Chapter 1: Structure and Function of Exercising Muscle - Vocabulary

Overview

• Anatomy of skeletal muscle
  • Muscle fibers
  • Myofibrils
• Muscle fiber contraction
• Skeletal muscle and exercise
  • Muscle fiber types
  • Muscle fiber (motor unit) recruitment
  • Orderly recruitment of muscle fibers (size principle)
  • Fiber type and athletic success

Three Types of Muscle Tissue

• Smooth muscle: involuntary, hollow organs
• Cardiac muscle: involuntary, heart
• Skeletal muscle: voluntary, skeleton

Connective Tissue Wrappings and Muscle Architecture

• Mysium: connective tissue sheaths that transfer force production
• Key components and arrangement
  • Epimysium (deep fascia) surrounding the whole muscle
  • Perimysium surrounding fasciculi
  • Endomysium between individual fibers
  • Fasciculus: a bundle of muscle fibers
  • Muscle belly: the bulk of the muscle
  • Capillary network within endomysium
• Structural sequence
  • Tendon connects muscle to bone
  • Bone

Muscle Cells and Subcellular Structures

• Single muscle fiber
• Sarcolemma: plasma membrane of a muscle cell
• Sarcoplasm: cytoplasm of a muscle cell
• Nuclei inside the fiber
• Myofibrils: contractile elements within muscle fibers
• Mitochondrion: powerhouse for ATP production
• Sarcoplasmic reticulum (SR): stores Ca^{2+}
• Transverse tubules (T-tubules): invaginations of the sarcolemma that propagate action potentials

Myofibrils and the Sarcomere

• A band: dark band containing thick and some thin filaments
• I band: light band containing thin filaments
• Z line: boundaries of a sarcomere
• sarcomere: functional unit of a myofibril; extends from Z line to Z line
• M band: center of the sarcomere
• The sarcomere is the primary contractile unit composed of myofilaments
• Myofilaments: contractile proteins in the sarcomere – actin (thin) and myosin (thick)

Functional Unit and Filaments

• Sarcomere unit: structure between two Z lines
• A band: region containing thick filaments (myosin) and overlapping thin filaments
• I band: region containing only thin filaments near Z line
• H zone: central region within the A band where thin filaments are absent
• Myofilaments
  • Myosin (thick filament)
  • Actin (thin filament)
• Accessory proteins
  • Troponin complex
  • Tropomyosin

Myosin, Actin, and Regulatory Proteins

• Myosin: thick filament
• Actin: thin filament
• Troponin complex: regulatory protein that binds Ca^{2+}
• Tropomyosin: regulatory protein that blocks or reveals myosin-binding sites on actin depending on Ca^{2+}
• Cross-bridge formation: myosin heads bind to actin when binding sites are exposed

Motor Units and Neuromuscular Transmission

• Motor units: a-motor neurons innervating muscle fibers
• Motor unit: more operating motor units = more contractile force
• Neuromuscular junction: synapse between an α-motor neuron and a muscle fiber
• Components of a motor unit circuit
  • Alpha-motor neuron (cell body)
  • Axon hillock
  • Axon terminals
  • Muscle fibers
  • Myofibrils
  • Dendrites
  • Motor end plates
• Direction of propagation of action potential along neuron

Ion Channels and Resting Potentials

• Type of ion channels
  • Ligand-gated channels: open in response to binding of a chemical messenger (e.g., neurotransmitter)
  • Voltage-gated channels: open with changes in membrane potential (depolarization)
• Skeletal muscle resting membrane potential: inside negative at about $-90~ ext{mV}$
• Depolarization during activation: charge can rise to about $+30~ ext{mV}$

Neuromuscular Junction Process (Stepwise)

1) Motor neuron action potential
2) Ca^{2+} enters voltage-gated channels
3) Acetylcholine (ACh) released from vesicles into cleft
4) ACh binds receptors; Na^{+} entry depolarizes motor end plate
5) Local current between depolarized end plate and adjacent muscle plasma membrane
6) Muscle fiber action potential initiation
7) Propagated action potential in muscle plasma membrane
8) Acetylcholinesterase degrades ACh to terminate signal

Excitation at the Muscle Fiber: T-Tubules and SR

• Opening of transverse tubules to extracellular fluid
• Transverse tubules (T-tubules)
• Lateral sacs of SR in close proximity to T-tubules
• Link between sarcolemma depolarization and Ca^{2+} release

The Excitation–Contraction Coupling Cascade

• Depolarization of T-tubule activates DHP receptor
• DHP receptor triggers Ryanodine receptor on SR
• Ca^{2+} release from lateral sacs into cytosol
• Ca^{2+} binds troponin, removing the blocking action of tropomyosin on actin
• Cross-bridge cycling begins with Ca^{2+} present
• Ca^{2+} removal from troponin (via SR Ca^{2+} reuptake) restores tropomyosin block
• Relaxation occurs when Ca^{2+} is removed and cross-bridges detach

Calcium Handling and Cross-Bridge Cycling

• Rest: low cytosolic Ca^{2+}; myosin head cannot bind to actin
  • Illustration: Troponin and Tropomyosin block actin; energised cross-bridge cannot form
  • ext{Low Ca}^{2+}
    ightarrow ext{relaxed state}
• Depolarization and Ca^{2+} release enable cross-bridge binding and force generation
• Cross-bridge cycling requires ATP

The Cross-Bridge Cycle and ATP Use

• ATP hydrolysis energizes the myosin head (ATP → ADP + Pi + energy)
• Cross-bridge binds to actin
• Power stroke occurs with ADP + Pi release
• Myosin head detaches when another ATP binds
• Re-energized cross-bridge ready for another cycle
• Overall reaction context:
  • ext{ATP}
    ightarrow ext{ADP} + ext{Pi} + ext{energy}
• Cytosolic Ca^{2+} rises to enable cross-bridge formation
• After binding, ATP binds to myosin causing detachment; ATP hydrolysis re-energizes the head

Contraction and Sarcomere Shortening

• Resting sarcomere length (example): $L_{ ext{rest}} = 4.0~ ext{μm}$
• During contraction, sarcomere length shortens to $L_{ ext{contracted}} = 2.7~ ext{μm}$
• When myosin heads bind actin and pull, the thick and thin filaments slide past each other, shortening the sarcomere
• Visualization: A-band and I-band changes reflect sarcomere shortening

Sliding Filament Theory: Steps to Movement

• Always begins with depolarization
  1) Ca^{2+} release from SR
  2) Ca^{2+} binds troponin, causing a conformational change in tropomyosin
  3) Actin binding sites are exposed on myosin-binding faces
  4) Energized myosin head binds actin (cross-bridge formation)
  5) ADP + Pi release from cross-bridge, producing the power stroke
  6) Sarcomere shortens
  7) ATP binds to myosin, causing cross-bridge detachment
  8) Cross-bridge detaches from actin
  9) Myosin ATPase breaks down ATP, returning energy to the head
  10) Myosin head is re-energized and ready for another cycle

Muscle Fiber Types and Their Characteristics

• Type I (Slow-twitch, SO)
  • About 50% of fibers in an average muscle
  • Peak tension in ~110 ms
• Type II (Fast-twitch)
  • Peak tension in ~50 ms
  • Subtypes: IIa (FOG; Fast oxidative/glycolytic) and IIx (FG; Fast glycolytic)
  • Type IIa ~25% of fibers; Type IIx ~25% of fibers in an average muscle

Classification Tables (Structural and Functional)

• Table 1.1: Fiber type classification (System 1: Slow-twitch, System 2: Fast-twitch, System 3: Slow oxidative)

  • Type I: Slow-twitch (SO)
  • Type IIa: Fast oxidative/glycolytic (FOG)
  • Type IIx: Fast glycolytic (FG)

• Key properties

  • Oxidative capacity: High in Type I and IIa; Low in Type IIx
  • Glycolytic capacity: Low in Type I; High in IIx; High in IIa
  • Contraction speed: Slow in Type I; Fast in IIa and IIx
  • Fatigue resistance: High in Type I; Moderate in IIa; Low in IIx
  • Motor unit strength: Low in Type I; High in IIa; High in IIx

• Table 1.2: Structural and Functional Characteristics

  • Number of fibers per motor neuron: Type I ≤ 300; Type IIa ≥ 300; Type IIx ≥ 300
  • Motor neuron size: Smaller for Type I; Larger for Type IIa and IIx
  • Conduction velocity: Slower for Type I; Faster for Type IIa and IIx
  • Contraction speed (ms): Type I ~110; Type IIa ~50; Type IIx ~50
  • Type of myosin ATPase: Slow in Type I; Fast in Type IIa and IIx
  • SR development: Low in Type I; High in Type IIa and IIx

Type I and Type II Fibers During Exercise

• Type I fibers during exercise
  • High aerobic endurance; can sustain activity for long periods
  • Require oxygen for ATP production
  • Recruited during low-intensity aerobic exercise and daily activities
  • Efficient at producing ATP from fat and carbohydrate
• Type II fibers during exercise
  • General: Poor aerobic endurance; fatigue quickly; rely on anaerobic ATP production
  • Type IIa: More force; faster fatigue than Type I; suited for short, high-intensity endurance events (e.g., 1,600 m run)
  • Type IIx: Rarely used for everyday activities; suited for short, explosive sprints (e.g., 100 m)

Fiber Type Determinants and Plasticity

• Genetic factors influence fiber type distribution
• Training factors can induce small (~10%) changes in fiber type (e.g., Type II → Type I, IIx → IIa)
• Aging: Muscles progressively lose type II motor units

Muscle Fiber Recruitment and the Size Principle

• Also called motor unit recruitment
• Method for altering force production
  • Fewer/smaller motor units for less force
  • More/larger motor units for more force
• Type I motor units are smaller than Type II
• Recruitment order is consistent: small to large as force requirements increase

Orderly Recruitment and the Size Principle

• Recruitment order as force increases:
  • Smallest motor units (Type I) recruited first
  • Midsized motor units (Type IIa) recruited next
  • Largest motor units (Type IIx) recruited last
• This order is consistent across activations
• Size principle: force requirements determine recruitment of progressively larger motor units in direct relation to a-motor neuron size

Recruitment vs Endurance and Strength Demands

• High recruitment thresholds correspond to higher force requirements
• Endurance exercises emphasize low-threshold motor units (Type I)
• Strength/power activities recruit high-threshold motor units (Type IIx, then IIa)

Fiber Type and Athletic Success

• Endurance athletes: Type I predominates
• Sprinters: Type II predominates
• Fiber type alone does not predict success
  • Cardiovascular function
  • Motivation
  • Training habits
  • Muscle size

Types of Muscle Contraction

• Static (isometric) contraction
  • Joint angle does not change
  • Myosin cross-bridges cycle, but no sliding occurs
• Dynamic contraction
  • Muscle produces force and changes length
  • Joint movement occurs

Dynamic Contraction Subtypes

• Concentric contraction
  • Most familiar; sarcomere shortens; filaments slide toward center
• Eccentric contraction
  • Cross-bridges form but sarcomere lengthens
  • Example: lowering a heavy weight

Generation of Force: Two Key Factors

1) Motor unit recruitment

• Type II motor units produce more force; Type I produce less
• Fewer small fibers versus more large fibers
  2) Frequency of stimulation (rate coding)
• Rate of action potentials to depolarize sarcolemma, leading to Ca^{2+} release and cross-bridge formation

Generation of Force: Length-Tension and Force-Velocity Relationships

• Length-Tension relationship
  • Optimal sarcomere length yields maximal force due to optimal overlap of thick and thin filaments
  • If sarcomere is too short or too stretched, force is reduced
  • Conceptual form: $F ext{ is maximized at } L = L<em>0 ext{ and decreases as } L \neq L</em>0$
• Force-Velocity relationship (speed-force relationship)
  • Maximal force development decreases at higher shortening speeds during concentric actions

Developed Tension and Sarcomere Geometry

• Developed tension varies with sarcomere length and cross-bridge cycling
• Visual cues from length-tension graphs show peak around optimal overlap

Summary Notes on Key Relationships

• Contraction type depends on joint angle changes and muscle length
• Cross-bridge cycling requires Ca^{2+}, ATP, and proper regulatory protein function
• Recruitment and rate coding together determine overall force output
• Fiber type distribution and motor unit recruitment shape athletic performance
• Aging and training can modulate motor unit recruitment and fiber type composition over time