Topic 2.3 – Physiology of Muscle

Overview

This section covers the microscopic structure and physiology of muscle tissue, focusing on skeletal muscle, but also comparing it to cardiac and smooth muscle. Key concepts include muscle cell anatomy, mechanisms of contraction, neuromuscular junctions, contractile properties, muscle metabolism, and differences among muscle types.

General Features of Muscle Cells

Muscle Cell Terminology and Structure

• Muscle fibers: Elongated cells specialized for contraction.

• Terminology: Prefixes myo- and sarco- refer to muscle.

• Contraction depends on actin and myosin myofilaments.

Comparison of Muscle Types

The three muscle types differ in structure, location, and function.

Functions and Characteristics of Muscle

Muscle Functions

• Movement: Locomotion, manipulation, blood pressure regulation, respiration, propulsion of food and urine.

• Posture: Maintains body position against gravity.

• Stabilization: Stabilizes joints (e.g., shoulders, knees).

• Heat Generation: Maintains body temperature (skeletal muscle is a major contributor).

Functional Characteristics of Muscle

• Excitability (Irritability): Ability to receive and respond to stimuli (usually chemical, e.g., neurotransmitter).

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to return to resting length after stretching.

Microscopic Anatomy of Skeletal Muscle Fiber

Structure and Terminology

• Sarcolemma: Plasma membrane of muscle cell.

• Sarcoplasm: Cytoplasm of muscle cell, rich in glycogen and myoglobin (oxygen-binding protein).

• Myofibrils: Rod-like contractile elements, make up ~80% of cell volume.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum, stores and releases Ca2+.

• T-tubules: Invaginations of sarcolemma, conduct action potentials into cell interior.

• Myofilaments: Actin (thin) and myosin (thick) filaments arranged in repeating units called sarcomeres.

Connective Tissue Organization

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles of fibers (fascicles).

• Epimysium: Surrounds entire muscle.

Sarcomere Structure

• Sarcomere: Functional contractile unit, extends from one Z disc to the next.

• A band: Dark band, contains thick filaments (myosin).

• I band: Light band, contains thin filaments (actin) only.

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

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

• Z disc: Anchors thin filaments, defines sarcomere boundaries.

Myofilament Composition

• Thick filaments (myosin): Each myosin molecule has a tail and two heads (with ATPase and actin-binding sites).

• Thin filaments (actin): Composed of F-actin (filamentous, polymer of G-actin monomers), tropomyosin (blocks myosin-binding sites), and troponin (binds Ca2+).

Excitation-Contraction Coupling

Neuromuscular Junction (NMJ)

• Site where motor neuron communicates with muscle fiber.

• Neurotransmitter: Acetylcholine (ACh).

• Action potential in neuron triggers ACh release, which binds to receptors on sarcolemma, initiating muscle action potential.

• ACh is broken down by acetylcholinesterase to terminate signal.

Excitation-Contraction Coupling Steps

1. Action potential travels along sarcolemma and down T-tubules.

2. Triggers Ca2+ release from sarcoplasmic reticulum.

3. Ca2+ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

4. Myosin heads bind to actin, initiating contraction.

5. Relaxation occurs when Ca2+ is pumped back into SR.

Sliding Filament Mechanism

• Muscle fiber shortens as sarcomeres shorten; filaments themselves do not change length.

• Thin filaments slide past thick filaments toward M line.

• ATP is required for myosin head detachment and re-cocking.

Key equation:

Contractile Properties of Skeletal Muscle

Motor Units and Graded Responses

• Motor unit: One motor neuron and all muscle fibers it innervates.

• Graded contractions achieved by varying frequency of stimulation (wave summation) and number of motor units recruited (multiple motor unit summation).

• Tetanus: Sustained contraction with no relaxation.

Types of Contractions

• Isotonic: Muscle changes length and moves a load (concentric = shortens, eccentric = lengthens).

• Isometric: Muscle develops tension but does not change length.

Length-Tension Relationship

• Optimal force generated when muscle is slightly stretched (sarcomeres at optimal length for cross-bridge formation).

Load and Contraction Velocity

• Heavier loads result in slower, shorter contractions.

Muscle Metabolism and Fatigue

Sources of ATP for Contraction

1. Stored ATP: Used for first few seconds of activity.

2. Direct phosphorylation: Creatine phosphate regenerates ATP quickly.

3. Anaerobic glycolysis: Glucose breakdown without oxygen, produces lactic acid.

4. Aerobic respiration: Most efficient, requires oxygen, supports prolonged activity.

Muscle Fatigue and Oxygen Debt

• Muscle fatigue: Physiological inability to contract, often due to ATP depletion, lactic acid buildup, or ionic imbalances.

• Oxygen debt (EPOC): Extra oxygen needed after exercise to restore metabolic conditions (replenish ATP, convert lactic acid, restore glycogen).

Muscle Fiber Types and Physical Activity

Smooth Muscle vs. Skeletal Muscle

Structural and Functional Differences

• Smooth muscle: Spindle-shaped, single nucleus, no striations, found in walls of hollow organs.

• Arranged in sheets, often with opposing orientations for peristalsis.

• Contraction is slow, sustained, and fatigue-resistant; regulated by autonomic nervous system, hormones, and local factors.

• Uses calmodulin (not troponin) for Ca2+ binding.

• Can contract over a greater length and generate more tension relative to length than skeletal muscle.

• Two types: Single-unit (visceral, with gap junctions) and multiunit (independent fibers, e.g., in large arteries, eye muscles).

Summary Table: Skeletal vs. Smooth Muscle

Example:

• Skeletal muscle is used for voluntary movements like walking or lifting.

• Smooth muscle is responsible for involuntary actions such as peristalsis in the digestive tract.

Additional info: Some details, such as the precise molecular steps of cross-bridge cycling and the Cori cycle in oxygen debt, were expanded for academic completeness.