Physiology of Muscle: Structure, Function, and Comparison of Muscle 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.

• 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
Special Features	Pacemaker sets rate; neural input can increase rate	Longest muscle fibers; can contract rapidly; tires easily; strong and 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). 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 of Skeletal Muscle Cell

• Cylindrical cell with many oval nuclei located just beneath the sarcolemma (plasma membrane).

• Muscle cell is a syncytium (multinucleated).

• Large cells: Diameter 10-100 μm, length up to several cm.

• Sarcoplasm (cytoplasm) contains glycogen and myoglobin (oxygen-binding protein).

• Contains hundreds to thousands of myofibrils (80% of cell volume).

• Extensive sarcoplasmic reticulum and T-tubules for calcium storage and signal conduction.

Connective Tissue Layers

• Endomysium: Thin connective tissue surrounding each muscle fiber.

• Perimysium: Connective tissue surrounding a fascicle (bundle of muscle fibers).

• Epimysium: Dense connective tissue surrounding the entire muscle.

Myofibril and Sarcomere Structure

• Myofibrils: Parallel rod-like structures, each containing repeating units called sarcomeres.

• Sarcomere: Contractile unit of muscle, extends from one Z-disc to the next.

• Contains myofilaments: Actin (thin) and myosin (thick).

• Bands and zones: A band (dark, thick filaments), I band (light, thin filaments), H zone (center of A band, no thin filaments), M line (center of sarcomere), Z disc (anchors thin filaments).

Ultrastructure and Molecular Composition of Myofilaments

Thick Filaments (Myosin)

• Composed of myosin molecules: Each has a tail (2 heavy chains) and two heads (ends of heavy chains + 2 light chains).

• Myosin heads form cross-bridges with actin and contain ATPase activity.

• Central part of thick filament lacks myosin heads; heads are present at ends where overlap with actin occurs.

Thin Filaments (Actin, Tropomyosin, Troponin)

• F-actin: Filamentous actin, two strands wound in a helix; made of G-actin (globular actin) subunits.

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

• Troponin: Three-polypeptide complex that binds calcium, causing conformational change to move tropomyosin and expose binding sites.

Sarcoplasmic Reticulum and T-Tubules

Calcium Storage and Release

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions ($Ca^{2+}$).

• SR surrounds each myofibril; terminal cisternae are enlarged areas for calcium storage.

• T-tubules: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber, triggering calcium release from SR.

Excitation-Contraction Coupling

Role of Action Potential and Calcium

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

• Triggers release of $Ca^{2+}$ from SR into sarcoplasm.

• $Ca^{2+}$ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

Sliding Filament Mechanism

• Muscle fiber shortens as sarcomeres shorten; filaments slide past each other, but do not change length.

• Myosin heads attach to actin, perform power stroke, detach, and reattach repeatedly, pulling thin filaments toward the center of the sarcomere.

• ATP is required for cross-bridge cycling and detachment.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (hydrolysis provides energy for myosin head movement)

Neuromuscular Junction (NMJ)

Structure and Function

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

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

• Acetylcholinesterase (AChE) terminates the signal by breaking down ACh.

Contractile Properties of Skeletal Muscle

Motor Units and Graded Responses

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

• Graded responses depend on frequency of stimulation (wave summation, tetanus) and number of motor units recruited (spatial summation).

• Muscle tone: Continuous, partial contraction of muscles for posture and joint stability.

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: Lifting a weight involves isotonic contraction; holding a heavy object in place involves isometric contraction.

Muscle Metabolism and Fatigue

Sources of Energy for Contraction

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

• Creatine phosphate (CP): Regenerates ATP rapidly; CP + ADP → creatine + ATP (via creatine kinase).

• Anaerobic glycolysis: Glucose → pyruvic acid → lactic acid; fast but less efficient, used during intense activity.

• Aerobic respiration: Glucose, fatty acids, or amino acids + O2 → CO2 + H2O + ATP; slower but more efficient, used during prolonged activity.

Equation:

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

Muscle Fatigue and Oxygen Debt

• Muscle fatigue: Physiological inability to contract, often due to relative lack of ATP, 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

Fiber Type	Features	Best Suited For
Slow Oxidative	Thin, red, high myoglobin, many mitochondria, aerobic, fatigue-resistant	Endurance activities (e.g., marathon running)
Fast Glycolytic	Large, pale, low myoglobin, few mitochondria, anaerobic, powerful, fatigues quickly	Short-term, rapid, intense movements (e.g., sprinting)
Fast Oxidative	Intermediate features, red/pink, moderate myoglobin, 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 layers 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).

• Contractile proteins spiral down the cell; dense bodies anchor filaments.

• Connected by gap junctions (in visceral smooth muscle) for coordinated contraction.

Contractile Response

• Contraction is slow, sustained, and fatigue-resistant.

• Calcium enters from extracellular fluid and SR, binds to calmodulin (not troponin), activating myosin.

• Can contract in response to stretch and hormones; some cells are pacemakers.

• Can undergo hyperplasia (increase in cell number), e.g., uterine smooth muscle during pregnancy.

Types of Smooth Muscle

• Visceral (single-unit): Most common, found in digestive, urinary, and reproductive tracts; contracts as a unit, electrically coupled by gap junctions.

• Multiunit: Found in large arteries, airways, arrector pili, and eye muscles; fibers act independently, each innervated by autonomic nerves.

Summary Table: Skeletal vs. Smooth Muscle

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

Key Equations and Concepts

• Creatine phosphate reaction:

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

• Aerobic respiration:

• $\text{Glucose} + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{ATP}$

• Anaerobic glycolysis:

• $\text{Glucose} \rightarrow 2\text{Pyruvic Acid} \rightarrow 2\text{Lactic Acid} + 2\text{ATP}$

Additional info: Some details and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.