Chapter 8: Joints (Articulations)

Types of Joints: Synarthrosis, Amphiarthrosis, and Diarthrosis

Joints, or articulations, are connections between bones that allow for varying degrees of movement. They are classified based on their structure and function.

• Synarthrosis: Immovable joints. Example: Sutures in the skull.

• Amphiarthrosis: Slightly movable joints. Example: Intervertebral discs between vertebrae.

• Diarthrosis: Freely movable joints. Example: Shoulder, hip, and knee joints.

Fibrous and Cartilaginous Joints

Joints can also be classified by the material connecting the bones.

• Fibrous Joints: Bones joined by dense connective tissue; mostly immovable. Example: Sutures of the skull.

• Cartilaginous Joints: Bones joined by cartilage; allow limited movement. Example: Pubic symphysis, intervertebral discs.

Synovial Joint Structure

Synovial joints are the most movable type of joint in the body. They have a unique structure that allows for a wide range of motion.

• Articular cartilage: Covers bone surfaces, reducing friction.

• Joint (synovial) cavity: Space filled with synovial fluid.

• Synovial membrane: Produces synovial fluid for lubrication.

• Fibrous capsule: Encloses the joint, providing stability.

• Ligaments: Reinforce and support the joint.

Types of Synovial Joints

There are six main types of synovial joints, each allowing different movements.

Body Movements

Joints allow for various types of body movements, which can be described and identified.

• 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.

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

Joint Injuries and Homeostatic Imbalances

Joints are susceptible to various injuries and disorders that can disrupt homeostasis.

• Sprain: Ligament injury due to overstretching.

• Dislocation: Bones forced out of alignment.

• Arthritis: Inflammation of joints, including osteoarthritis and rheumatoid arthritis.

Homeostatic imbalance in joints can lead to pain, reduced mobility, and chronic conditions.

Chapter 9: Muscle Structure and Function

Unique Traits of Muscle Cells

Muscle cells, or fibers, have specialized properties that enable contraction and movement.

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original shape.

Gross Anatomy of Skeletal Muscle

Skeletal muscles are composed of bundles of muscle fibers surrounded by connective tissue.

• Epimysium: Surrounds entire muscle.

• Perimysium: Surrounds fascicles (bundles of fibers).

• Endomysium: Surrounds individual muscle fibers.

Microscopic Structure and Functional Roles

Muscle fibers contain specialized organelles for contraction.

• Myofibrils: Rod-like units containing contractile proteins.

• Sarcoplasmic reticulum: Stores and releases calcium ions.

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

Sliding Filament Model of Muscle Contraction

The sliding filament model explains how muscles contract at the molecular level.

• Actin and myosin filaments slide past each other, shortening the muscle.

• ATP is required for myosin heads to detach and reattach to actin.

Equation:

Major Steps of Muscle Activation

Muscle contraction involves several key steps:

1. Action potential

2. Release of acetylcholine

3. Calcium release

4. Cross-bridge formation

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.

• Presynaptic terminal: Releases neurotransmitter (acetylcholine).

• Synaptic cleft: Space between neuron and muscle.

• Postsynaptic membrane: Contains receptors for acetylcholine.

Depolarization vs. Repolarization

These processes are essential for muscle fiber activation.

• Depolarization: Sodium ions enter the cell, making the inside more positive.

• Repolarization: Potassium ions exit the cell, restoring the negative resting potential.

ATP Sources in Muscle Contraction

ATP is the energy currency for muscle contraction, supplied by several mechanisms.

• Creatine phosphate: Rapidly regenerates ATP.

• Anaerobic glycolysis: Produces ATP without oxygen.

• Aerobic respiration: Produces ATP with oxygen in mitochondria.

Factors Influencing Force of Muscle Contraction

Several factors affect the strength of muscle contraction.

• Number of muscle fibers recruited

• Size of muscle fibers

• Frequency of stimulation

• Degree of muscle stretch

Chapter 10: Muscle Actions and Naming

Muscle Roles: Origin, Insertion, Prime Mover, Antagonist, Synergist

Muscles are classified by their roles in movement.

• Origin: Fixed attachment point.

• Insertion: Movable attachment point.

• Prime mover (agonist): Main muscle responsible for movement.

• Antagonist: Opposes the prime mover.

• Synergist: Assists the prime mover.

Muscle Locations and Actions

Muscles are named and identified based on their location and the movements they produce.

• Flexion: Biceps brachii (arm flexion)

• Extension: Triceps brachii (arm extension)

• Abduction: Deltoid (arm abduction)

• Adduction: Adductor longus (thigh adduction)

Criteria for Naming Muscles

Muscles are named according to several criteria:

• Location: e.g., tibialis anterior (front of tibia)

• Shape: e.g., deltoid (triangular)

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

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

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

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