes).

• Inversion: Turning the sole of the foot inward.

• Eversion: Turning the sole of the foot outward.

• Gliding: Sliding one bone over another (e.g., wrist movements).

• Protraction: Moving a body part forward (e.g., jutting the jaw).

• Retraction: Moving a body part backward (e.g., pulling the jaw back).

Example: The shoulder joint (a ball and socket joint) allows for flexion, extension, abduction, adduction, rotation, and circumduction.

Muscle Tissue Types and Characteristics

Overview of Muscle Types

Muscle tissue is specialized for contraction and is classified into three main types, each with distinct structure and function.

General Characteristics of Muscle Tissue

• Excitability (Responsiveness): Ability to respond to stimuli (chemical, electrical, stretch).

• Conductivity: Ability to propagate electrical signals along the membrane.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched without damage.

• Elasticity: Ability to return to original length after stretching.

Muscle Fiber Structure

Key Components

• Sarcolemma: Plasma membrane of a muscle fiber.

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

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

• Myofibrils: Bundles of contractile proteins (actin and myosin) organized into sarcomeres.

• Sarcomere: The functional contractile unit of muscle, defined by Z-lines.

Muscle Contraction and Action Potential

Steps in Muscle Contraction

1. Action Potential Initiation: A nerve impulse triggers the opening of Na+ channels, depolarizing the sarcolemma.

2. Propagation: The action potential spreads along the sarcolemma and down T-tubules.

3. Calcium Release: Depolarization causes the SR to release Ca2+ into the sarcoplasm.

4. Cross-Bridge Formation: Ca2+ binds to troponin, exposing binding sites on actin for myosin heads to attach, forming cross-bridges.

5. Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

6. Detachment: ATP binds to myosin, causing it to detach from actin and reset for another cycle.

Equation (Sliding Filament Theory):

Types of Muscle Fibers

Slow vs. Fast Muscle Fibers

Muscle fibers differ in their contraction speed and resistance to fatigue.

Additional info: Most muscles contain a mix of both fiber types, such as the gastrocnemius.

Muscle Contraction Patterns

Myogram and Contraction Phases

• Latent Period: Time between stimulus and contraction onset.

• Contraction Period: Muscle fibers shorten and tension increases.

• Relaxation Period: Muscle tension decreases as fibers return to resting length.

Treppe (Staircase Effect): Repeated stimuli cause a gradual increase in contraction strength.

Tetanus: Rapid, repeated stimulation prevents relaxation, resulting in sustained contraction.

Summary Table: Joint Movements