Muscle Structure and Function in Vertebrates (Lecture 12)

Muscle Types in Vertebrates

Overview of Muscle Types

Vertebrates possess three primary types of muscle tissue: cardiac, smooth, and skeletal. Each type is specialized for distinct physiological functions and is characterized by unique structural and regulatory features.

• Cardiac Muscle: Found only in the heart, responsible for pumping blood. Cells are branched, striated, and interconnected by gap junctions.

• Smooth Muscle: Located in walls of internal organs (e.g., digestive tract, blood vessels). Cells are spindle-shaped, non-striated, and contract involuntarily.

• Skeletal Muscle: Attached to bones, responsible for voluntary movements. Cells are long, multinucleate, and striated.

Structural Comparison

All muscle types utilize actin and myosin filaments for contraction, but their arrangement and control mechanisms differ.

• Cardiac and Skeletal Muscle: Striated due to regular arrangement of actin and myosin.

• Smooth Muscle: Non-striated; actin and myosin are irregularly arranged.

Skeletal Muscle Structure

Muscle Fiber Organization

Skeletal muscle fibers are large, multinucleate cells packed with myofibrils. Each myofibril consists of repeating units called sarcomeres, the fundamental contractile unit.

• Muscle fibers: Single cells containing many nuclei.

• Myofibrils: Bundles of actin (thin) and myosin (thick) filaments.

• Sarcomere: Bounded by Z lines, contains A band (myosin), I band (actin), H zone (non-overlapping region), and M band (myosin support proteins).

Antagonistic Muscle Pairs

Movement around joints is achieved by antagonistic pairs of muscles: one contracts while the other relaxes. Ligaments hold bones together, and tendons attach muscles to bones.

Sarcomere Structure and Function

Detailed Sarcomere Anatomy

The sarcomere is the basic unit of muscle contraction. Its structure is defined by the arrangement of actin and myosin filaments, supported by proteins such as titin.

• Z line: Anchors actin filaments.

• A band: Contains all myosin filaments.

• M band: Supports myosin filaments.

• H zone and I band: Regions where actin and myosin do not overlap in relaxed muscle.

• Titin: Runs from Z line to Z line, stabilizing myosin bundles.

Muscle Contraction Mechanism

Sliding Filament Theory

Muscle contraction occurs as actin and myosin filaments slide past each other, shortening the sarcomere. This process is known as the sliding filament theory.

• During contraction, the H zone and I band narrow, and Z lines move toward the A band.

• Myosin heads bind to actin, forming cross-bridges and pulling actin filaments inward.

Actin and Myosin Structure

Actin filaments are composed of two chains of actin molecules twisted together, associated with regulatory proteins troponin and tropomyosin. Myosin filaments consist of many myosin molecules, each with a globular head capable of binding actin and hydrolyzing ATP.

The Contractile Cycle

The contractile cycle involves several steps:

1. Myosin head binds ATP and is in a low-energy state.

2. ATP is hydrolyzed to ADP and Pi, energizing the myosin head.

3. Myosin head binds actin, forming a cross-bridge.

4. Release of ADP and Pi triggers the power stroke, sliding actin past myosin.

5. Binding of new ATP releases myosin from actin, restarting the cycle.

Regulation by Calcium, Troponin, and Tropomyosin

Muscle contraction is regulated by the presence of Ca2+ ions. At rest, troponin and tropomyosin block myosin binding sites on actin. When Ca2+ binds to troponin, the complex changes shape, exposing binding sites and allowing contraction.

Neural Control and Excitation-Contraction Coupling

Motor Neurons and Action Potentials

Skeletal muscle contraction is initiated by action potentials from motor neurons. Each motor neuron can stimulate multiple muscle fibers, forming a motor unit. Depolarization opens sodium channels, generating action potentials in the muscle fiber.

T-tubules and Sarcoplasmic Reticulum

Action potentials travel deep into the muscle fiber via T-tubules, which are continuous with the plasma membrane. T-tubules are closely associated with the sarcoplasmic reticulum (SR), a specialized organelle that stores Ca2+. Upon stimulation, Ca2+ is released from the SR into the cytoplasm, triggering contraction.

Muscle Fiber Types and Athletic Performance

Types of Skeletal Muscle Fibers

Skeletal muscle fibers are classified based on their metabolic properties and contraction speed:

• Type I (Slow Oxidative, Red): High myoglobin, many mitochondria, fatigue-resistant, optimal for endurance activities.

• Type IIa (Fast Oxidative, Red): High myoglobin, rapid ATP splitting, resistant to fatigue, suited for middle-distance activities.

• Type IIb (Fast Glycolytic, White): Low myoglobin, few mitochondria, fatigues easily, optimal for short, intense activities.

Fiber Type	Myoglobin	Mitochondria	Fatigue Resistance	Optimal Activity
Type I (Slow Oxidative)	High	Many	High	Endurance (e.g., marathon)
Type IIa (Fast Oxidative)	High	Many	Moderate	Middle distance
Type IIb (Fast Glycolytic)	Low	Few	Low	Sprinting, weightlifting

Exercise and Muscle Adaptation

Exercise influences muscle fiber composition and function:

• Strength training: Increases actin and myosin filaments, enlarging muscles.

• Aerobic exercise: Enhances oxidative capacity, increases capillary density, myoglobin content, and mitochondria.

Drugs Affecting Muscle Function

Curare and Neuromuscular Blockade

Curare is a plant-derived toxin that blocks acetylcholine (ACh) receptors at the neuromuscular junction, preventing muscle contraction and causing paralysis.

Botulinum Toxin (Botox)

Botulinum toxin, produced by Clostridium botulinum, inhibits ACh release, causing paralysis. Used therapeutically for muscle spasms and cosmetically for wrinkle reduction. Effects are temporary and localized, but improper administration can cause serious side effects.

Summary Table: Key Muscle Components

Component	Function
Tropomyosin	Blocks myosin-binding sites on actin
Sarcoplasmic reticulum	Stores calcium ions in muscle
Troponin	Calcium binding protein involved in muscle contraction
T-tubule	Propagates action potential into the middle of a myofiber
Thick filament	Responsible for higher density of A-band in striated muscle
Acetylcholine	Initiates depolarization of a myofiber

Practice Questions

Multiple Choice Example

Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?

1. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane.

2. Transverse (T) tubules transmit the depolarization to the sarcoplasmic reticulum.

3. Calcium is released into the cytoplasm.

4. Tropomyosin is shifted, which unblocks the cross-bridge binding sites.

5. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.

Correct answer: E) 5 → 3 → 2 → 1 → 4

Matching Example

Function	Component
Blocks myosin-binding sites on actin	Tropomyosin
Stores calcium ions in muscle	Sarcoplasmic reticulum
Calcium binding protein involved in muscle contraction	Troponin
Propagates action potential into the middle of a myofiber	T-tubule
Responsible for higher density of A-band in striated muscle	Thick filament
Initiates depolarization of a myofiber	Acetylcholine

Key Equations

ATP Hydrolysis in Muscle Contraction

The hydrolysis of ATP by myosin provides energy for muscle contraction:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Conclusion

Muscle structure and function are central to vertebrate physiology, enabling movement, circulation, and organ function. Understanding the molecular mechanisms of contraction, regulation, and adaptation is essential for biology students.