Study Guide: Joints and Muscle Tissue (Chapters 8-9)

Joints

General Structure of Joints

Joints, also known as articulations, are sites where two or more bones meet. They are classified based on their structure and function. The three main structural types are fibrous joints, cartilaginous joints, and synovial joints.

• Fibrous Joints: Bones are joined by dense connective tissue, and there is no joint cavity. Most are immovable. Examples: Sutures of the skull, syndesmoses between certain long bones, and gomphoses (tooth sockets).

• Cartilaginous Joints: Bones are united by cartilage (hyaline or fibrocartilage), without a joint cavity. They allow more movement than fibrous joints but less than synovial joints. Examples: Synchondroses (epiphyseal plates), symphyses (pubic symphysis, intervertebral discs).

• Synovial Joints: Bones are separated by a fluid-filled joint cavity. These joints are freely movable (diarthroses) and are the most common type in the body. Examples: Shoulder, knee, elbow, hip joints.

Muscles and Muscle Tissue

Ion Channels in the Plasma Membrane

Ion channels are proteins that allow ions to move across the plasma membrane, crucial for muscle and nerve function.

• Leak Channels: Always open, allowing ions to move according to their concentration gradients. Help establish the resting membrane potential.

• Voltage-Gated Channels: Open or close in response to changes in membrane potential. Essential for action potentials in excitable tissues.

• Ligand-Gated Channels: Open when a specific chemical (ligand) binds to the channel protein. Important at synapses and neuromuscular junctions.

Excitable Tissues and Membrane Potentials

Excitable tissues, such as muscle and nerve cells, can generate and propagate electrical signals.

• Resting Membrane Potential: The voltage difference across the plasma membrane in a resting cell, typically around -70 mV in neurons. Originates from differences in ion concentrations and membrane permeability.

• Depolarization: Membrane potential becomes less negative (more positive), often due to Na+ influx.

• Repolarization: Return of the membrane potential to its resting value, usually by K+ efflux.

• Hyperpolarization: Membrane potential becomes more negative than the resting potential.

Types of Muscle Tissue

There are three basic types of muscle tissue, each with distinct structure and function.

• Skeletal Muscle: Voluntary, striated, multinucleated. Responsible for body movement.

• Cardiac Muscle: Involuntary, striated, branched, with intercalated discs. Found only in the heart.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped cells. Found in walls of hollow organs.

Functions of Muscle Tissue

• Producing movement

• Maintaining posture

• Stabilizing joints

• Generating heat

Gross Structure of Skeletal Muscle

Skeletal muscle is organized into bundles and connective tissue layers:

• Fascicles: Bundles of muscle fibers within a muscle.

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds fascicles.

• Epimysium: Surrounds the entire muscle.

• Fascia: Connective tissue that separates muscles from each other.

• Aponeuroses: Broad, flat tendons connecting muscles to bones or other muscles.

• Tendons: Cord-like structures attaching muscle to bone.

Microscopic Structure of Skeletal Muscle Fiber

• Myofibrils: Rod-like units within muscle fibers, composed of sarcomeres.

• Sarcomeres: The contractile units of muscle, defined by Z-lines.

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions for contraction.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Sarcolemma: Plasma membrane of a muscle fiber.

• T Tubules: Invaginations of the sarcolemma that conduct impulses deep into the fiber.

Sarcomere Structure

The sarcomere contains both contractile and non-contractile components:

• I-band: Light region containing only thin filaments (actin).

• A-band: Dark region containing thick filaments (myosin), including areas of overlap with thin filaments.

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

• M-line: Center of the H-zone, where thick filaments are linked.

• Z-line (Z-disc): Boundary of each sarcomere; anchors thin filaments.

Sliding Filament Model of Muscle Contraction

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere without changing the length of the filaments themselves.

• Myosin heads bind to actin, forming cross-bridges and pulling the thin filaments toward the center of the sarcomere.

• ATP is required for both the power stroke and the detachment of myosin from actin.

Changes in Sarcomere Bands During Contraction

• I-band: Shortens

• A-band: Remains the same length

• H-zone: Shortens or disappears

• Z-lines: Move closer together

• M-line: Remains central

Neuromuscular Junction and Muscle Fiber Stimulation

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber to initiate contraction.

• Action potential arrives at the axon terminal, causing release of acetylcholine (ACh).

• ACh binds to receptors on the sarcolemma, opening ligand-gated channels and generating an action potential in the muscle fiber.

Excitation-Contraction Coupling

This process links the muscle fiber action potential to the activation of cross-bridge cycling and contraction.

• Action potential travels along the sarcolemma and T tubules.

• Triggers release of Ca2+ from the sarcoplasmic reticulum.

• Ca2+ binds to troponin, shifting tropomyosin and exposing binding sites on actin.

Cross-Bridge Cycle

• Myosin head attaches to actin (cross-bridge formation).

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

• ATP binds to myosin, causing detachment from actin.

• ATP hydrolysis re-cocks the myosin head.

• Relaxation occurs when Ca2+ is pumped back into the SR and binding sites are covered.

Contraction of Smooth, Cardiac, and Skeletal Muscle

• Skeletal Muscle: Voluntary, rapid, and forceful contractions; requires neural stimulation.

• Cardiac Muscle: Involuntary, rhythmic contractions; cells connected by intercalated discs; can contract without neural input due to pacemaker cells.

• Smooth Muscle: Involuntary, slow, sustained contractions; found in walls of hollow organs; can be stimulated by hormones, stretch, or autonomic nerves.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of hollow organs
Contraction Speed	Fast	Moderate	Slow