Physiology of Muscle: Structure, Function, and Types

Topic 2.3 - Physiology of Muscle

Overview

This section covers the microscopic structure and physiology of muscle tissue, including skeletal, cardiac, and smooth muscle. Key concepts include muscle cell anatomy, mechanisms of contraction, neuromuscular junctions, contractile properties, muscle metabolism, and comparative features of 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 and flesh, respectively.

• Muscle contraction depends on actin and myosin myofilaments.

Comparison of Muscle Types

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

Feature	Cardiac Muscle	Skeletal Muscle	Smooth Muscle
Location	Only in heart	Attached to and covers bony skeleton	Walls of hollow, visceral organs
Striations	Striated	Striated	Nonstriated
Control	Involuntary	Voluntary	Involuntary
Contraction	Pacemaker sets rate; neural input can increase rate	Can contract rapidly; tires easily & must rest; strong, adaptable	Slow, sustained contractions

Functions and Characteristics of Muscle

Muscle Functions

• Generate movement: Locomotion, manipulation, blood pressure control, respiration, propulsion of food and urine.

• Maintain posture: Muscles work constantly against gravity.

• Joint stabilization: Stabilize joints during movement (e.g., shoulders, knees).

• Generation of heat: Maintains body temperature, especially via skeletal muscle (about 40% of body mass).

Functional Characteristics of Muscle

• Excitability (Irritability): Ability to receive and respond to a stimulus, usually a chemical (neurotransmitter, hormone, pH change). Response is an action potential along the sarcolemma, leading to contraction.

• Contractility: Ability to shorten forcibly when adequately stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to resume resting length after being stretched.

Microscopic Anatomy of Skeletal Muscle Fiber

Structure and Components

• Skeletal muscle fiber: Cylindrical cell with multiple oval nuclei located just beneath the sarcolemma (plasma membrane).

• Syncytium: Muscle cell formed by fusion of multiple cells, resulting in many nuclei.

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

• Myofibrils: Parallel rod-like structures (~80% of cell volume) composed of myofilaments (actin, myosin) arranged in repeating units called sarcomeres.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum for Ca2+ storage and release.

• T-tubules: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber.

Connective Tissue Layers

• Endomysium: Thin connective tissue surrounding each muscle fiber.

• Perimysium: Connective tissue surrounding bundles of muscle fibers (fascicles).

• Epimysium: Dense connective tissue surrounding the entire muscle.

Organization of Myofibrils and Sarcomeres

Sarcomere Structure

• Sarcomere: The contractile unit of muscle, defined as the region between two Z discs.

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

• Bands and Zones:

  • A band: Dark band, contains thick filaments.

  • I band: Light band, contains thin filaments.

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

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

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

Myofilament Composition

• Myosin (thick filament): Composed of two heavy chains (tail) and four light chains (heads). Heads have actin-binding sites and ATPase activity, forming cross-bridges with actin.

• Actin (thin filament): Made of G-actin (globular) subunits polymerized into F-actin (filamentous) strands. Each G-actin has a myosin-binding site.

• Tropomyosin: Rod-shaped protein that blocks myosin-binding sites on actin in resting muscle.

• Troponin: Three-polypeptide complex that binds Ca2+ and regulates tropomyosin position.

Excitation-Contraction Coupling

Role of Action Potential and Ca2+

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

• Triggers release of Ca2+ from sarcoplasmic reticulum.

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

Sliding Filament Mechanism

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

• Power stroke: Myosin heads pivot, pulling thin filaments toward the center of the sarcomere.

• ATP binds to myosin, causing detachment from actin; ATP hydrolysis re-cocks the myosin head.

• Process repeats as long as Ca2+ and ATP are available.

• Muscle relaxation occurs when Ca2+ is pumped back into SR.

Key Equation:

$\text{CP} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

Neuromuscular Junction (NMJ)

Structure and Function

• NMJ is the synapse between a motor neuron and a muscle fiber.

• Neurotransmitter acetylcholine (ACh) is released from the neuron, binds to receptors on the sarcolemma, and initiates an action potential.

• ACh is broken down by acetylcholinesterase (AChE) to terminate the signal.

Contractile Properties of Skeletal Muscle

Motor Units and Graded Responses

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

• Graded muscle responses are achieved by varying the frequency of stimulation (wave summation, tetanus) and the number of motor units recruited (multiple motor unit summation).

Types of Contractions

• Isotonic contraction: Muscle changes length and moves a load. Includes concentric (shortening) and eccentric (lengthening) contractions.

• Isometric contraction: Muscle generates tension but does not change length.

Example: Holding up a heavy couch involves isometric contraction; lifting it involves isotonic contraction.

Muscle Metabolism and Fatigue

Energy Sources for Contraction

• Stored ATP: Immediate source, lasts 4-6 seconds.

• Creatine phosphate (CP): Regenerates ATP rapidly, lasts 15-20 seconds.

• Anaerobic glycolysis: Produces ATP quickly without oxygen, but yields lactic acid.

• Aerobic respiration: Slow, requires oxygen, produces most ATP.

Muscle Fatigue and Oxygen Debt

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

• Oxygen debt (Excess Post-Exercise Oxygen Consumption, EPOC): Extra oxygen required after exercise to replenish reserves, convert lactic acid to pyruvic acid, restore glycogen, and resynthesize ATP and CP.

Muscle Fiber Types and Physical Activity

Classification of Muscle Fibers

Type	Features	Best for
Slow oxidative fibers	Thin, red, high myoglobin, many mitochondria, aerobic, fatigue-resistant	Endurance activities (e.g., marathon running)
Fast glycolytic fibers	Large, pale, low myoglobin, few mitochondria, anaerobic, powerful but fatigues quickly	Short-term, rapid, intense movements (e.g., sprinting)
Fast oxidative fibers	Intermediate features, red/pink, aerobic, fairly fatigue-resistant	Intermediate activities (e.g., walking, middle-distance running)

Comparison of Smooth and Skeletal Muscle

Structural Organization

• Smooth muscle: Spindle-shaped cells, single central nucleus, arranged in sheets in walls of hollow organs.

• No striations or sarcomeres; thick and thin filaments are present but arranged differently (ratio of thick:thin is 1:13).

• Contractions are slow, sustained, and fatigue-resistant.

• Regulated by autonomic nervous system, hormones, and local factors.

Contractile Response

• Contraction via sliding filament mechanism; Ca2+ binds to calmodulin (not troponin).

• Gap junctions allow synchronized contraction in unitary smooth muscle (e.g., gut, uterus).

• Multiunit smooth muscle (e.g., large arteries, arrector pili) contracts independently.

Summary Table: Skeletal vs. Smooth Muscle

Feature	Skeletal Muscle	Smooth Muscle
Cell shape	Long, cylindrical, multinucleate	Spindle-shaped, single nucleus
Striations	Present	Absent
Control	Voluntary	Involuntary
Contraction speed	Fast, powerful, fatigues easily	Slow, sustained, fatigue-resistant
Regulation	Somatic nervous system	Autonomic nervous system, hormones

Additional info: Some details, such as the precise molecular steps of contraction and the role of specific proteins, have been expanded for clarity and completeness.