Muscle Tissue Types and Terminology

Terminology and Muscle Tissue Types

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Prefixes such as myo-, mys-, and sarco- are commonly used in muscle-related terminology. For example, sarcoplasm refers to the cytoplasm of a muscle cell.

• Skeletal Muscle: Attached to bones and skin, responsible for voluntary movements, and characterized by striations.

• Cardiac Muscle: Found only in the heart, involuntary, striated, and contracts at a steady rate due to the heart's pacemaker.

• Smooth Muscle: Located in the walls of hollow organs (e.g., stomach, urinary bladder, airways), involuntary, and non-striated.

Comparison of Muscle Tissue Types

The three muscle tissue types differ in structure, function, and control mechanisms.

Characteristics and Functions of Muscle Tissue

Muscle Tissue Characteristics

All muscle tissues share four main characteristics:

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

Functions of Muscle Tissue

• Produce Movement: Responsible for locomotion and manipulation (e.g., walking, digesting, pumping blood).

• Maintain Posture and Body Position: Stabilizes the body during activity.

• Stabilize Joints: Supports and reinforces joints.

• Generate Heat: Muscle contraction produces heat, helping maintain body temperature.

Skeletal Muscle Structure and Organization

Connective Tissue Sheaths

Each skeletal muscle and muscle fiber is covered by connective tissue sheaths that support and reinforce the muscle:

• Epimysium: Dense irregular connective tissue surrounding the entire muscle.

• Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).

• Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Organizational Levels of Skeletal Muscle

Skeletal muscle is organized into hierarchical levels:

• Muscle (organ)

• Fascicle (bundle of muscle fibers)

• Muscle fiber (cell)

• Myofibril (organelle)

• Myofilament (protein filaments: actin and myosin)

Microscopic Anatomy of Skeletal Muscle

Myofibrils and Myofilaments

Myofibrils are densely packed, rodlike elements within muscle fibers, accounting for about 80% of cell volume. They contain repeating units called sarcomeres, which are the basic contractile units.

• Striations: Alternating dark (A bands) and light (I bands) regions.

• Sarcomere: Distance between two Z discs; site of contraction.

• Myofilaments: Actin (thin) and myosin (thick) filaments arranged in a hexagonal pattern.

• Regulatory proteins: Tropomyosin and troponin complex regulate contraction.

Sarcomere Structure

The sarcomere is composed of overlapping thick (myosin) and thin (actin) filaments. The arrangement of these filaments creates the striated appearance and is essential for muscle contraction.

Sarcoplasmic Reticulum and T Tubules

Structure and Function

The sarcoplasmic reticulum (SR) is a network of smooth endoplasmic reticulum tubules surrounding each myofibril. It stores and releases calcium ions (Ca2+), which are essential for muscle contraction. T tubules are invaginations of the sarcolemma that conduct action potentials into the muscle fiber.

• Triad: Consists of a T tubule and two terminal cisterns of the SR.

• Muscle action potential: Propagates along the sarcolemma and down T tubules, triggering Ca2+ release.

Sliding Filament Theory of Muscle Contraction

Mechanism of Contraction

Muscle contraction occurs when myosin heads bind to actin, forming cross bridges. The sliding filament theory explains how sarcomeres shorten:

• Cross bridge attachments form and break, pulling thin filaments toward the center of the sarcomere.

• I bands shorten, Z discs move closer, H zones disappear, and A bands move closer together.

Neuromuscular Junction and Muscle Contraction

Ion Channels and Action Potentials

Ion channels play a major role in changing membrane potentials:

• Chemically gated ion channels: Opened by neurotransmitters (e.g., acetylcholine).

• Voltage-gated ion channels: Open or close in response to voltage changes.

Neuromuscular Junction (NMJ)

The NMJ is the site where a motor neuron stimulates a muscle fiber. The axon terminal and muscle fiber are separated by the synaptic cleft. Synaptic vesicles contain acetylcholine (ACh), which binds to receptors on the sarcolemma, triggering muscle contraction.

Events at the Neuromuscular Junction

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters motor neuron.

3. Ca2+ entry causes release of ACh into synaptic cleft.

4. ACh binds to receptors, opening Na+ channels and generating end plate potential (EPP).

5. Acetylcholinesterase degrades ACh, terminating the signal.

Generation of Action Potential Across the Sarcolemma

Depolarization and Repolarization

• Depolarization: If EPP reaches threshold, voltage-gated Na+ channels open, triggering an action potential.

• Repolarization: Na+ channels close, K+ channels open, restoring resting membrane potential.

• Refractory period: Muscle fiber cannot be stimulated until repolarization is complete.

• Na+-K+ pump: Restores ionic conditions.

Excitation-Contraction (E-C) Coupling

Steps in E-C Coupling

E-C coupling refers to the events that transmit the action potential along the sarcolemma and trigger contraction:

1. Muscle action potential propagates along sarcolemma and down T tubules.

2. Stored Ca2+ is released from the SR.

3. Ca2+ binds to troponin, moving tropomyosin and exposing myosin binding sites on actin.

4. Myosin binds to actin, initiating contraction (cross bridge cycling).

Cross Bridge Cycle

Steps of the Cross Bridge Cycle

1. Cross bridge formation: High-energy myosin head attaches to actin.

2. Power stroke: Myosin head pivots, pulling actin toward the M line.

3. Cross bridge detachment: ATP binds to myosin, causing detachment.

4. Cocking of myosin head: ATP hydrolysis re-cocks myosin head for the next cycle.

Clinical Applications

Botulinum Toxin (Botox)

• Used to treat migraines, muscle spasms, and wrinkles by paralyzing muscles.

Black Widow Spider Venom

• Enhances ACh secretion, causing excessive muscle contraction and abdominal cramps.

Rigor Mortis

• Postmortem muscle stiffness due to lack of ATP, resulting in sustained cross bridge formation.

Additional info: The notes have been expanded with academic context and examples for clarity and completeness.