Joints

Structures of Synovial Joints

Synovial joints are the most movable type of joint in the human body, characterized by the presence of a fluid-filled joint cavity. The five main structures are:

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

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

• Articular Capsule: Double-layered capsule enclosing the joint cavity; 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 and stabilize the joint.

Additional info: Other structures may include bursae and menisci in certain joints.

Types of Joints

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

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

• Synovial Joints: Freely movable joints with a synovial cavity (e.g., knee, shoulder).

Joint Stability

Joint stability is influenced by:

• Shape of Articular Surfaces

• Ligament Number and Location

• Muscle Tone

Ligaments Stabilizing the Knee

• Anterior Cruciate Ligament (ACL): Prevents anterior displacement of tibia.

• Posterior Cruciate Ligament (PCL): Prevents posterior displacement of tibia.

• Medial Collateral Ligament (MCL): Stabilizes medial aspect.

• Lateral Collateral Ligament (LCL): Stabilizes lateral aspect.

• Patellar Ligament: Connects patella to tibia.

Shoulder Joint Features

• Glenoid Cavity: Shallow socket for humeral head.

• Rotator Cuff Muscles: Provide dynamic stability.

• Labrum: Fibrocartilaginous rim deepening the socket.

• Loose Capsule: Allows wide range of motion.

Joint Injuries and Conditions

• Sprains: Ligament damage.

• Dislocations: Bone displacement.

• Arthritis: Inflammatory joint disease (e.g., osteoarthritis, rheumatoid arthritis).

Fracture Repair Process

1. Hematoma Formation: Blood clot forms at fracture site.

2. Fibrocartilaginous Callus Formation: Soft callus bridges the gap.

3. Bony Callus Formation: New bone replaces soft callus.

4. Bone Remodeling: Bone is reshaped to original form.

Muscle

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, attached to bones.

• Cardiac Muscle: Involuntary, striated, found in heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

Characteristics of Muscle

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original length.

Functions of Muscle

• Movement: Locomotion and manipulation.

• Posture Maintenance

• Joint Stabilization

• Heat Generation

Myofilaments in Muscle

• Actin (Thin Filament): Contains binding sites for myosin.

• Myosin (Thick Filament): Motor protein with heads for cross-bridge formation.

• Regulatory Proteins: Tropomyosin and Troponin control actin-myosin interaction.

Neuromuscular Junction Events

1. Action potential arrives at axon terminal.

2. Acetylcholine (ACh) released into synaptic cleft.

3. ACh binds to receptors on sarcolemma.

4. Depolarization of muscle fiber membrane.

Action Potential Propagation

The action potential travels along the sarcolemma, triggering calcium release from the sarcoplasmic reticulum, leading to muscle contraction.

Cross Bridge Cycling

1. Myosin head attaches to actin (cross bridge formation).

2. Power stroke: Myosin head pivots, pulling actin.

3. ATP binds to myosin, detaching it from actin.

4. ATP hydrolysis re-cocks myosin head.

Skin

Cutaneous Sensation and Receptors

• Meissner's Corpuscles: Light touch, found in dermal papillae.

• Pacinian Corpuscles: Deep pressure, vibration, found in deep dermis.

• Merkel Cells: Light pressure, found in epidermis.

• Free Nerve Endings: Pain, temperature, found throughout skin.

Layers of the Epidermis

• Stratum Basale: Deepest layer, mitotically active.

• Stratum Spinosum: Provides strength and flexibility.

• Stratum Granulosum: Keratinization begins.

• Stratum Lucidum: Present in thick skin only.

• Stratum Corneum: Outermost, dead keratinized cells.

Cells of the Epidermis

• Keratinocytes: Produce keratin.

• Melanocytes: Produce melanin pigment.

• Langerhans Cells: Immune defense.

• Merkel Cells: Sensory receptors.

Pigments Giving Skin Color

• Melanin: Brown-black pigment.

• Carotene: Yellow-orange pigment.

• Hemoglobin: Red pigment from blood.

Types of Sweat Glands

• Eccrine Glands: Widely distributed, regulate temperature.

• Apocrine Glands: Found in axilla and groin, active after puberty.

Skin Protection Mechanisms

• Physical barrier (keratinized cells)

• Chemical barrier (acid mantle, antimicrobial proteins)

• Biological barrier (immune cells)

Types of Skin Cancer

• Basal Cell Carcinoma: Most common, least dangerous.

• Squamous Cell Carcinoma: Arises from keratinocytes.

• Melanoma: Most dangerous, arises from melanocytes.

Bone

Functions of Bone

• Support

• Protection

• Movement

• Mineral Storage

• Blood Cell Formation (hematopoiesis)

Types of Bone Cells

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells, maintain bone matrix.

• Osteoclasts: Bone-resorbing cells.

• Bone Lining Cells: Maintain bone surfaces.

Organic and Inorganic Components of Bone

• Organic: Collagen fibers, ground substance (provides flexibility).

• Inorganic: Hydroxyapatite (calcium phosphate crystals, provide hardness).

Bone Remodeling

Bone is continuously renewed through resorption (by osteoclasts) and deposition (by osteoblasts), regulated by hormones and mechanical stress.

Fracture Repair and Growth

• Intramembranous Ossification: Bone develops from fibrous membrane (e.g., flat bones).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

• Postnatal Growth: Bones grow in length at epiphyseal plates and in width by appositional growth.

Types of Skeletal Cartilages

• Hyaline Cartilage: Most abundant, found in nose, ribs, joints.

• Elastic Cartilage: Flexible, found in ear and epiglottis.

• Fibrocartilage: Strong, found in intervertebral discs, menisci.

Summary Table: Types of Bone Cells and Their Functions

Key Equations

• Muscle Contraction:

• Bone Remodeling:

Additional Info

Some questions required inference and expansion for completeness. For example, the process of bone repair and the types of skin receptors were elaborated based on standard Anatomy & Physiology knowledge.