⚡ Contraction + Fiber Types

Muscle Contraction – Action Potential

Neuromuscular Junction
 Synaptic connection between the terminal end of a motor nerve and a muscle fiber

Action Potential
 Muscle contraction begins with an action potential
 Signal is transmitted through the neuromuscular junction to the muscle fiber

Key Point
 Neuromuscular junction converts nerve signals into muscle activation)

Muscle Contraction – Action Potential Transmission

Sarcolemma
 Action potential travels along the muscle cell membrane (sarcolemma)

T-Tubules
 Action potential is also transmitted through invaginations of the sarcolemma called T-tubules
 Allows signal to reach deep into the muscle fiber

Key Point
 T-tubules ensure the action potential rapidly reaches the interior of the muscle fiber for coordinated contraction)

Muscle Contraction – Calcium Release

Sarcoplasmic Reticulum (SR)
 Action potential triggers release of calcium (Ca²⁺) from the SR into the sarcoplasm (cytoplasm of skeletal muscle cell)

Effect on Filaments
 Increased Ca²⁺ levels in the sarcoplasm cause actin and myosin filaments to slide

Key Point
 Calcium release from SR is essential for initiating filament sliding and muscle contraction)

Muscle Contraction – Thin Filament Activation

Relaxed Stage
 Tropomyosin blocks myosin binding sites on actin subunits

Calcium Effect
 Ca²⁺ released from the sarcoplasmic reticulum binds to troponin
 Changes troponin conformation
 Pushes tropomyosin off myosin binding sites

Key Point
 Calcium binding to troponin exposes actin sites for myosin attachment, enabling contraction)

Muscle Contraction – Thin Filament Conformation

Relaxed Muscle
 Tropomyosin covers myosin binding sites on actin
 Myosin cannot bind, muscle remains relaxed

Contracted Muscle
 Ca²⁺ binds to troponin
 Troponin changes conformation and pulls tropomyosin away
 Tropomyosin moves, exposing myosin binding sites on actin
 Myosin heads attach, initiating contraction

Key Point
 Calcium-induced conformational change in thin filaments allows myosin–actin interaction for muscle contraction)

Muscle Contraction – Filament Sliding

Myosin Action
 Myosin heads rotate and bend during contraction
 Attach to actin filaments and pull them

ATP Role
 ATP provides the energy for myosin movement and filament sliding

Effect on Filaments
 Thin (actin) and thick (myosin) filaments slide past each other
 Shortens the sarcomere, generating muscle contraction

Key Point
 ATP-powered myosin movement causes actin–myosin sliding, which shortens the sarcomere and contracts the muscle)

Muscle Contraction – Sarcomere Sliding

Relaxed Sarcomere
 Actin (thin filaments) and myosin (thick filaments) are not bound
 Tropomyosin blocks myosin binding sites on actin

Activation
 Ca²⁺ binds to troponin
 Troponin changes conformation, moves tropomyosin, exposing myosin binding sites

Myosin Binding and Power Stroke
 ADP-bound myosin heads attach to actin
 Myosin head rotates, producing a power stroke
 Thin filaments are pulled past thick filaments, shortening the sarcomere

ATP Role
 ADP is released and ATP binds to myosin head, causing detachment from actin
 ATP hydrolysis re-cocks the myosin head for the next power stroke

Key Point
 Cycle of Ca²⁺ binding, myosin attachment, power stroke, and ATP hydrolysis drives sarcomere shortening and muscle contraction)

Muscle Contraction – H Zone

H Zone
 Central region of the A band containing only thick filaments (myosin)
 Shortens during contraction as filaments slide

ATP and Ca²⁺ Role
 Ca²⁺ exposes myosin binding sites on actin by binding to troponin which moves tropomyosin
 ATP powers myosin head movement and detachment

Key Point
 H zone shortens, titin maintains alignment, and ATP + Ca²⁺ enable contraction)

Muscle Contraction – Stepwise Process (Structural Hierarchy & ATP Role)

1 – Action Potential Initiation
 Signal starts at motor neuron
 Transmitted through the neuromuscular junction to the muscle fiber (cell)

2 – Action Potential Propagation
 Travels along the sarcolemma (muscle membrane)
 Also transmitted deep into the fiber via T-tubules
 Ensures signal reaches interior of muscle fiber

3 – Calcium Release from Sarcoplasmic Reticulum (SR)
 Action potential triggers Ca²⁺ release from SR into the sarcoplasm (cytoplasm of muscle cell)
 Inside the muscle fiber surrounding the myofibrils

4 – Thin Filament Activation
 Ca²⁺ binds troponin on the thin filament (actin)
 Troponin changes conformation, moves tropomyosin
 Exposes myosin binding sites on actin

5 – Cross-Bridge Formation
 ADP-bound myosin heads on thick filament attach to exposed actin sites
 Inside the sarcomere, thick and thin filaments begin interaction

6 – Power Stroke
 Myosin heads rotate and bend, pulling actin filaments toward sarcomere center
 Shortens the sarcomere, A band remains constant, H zone and I band shorten

7 – ATP Role in Detachment
 ATP binds myosin head, causing it to detach from actin
 ATP hydrolysis re-cocks the myosin head into a ready position (ADP + Pi-bound)
 Cycle can repeat as long as Ca²⁺ and ATP are present

8 – Sarcomere Shortening
 Multiple myofibrils within the muscle fiber shorten
 Muscle fiber contracts, generating force

9 – Relaxation
 Ca²⁺ pumped back into SR
 Troponin/tropomyosin re-block myosin binding sites
 Sarcomere returns to resting length, H zone and I band lengthen

Key Point
 Muscle contraction is a coordinated process:
  Signal (T-tubules) → Ca²⁺ release (SR) → thin filament activation → myosin cross-bridge cycling → sarcomere shortening
 ATP controls myosin attachment/detachment and resets the head, enabling repeated cycles of contraction)

Muscle Fiber Types – Classification and Characteristics

Type I – Slow Twitch, Oxidative Fibers
 Contract slowly
 Use oxygen for energy production (aerobic metabolism)
 Resistant to fatigue, suited for endurance activities

Type II – Fast Twitch Fibers

 Type II β – Fast Glycolytic
  Contract quickly
  Rely primarily on anaerobic (glycolytic) metabolism for energy
  Fatigue rapidly, suited for short, intense bursts

 Type II α – Fast Oxidative-Glycolytic
  Use both aerobic and anaerobic metabolism
  Intermediate speed and fatigue resistance

Muscle Fiber Distribution – Characteristics

Mixed Fiber Composition
 Most muscles contain all three fiber types
 Relative distribution varies among specific muscles

Slow Twitch Fibers (Type I)
 Muscles with more slow twitch fibers appear red
 High myoglobin content (oxygen-binding protein)
 Suited for endurance and continuous activity

Fast Twitch Fibers (Type II)
 Muscles with more fast twitch fibers appear white
 Lower myoglobin content
 Suited for short, intense bursts of activity

Muscle Fiber Energy Metabolism – Glycolytic Proportion

23% Glycolytic Fibers (77% Oxidative)
 Majority are oxidative fibers (Type I, slow twitch)
 Primary energy metabolism is aerobic (oxidative)

75% Glycolytic Fibers
 Majority are glycolytic fibers (Type II, fast twitch)
 Primary energy metabolism is anaerobic (glycolytic)

Muscle Fiber Energy Metabolism – Aerobic vs Anaerobic

Type I Fibers – Slow Twitch, Oxidative
 Use aerobic respiration
 Glucose + O₂ → CO₂ + H₂O + ~30 ATP
 Pathway: Glycolysis → Pyruvate → Acetyl-CoA → TCA cycle → Electron Transport Chain
 High ATP yield, fatigue-resistant

Type IIa Fibers – Fast Oxidative-Glycolytic
 Use both aerobic and anaerobic metabolism
 Intermediate ATP yield
 Can sustain moderate force and activity

Type IIb Fibers – Fast Glycolytic
 Use anaerobic respiration
 Glucose → Lactate + 2 ATP
 Pathway: Glycolysis → Fermentation to Lactate
 Rapid energy production, fatigues quickly

Key Point
 Fiber type determines energy pathway:
  Type I → aerobic (needs O₂), oxidation of glucose
  Type IIa → mixed metabolism, aerobic oxidation of glucose / anaerobic fermentation
  Type IIb → anaerobic fermentation to lactate

Muscle Fiber Types – Summary

Type I (Slow Twitch)
 Slow contraction speed
 Uses aerobic metabolism
 Highest myoglobin content
 Low glycogen concentration (since glycogen is used to make the glucose)
 Red color

Type IIβ (Fast Glycolytic)
 Fast contraction speed
 Uses anaerobic metabolism
 Least myoglobin
 High glycogen content
 White color

Type IIα (Fast Oxidative-Glycolytic)
 Fast contraction speed
 Uses both aerobic and anaerobic metabolism
 Intermediate myoglobin
 Intermediate glycogen
 Intermediate (pink) color