Classification of Joints

Types and Subdivisions of Joints

Joints, also known as articulations, are the sites where two or more bones meet. They are classified based on their structure and function.

• Fibrous Joints (Synarthroses): These joints are connected by dense connective tissue and allow little to no movement. Examples: Sutures of the skull.

• Cartilaginous Joints (Amphiarthroses): Bones are joined by cartilage, allowing limited movement. Examples: Intervertebral discs, pubic symphysis.

• Synovial Joints (Diarthroses): These joints have a synovial cavity and are freely movable. Examples: Shoulder, knee, hip joints.

Subdivision Types: Joints can be further classified by their shape and movement:

• Plane (Gliding) Joints

• Hinge Joints

• Pivot Joints

• Condyloid Joints

• Saddle Joints

• Ball-and-Socket Joints

Movement of Joints: Joints allow various types of movement such as flexion, extension, abduction, adduction, rotation, and circumduction.

Bursae

Bursae are small fluid-filled sacs that reduce friction between moving structures in joints, such as tendons and bones. They are commonly found in areas of high movement, like the shoulder and knee.

Homeostatic Imbalances of Joints

• Arthritis: Inflammation of joints, leading to pain and stiffness.

• Bursitis: Inflammation of a bursa.

• Dislocation: Displacement of bones at a joint.

Muscular System

Types of Muscle Tissue

There are three main types of muscle tissue, each with distinct characteristics and functions.

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs; responsible for movements like peristalsis.

Comparison Table:

Functions and Special 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 shape.

Anatomy of Skeletal Muscle

Skeletal muscle is organized into bundles of fibers surrounded by connective tissue layers:

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers.

Microscopically, muscle fibers contain myofibrils composed of repeating units called sarcomeres, which are the functional units of contraction.

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber. When an action potential reaches the axon terminal, it triggers the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the muscle fiber, initiating a muscle action potential.

Events at the Neuromuscular Junction and Muscle Contraction

• Generation of Action Potential: Action potential travels along the sarcolemma.

• Excitation-Contraction Coupling: The process by which the action potential leads to muscle contraction.

• Cross Bridge Cycle: Myosin heads bind to actin, pull, and release, causing muscle shortening.

Key Steps in the Cross Bridge Cycle:

1. Myosin binds to actin forming a cross-bridge.

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

3. ATP binds to myosin, causing detachment from actin.

4. ATP hydrolysis re-cocks the myosin head.

Equation for ATP Hydrolysis:

Muscle Twitch and Tone

• Muscle Twitch: A single, brief contraction and relaxation cycle in a muscle fiber.

• Muscle Tone: The continuous and passive partial contraction of muscles, important for posture.

Energy for Contraction

Muscle contraction requires energy, primarily supplied by ATP. Sources include:

• Stored ATP

• Creatine phosphate

• Anaerobic glycolysis

• Aerobic respiration

Types of Muscle Contractions

• Isometric Contraction: Muscle tension increases, but length does not change.

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

Force of Muscle Contraction

The force generated by a muscle depends on:

• Number of muscle fibers recruited

• Size of muscle fibers

• Frequency of stimulation

• Degree of muscle stretch

Refractory Period

The refractory period is the time after an action potential during which a muscle fiber cannot respond to another stimulus.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs and is responsible for involuntary movements such as peristalsis. It contracts differently from skeletal muscle, using a sliding filament mechanism but with different regulatory proteins.

Muscle Naming and Classification

How Muscles Are Named

Muscles are named based on:

• Location (e.g., biceps brachii in the arm)

• Shape (e.g., deltoid is triangular)

• Size (e.g., gluteus maximus is large)

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

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

• Location of attachments

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

Muscle Mechanics: Arrangement of Fascicles

Fascicle arrangement affects muscle function and range of motion:

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

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

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

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

Muscle Roles: Agonist, Antagonist, Synergist, Fixator

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

• Antagonist: Opposes the action of the agonist.

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

• Fixator: Stabilizes the origin of the agonist.

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

Additional info: For practical exams, students should review the specific actions of muscles listed in their course's muscle sheet.