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

Joints

General Structure of Joints

Joints, or articulations, are sites where two or more bones meet. They are classified based on their structure and the type of tissue that connects the bones.

• Fibrous Joints: Bones are joined by dense fibrous connective tissue. These joints are mostly immovable (synarthroses). Examples include sutures of the skull.

• Cartilaginous Joints: Bones are united by cartilage. These joints allow more movement than fibrous joints but less than synovial joints. Examples include synchondroses (hyaline cartilage, e.g., epiphyseal plates) and symphyses (fibrocartilage, e.g., intervertebral discs).

• Synovial Joints: Bones are separated by a fluid-filled joint cavity. These are freely movable joints (diarthroses) and include most joints of the limbs. Key features include articular cartilage, joint (synovial) cavity, articular capsule, synovial fluid, reinforcing ligaments, and nerves and blood vessels.

Example: The knee is a synovial joint, while the sutures of the skull are fibrous joints.

Muscles and Muscle Tissue

Ion Channels in the Plasma Membrane

Ion channels regulate the movement of ions across the plasma membrane, crucial for muscle and nerve function.

• Leak Channels: Always open, allowing ions to move according to their concentration gradients.

• Voltage-Gated Channels: Open or close in response to changes in membrane potential.

• Ligand-Gated Channels: Open when a specific chemical (ligand) binds to the channel protein.

Comparison Table:

Channel Type	Stimulus for Opening	Example
Leak	None (always open)	K+ leak channels
Voltage-Gated	Change in membrane potential	Na+ channels in neurons
Ligand-Gated	Binding of neurotransmitter	ACh receptor channels

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).

• Repolarization: Return of the membrane potential to its resting value after depolarization.

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

Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

(Nernst equation for K+ equilibrium potential)

Types of Muscle Tissue

There are three basic types of muscle tissue, each with distinct characteristics.

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

• Cardiac Muscle: Involuntary, striated, branched, found in the heart, contains intercalated discs.

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

Comparison Table:

Feature	Skeletal	Cardiac	Smooth
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Location	Bones	Heart	Hollow organs

Functions of Muscle Tissue

Muscle tissue performs several essential functions:

• 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.

• 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.

• Tendons: Connect muscle to bone.

Microscopic Structure of Skeletal Muscle Fiber

Muscle fibers contain specialized structures for contraction:

• Myofibrils: Rod-like units containing contractile proteins.

• Sarcomeres: Functional contractile units within myofibrils.

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

• 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 is the basic contractile unit of muscle fiber, defined by specific bands and lines:

• I-band: Light band, contains thin filaments only.

• A-band: Dark band, contains thick filaments (with overlapping thin filaments).

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

• M-line: Center of H-zone, holds thick filaments together.

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

Sliding Filament Model of Muscle Contraction

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

• Myosin heads bind to actin, forming cross-bridges.

• ATP hydrolysis powers the movement of myosin heads, pulling actin filaments toward the center.

Equation:

$\text{Muscle Tension} = \text{Number of Cross-Bridges} \times \text{Force per Cross-Bridge}$

Changes in Sarcomere During Contraction

During contraction:

• I-band: Shortens

• A-band: Remains the same

• H-zone: Disappears

• Z-lines: Move closer together

• M-line: Remains in the center

Stimulation of Muscle Fibers: Neuromuscular Junction

The neuromuscular junction is the site where a motor neuron stimulates a muscle fiber.

• Action potential arrives at axon terminal.

• Acetylcholine (ACh) is released into the synaptic cleft.

• ACh binds to receptors on the sarcolemma, opening ligand-gated channels.

• Sodium ions enter, depolarizing the membrane and initiating an action potential in the muscle fiber.

Excitation-Contraction Coupling

This process links the muscle fiber action potential to contraction:

• Action potential travels along 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 cycling begins.

Cross-Bridge Cycle

The cross-bridge cycle describes the sequence of events during muscle contraction:

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

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.

5. Cycle repeats as long as Ca2+ and ATP are present.

6. Relaxation occurs when Ca2+ is pumped back into the SR and cross-bridges detach.

Contraction of Smooth and Cardiac Muscle

Smooth and cardiac muscle contractions differ from skeletal muscle:

• Smooth Muscle: Contraction is slower, can be sustained for long periods, regulated by autonomic nervous system and hormones, lacks sarcomeres but contains actin and myosin.

• Cardiac Muscle: Contraction is rhythmic and involuntary, cells are connected by intercalated discs, action potentials can spread from cell to cell.

• Skeletal Muscle: Contraction is rapid and voluntary, controlled by somatic nervous system, organized into sarcomeres.

Example: Cardiac muscle contracts to pump blood, smooth muscle contracts to move food through the digestive tract, skeletal muscle contracts to move the skeleton.

Additional info: Where the original content was brief, academic context and definitions were added for clarity and completeness.