Chapter 9: Muscles – Structure, Function, and Physiology

Muscle Tissue: Overview

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various physiological processes. There are three main types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal Muscle: Large, striated cells attached to bones; under voluntary control.

• Cardiac Muscle: Small, striated cells found only in the heart; under involuntary control.

• Smooth Muscle: Small, spindle-shaped, non-striated cells found in walls of hollow organs; under involuntary control.

Special Characteristics of Muscle Tissue

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

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original length after stretching or contracting.

Functions of Muscle Tissue

• Movement: Muscles move bones, pump blood, and propel substances through hollow organs.

• Posture Maintenance: Muscles adjust body position relative to gravity.

• Joint Stabilization: Muscles exert tension to stabilize joints.

• Heat Generation: Muscle metabolism produces heat, aiding in temperature regulation.

Skeletal Muscle

Gross Anatomy of Skeletal Muscle

• Each muscle is supplied by nerves and blood vessels for control and nutrient/waste exchange.

• Connective tissue sheaths:

  • Endomysium: Surrounds individual muscle fibers.

  • Perimysium: Surrounds groups of fibers (fascicles).

  • Epimysium: Surrounds the entire muscle.

• Muscle attachments:

  • Origin: Less movable attachment.

  • Insertion: More movable attachment.

  • Direct attachment: Epimysium fused to periosteum.

  • Indirect attachment: Connective tissue extends as tendon or aponeurosis.

Microscopic Anatomy of Skeletal Muscle Fiber

• Muscle Fiber: Long, cylindrical, multinucleated cell.

• Myofibrils: Cylindrical structures making up ~80% of cell volume; contain contractile elements.

• Striations: Alternating dark (A) and light (I) bands due to arrangement of myofilaments.

• Myofilaments:

  • Thick filaments: Composed of myosin.

  • Thin filaments: Composed of actin, with regulatory proteins tropomyosin and troponin.

• Sarcoplasmic Reticulum (SR): Smooth ER surrounding myofibrils; stores and releases calcium ions.

• T Tubules: Invaginations of sarcolemma; conduct electrical impulses deep into the fiber.

The Sliding Filament Model of Contraction

During contraction, thin filaments slide past thick filaments, increasing overlap and shortening the sarcomere.

• Myosin heads bind to actin, forming cross-bridges and pulling thin filaments toward the center.

• ATP is required for cross-bridge cycling.

Neuromuscular Junction and Muscle Fiber Stimulation

• Neuromuscular Junction (NMJ): Synapse between motor neuron and muscle fiber.

• Nerve impulse triggers release of acetylcholine (ACh) into synaptic cleft.

• ACh binds to receptors on motor end plate, initiating an action potential in the sarcolemma.

• ACh is rapidly degraded by acetylcholinesterase (AChE) to prevent continuous stimulation.

Generation of Action Potential

• Binding of ACh increases sarcolemma permeability to sodium ions, depolarizing the membrane.

• Action potential propagates along sarcolemma and T tubules.

Excitation-Contraction Coupling

• Sequence linking action potential to muscle contraction.

• Action potential triggers calcium release from SR.

• Calcium binds to troponin, shifting tropomyosin and exposing myosin-binding sites on actin.

• Cross-bridge cycling and contraction occur.

Motor Units and Muscle Twitch

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Muscle Twitch: Response of a muscle to a single action potential.

• Three phases: latent period, contraction, relaxation.

Graded Muscle Responses

• Wave Summation: Increased stimulus frequency increases contraction strength.

• Recruitment (Multiple Motor Unit Summation): Increasing number of active motor units increases force.

• Treppe: Gradual increase in contraction strength with repeated stimulation.

• Muscle Tone: Slight, constant contraction even at rest.

Types of Contractions

• Isotonic: Muscle changes length (shortens or lengthens) and moves a load.

• Isometric: Muscle tension increases, but muscle does not change length.

Muscle Metabolism

• ATP is regenerated by:

  • Phosphorylation by creatine phosphate

  • Glycolysis and anaerobic respiration

  • Aerobic respiration

• Muscle fatigue results from ATP deficit, lactic acid buildup, and ionic imbalances.

• Oxygen Deficit: Extra oxygen needed to restore reserves after exercise.

• Heat is produced during muscle activity and must be dissipated to maintain homeostasis.

Force, Velocity, and Duration of Contraction

• Force increases with number and size of fibers stimulated, frequency of stimulation, and optimal muscle length.

• Velocity and duration depend on muscle fiber type, load, and recruitment.

Types of Skeletal Muscle Fibers

Fiber Type	Contraction Speed	ATP Pathway	Fatigue Resistance
Slow Oxidative	Slow	Aerobic	High
Fast Oxidative	Fast	Aerobic (some anaerobic)	Intermediate
Fast Glycolytic	Fast	Anaerobic glycolysis	Low

Note: Most muscles contain a mix of fiber types, but proportions vary by muscle and individual genetics.

Effects of Exercise

• Aerobic (Endurance) Exercise: Increases capillaries, mitochondria, and myoglobin; improves metabolism but does not cause hypertrophy.

• Resistance Exercise: Increases muscle size (hypertrophy), myofilaments, and glycogen stores.

Smooth Muscle

Microscopic Structure

• Small, spindle-shaped cells with one central nucleus.

• Arranged in sheets (longitudinal and circular layers).

• No striations or sarcomeres; lower ratio of thick to thin filaments.

• Contains tropomyosin but no troponin.

• Intermediate filaments and dense bodies anchor actin filaments.

• Less developed SR; calcium stored in caveolae of cell membrane.

• No neuromuscular junctions; neurotransmitters released from varicosities.

Contraction Mechanism

• Slow, synchronized contractions via gap junctions.

• Sliding filament mechanism (actin and myosin interaction).

• Calcium binds to calmodulin, activating myosin light chain kinase for cross-bridge cycling.

• Consumes less ATP and contracts more slowly than skeletal muscle.

Regulation of Contraction

• Autonomic nerves release acetylcholine or norepinephrine, causing excitation or inhibition.

• Hormones and local factors (e.g., low oxygen, pH changes) can trigger contraction.

Special Features

• Can stretch and still contract efficiently (important for hollow organs).

• Capable of hyperplasia (increase in cell number) as well as hypertrophy (increase in cell size).

Types of Smooth Muscle

• Single-unit (Visceral) Smooth Muscle: Most common; contracts as a unit, electrically coupled by gap junctions, exhibits spontaneous action potentials.

• Multiunit Smooth Muscle: Found in large airways, arteries, arrector pili, and iris; cells act independently, allowing graded contractions.

Key Definitions and Concepts

• Sarcomere: Smallest contractile unit of muscle, region between two Z-lines.

• ATP: Main energy source for muscle contraction.

• Motor Unit: Motor neuron and all muscle fibers it innervates.

• Muscle Twitch: Response to a single stimulus.

• Muscle Fatigue: Inability to contract due to ATP deficit, lactic acid, or ionic imbalance.

• Oxygen Deficit: Extra oxygen needed after exercise to restore reserves.

Sample Equations

• ATP Hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Creatine Phosphate Reaction: $\mathrm{Creatine~phosphate} + \mathrm{ADP} \rightarrow \mathrm{Creatine} + \mathrm{ATP}$

• Aerobic Respiration (simplified): $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + 36~\mathrm{ATP}$

Comparisons: Skeletal vs. Smooth Muscle

Feature	Skeletal Muscle	Smooth Muscle
Striations	Present	Absent
Control	Voluntary	Involuntary
Location	Attached to bones	Walls of hollow organs
Contraction Speed	Fast	Slow
Regeneration	Poor	Good (in some tissues)

Clinical and Applied Concepts

• Rigor Mortis: Post-mortem muscle stiffness due to lack of ATP for cross-bridge detachment.

• Muscle Atrophy: Decrease in muscle size due to disuse or immobilization.

• Exercise: Endurance training increases efficiency; resistance training increases size and strength.

• Muscle Spasm vs. Cramp: Both involve involuntary contractions; cramps are often more painful and prolonged.

Summary Table: Muscle Fiber Types

Type	Color	Myoglobin Content	Mitochondria	Fatigue Resistance	Example
Slow Oxidative	Red	High	Many	High	Postural muscles
Fast Oxidative	Red to pink	Intermediate	Many	Intermediate	Leg muscles
Fast Glycolytic	White	Low	Few	Low	Arm muscles

Additional info:

• Muscle contraction is essential for movement, posture, and heat production.

• Muscle fiber types and their distribution are genetically determined but can be influenced by training.

• Muscle fatigue and oxygen deficit are important considerations in exercise physiology.

• Smooth muscle is critical for functions such as peristalsis and blood vessel regulation.