Muscle Tissue: Structure, Function, and Types (Chapter 12 Study Notes)

Chapter 12: Muscles

Overview

This chapter explores the anatomy and physiology of muscle tissue, focusing on skeletal, cardiac, and smooth muscle. It covers muscle structure, contraction mechanisms, energy requirements, and classification by function and fatigue resistance.

Three Types of Muscle

Classification and Functions

• Skeletal Muscle

  • Striated muscle attached to bones of the skeleton

  • Responsible for voluntary control of body movement

  • Innervated by somatic motor neurons

• Cardiac Muscle

  • Striated muscle found only in the heart

  • Moves blood through the circulatory system

  • Involuntary control; responds to autonomic innervation and is modulated by the endocrine system

• Smooth Muscle

  • Primary muscle of internal organs and tubes

  • Influences movement of material into, out of, and within the body

  • Involuntary control; responds to autonomic innervation and is modulated by the endocrine system

12.1 Skeletal Muscle

Structure and Organization

• Muscle fibers are the cells of skeletal muscle

  • Long, cylindrical, and multinucleated due to fusion of precursor cells

• Satellite cells (muscle stem cells)

  • Differentiate into muscle for growth or repair

• Fascicles

  • Bundles of muscle fibers surrounded by connective tissue sheath

• Connective tissue

  • Surrounds entire muscle and is continuous with the sheath

  • Connects muscle to bone via tendons

Muscle Fiber Anatomy

• Myofibrils: Contractile structures within muscle fibers

• Sarcomere: The functional contractile unit of myofibril

  • Z disks: Boundaries of each sarcomere; anchor thin filaments

  • I band: Contains only thin filaments (actin)

  • A band: Overlap of thick (myosin) and thin (actin) filaments

  • H zone: Center of A band; thick filaments only

  • M line: Proteins anchoring thick filaments

Accessory Proteins

• Titin

  • Elastic protein that stabilizes contractile elements

  • Returns stretched muscles to resting length

• Nebulin

  • Inelastic protein that aligns actin filaments

T-tubules and Sarcoplasmic Reticulum

• T-tubules: Extensions of the sarcolemma that bring action potentials into the interior of the muscle fiber

• Sarcoplasmic reticulum: Stores Ca2+ needed for contraction

Muscle Contraction Mechanisms

Major Steps Leading to Skeletal Muscle Contraction

1. Events at the neuromuscular junction

2. Excitation-contraction (E-C) coupling

3. Contraction-relaxation cycle

Sliding Filament Theory

• Actin and myosin filaments overlap and slide past each other during contraction

• Fibrils remain fixed in length; movement is energy-dependent

Crossbridge Cycle

• Power stroke: Myosin crossbridge swivels, pulling actin toward the M line

• Myosin releases actin, resets, and binds another actin molecule

• Process is repeated multiple times; heads do not release in unison

• Myosin ATPase hydrolyzes ATP, energizing the myosin head

Role of Calcium and Regulatory Proteins

• Troponin: Complex of three proteins; Troponin C binds Ca2+ reversibly

• Tropomyosin: Covers myosin binding sites on actin, preventing interaction

• Calcium release from the sarcoplasmic reticulum binds troponin, shifting tropomyosin and exposing binding sites

• Contraction continues as long as Ca2+ and ATP are available

Contraction Cycle Steps

1. ATP binds myosin, causing detachment from actin

2. ATP hydrolysis re-cocks the myosin head

3. Myosin binds weakly to new actin site

4. Release of Pi allows the power stroke

5. ADP is released, and the cycle repeats

Rigor State and Rigor Mortis

• Rigor state: Occurs when no ATP or ADP is bound to myosin; very brief

• Rigor mortis: Muscles freeze if no ATP is available to release myosin from actin

Excitation-Contraction Coupling

Role of Acetylcholine (ACh)

• ACh released from somatic motor neuron

• Binds to receptors on sarcolemma, initiating muscle action potential

• Depolarization (end-plate potential) triggers Ca2+ release from sarcoplasmic reticulum

• Key channels: L-type calcium channel (DHP receptor) on T-tubule, ryanodine receptor (RyR) on sarcoplasmic reticulum

• Ca2+ combines with troponin to initiate contraction

Relaxation

• Ca2+ pumped back into sarcoplasmic reticulum via Ca2+-ATPase

• A muscle twitch is a single contraction-relaxation cycle

• Latent period: Delay between action potential and tension development (time for Ca2+ release and binding)

Energy for Muscle Contraction

ATP Sources

• Phosphocreatine breakdown provides a short burst of energy

  • Enzyme: Creatine kinase (CK)

• Carbohydrates (glucose) are the most rapid and efficient energy source

• Anaerobic glycolysis

  • Quick, no oxygen required, produces lactate and acid

  • Small amount of energy released

• Aerobic respiration

  • Slow, oxygen required, large amount of energy released

Muscle Fatigue

Causes of Fatigue

• Central fatigue: Originates in the CNS

• Peripheral fatigue: Due to neuron or muscle

  • Depletion of glycogen stores (extended submaximal exercise)

  • Increased levels of Pi (short-duration maximal exertion)

  • Leads to ion imbalances (maximal exercise)

  • K+ leaves muscle fiber, increasing extracellular [K+], altering membrane potential

  • Changes Na+-K+-ATPase activity

Classification of Skeletal Muscle Fibers

Speed and Fatigue Resistance

• Slow-twitch fibers (ST or type I)

  • Rely on oxidative phosphorylation

• Fast-twitch fibers

  • Develop tension faster, split ATP more rapidly, pump Ca2+ more rapidly

  • Fast-twitch oxidative-glycolytic (FOG or type IIA): Use both oxidative and glycolytic metabolism

  • Fast-twitch glycolytic (FG or type IIB/IX): Rely on anaerobic glycolysis

• Myoglobin binds oxygen in muscle, supporting aerobic processes; oxidative fibers have more myoglobin

Table: Characteristics of Muscle Fiber Types

Characteristic	Slow-Twitch Oxidative (Type I)	Fast-Twitch Oxidative-Glycolytic (Type IIA)	Fast-Twitch Glycolytic (Type IIB/IX)
Speed of Maximum Tension	Slowest	Intermediate	Fastest
Myosin ATPase Activity	Slow	Fast	Fast
Diameter	Small	Medium	Large
Contraction Duration	Longest	Short	Short
Ca2+-ATPase Activity in SR	Moderate	High	High
Endurance	Fatigue resistant	Fatigue resistant	Easily fatigued
Use	Most used: posture	Standing, walking	Least used: jumping, quick, fine movements
Metabolism	Oxidative; aerobic	Glycolytic but becomes more oxidative with training	Glycolytic; more anaerobic than oxidative type
Capillary Density	High	High	Low
Mitochondria	Numerous	Numerous	Few
Color	Dark red (myoglobin)	Red	Pale

Additional info: These notes expand on the provided slides with definitions, examples, and a summary table for muscle fiber types, suitable for exam preparation in Anatomy & Physiology.