Muscle Fiber Contraction and Types

Week 6

Review from Last Lecture

• Events Leading to Muscle Fiber Contraction:
  1. A motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
  2. ACh binds to receptors on the sarcolemma (muscle cell membrane).
  3. If sufficient ACh binds, an action potential transmits along the muscle fiber.
  4. This action potential triggers the release of Ca2+ ions from the sarcoplasmic reticulum (SR).
  5. Ca2+ binds to troponin on the actin filament, causing troponin to move tropomyosin, which uncovers active sites on actin for myosin binding.

More on Muscle Structure

Myosin (Thick Filaments)

• Structure: Two intertwined protein strands (filaments).
• Globular heads:
  • Protrude 360° from the thick filament axis and interact with actin filaments for contraction.
• Titin: Fine filaments that stabilize myosin.

Actin (Thin Filaments)

• Actin: Contains myosin-binding site.
• Composed of three proteins:
  • Actin
  • Tropomyosin: Covers active site at rest.
  • Troponin: Anchored to actin and moves tropomyosin.
• Anchoring: Actin is anchored at the Z-disk with nebulin as the anchoring protein.

Structure of Sarcomere

• Components:
  • Thin filament: Actin, troponin, tropomyosin.
  • Thick filament: Myosin.
• Key structures: Z-disk, M-line, H-zone, A-band, I-band, Titin.

Titin (Third Myofilament)

• Acts like a spring; stiffness increases with muscle activation and force development.
  • Extends from Z-disk to M-band.
  • Ca2+ binds to titin, increasing muscle force when stretched.
  • The action of titin is addressed by the “winding filament theory.” Activated by Ca2+ influx, winding around thin filaments employs rotation.
• Functions:
  1. Stabilizes sarcomeres and centers myosin in the sarcomere.
  2. Prevents overstretching.

Muscle Contraction

Excitation–Contraction Coupling (1 of 2)

1. Action potential (AP) initiates in the brain.
2. AP arrives at the axon terminal, releasing ACh.
3. ACh crosses the synapse and binds to ACh receptors on the plasmalemma.

Excitation–Contraction Coupling (2 of 2)

4. AP travels along plasmalemma and T-tubules.
5. Triggers Ca2+ release from the sarcoplasmic reticulum (SR).
6. Ca2+ enables actin-myosin contraction.

Role of Ca2+ in Muscle Fiber

• AP arrives at SR from T-tubule, prompting a mass release of Ca2+ into the sarcoplasm.
• Ca2+ binds to troponin, unblocking the binding site that tropomyosin covers.
• Myosin binding occurs, permitting contraction.

Sliding Filament Theory: How Muscles Create Movement (1 of 2)

• Relaxed state: No actin-myosin interaction; myofilaments overlap minimally.
• Contracted state: Myosin heads pull actin toward the sarcomere center (power stroke); the filaments slide past each other; sarcomeres, myofibrils, and muscle fiber all shorten.

Sliding Filament Theory: How Muscles Create Movement (2 of 2)

• After the power stroke:
  1. Myosin detaches from the active site.
  2. The myosin head rotates back to its original position.
  3. Myosin attaches to a further active site down the actin filament.
• This cycle continues until:
  1. Z-disk reaches myosin filaments, or
  2. AP ends and Ca2+ pumped back into SR - without Ca2+, troponin and tropomyosin revert to resting conformation, covering myosin-binding sites.

Muscle Fiber Contraction Summary Key Points

• Muscle contraction begins with an action potential from an α-motor neuron.
• The motor neuron releases ACh, opening ion gates in the muscle cell membrane.
• Sodium enters the muscle cell, causing depolarization of plasmalemma.
• The action potential travels through plasmalemma and T-tubules, releasing Ca2+ ions from SR.
• Ca2+ ions bind with troponin, exposing active sites on actin.
• Myosin head binds to these active sites.
• Myosin heads bind ATP, causing ATPase on the myosin heads to split ATP into ADP and Pi, which releases energy for muscle contraction.
• Myosin head exerts force towards sarcomere center (power stroke), and contraction concludes when Ca2+ is actively pumped back into SR.

Muscle Fiber Types

• Type I fibers:
  • Approximately 50% of fibers in an average muscle.
  • Peak tension in 110 ms (slow twitch).
• Type II fibers:
  • Peak tension in 50 ms (fast twitch).
  • Type IIa: ~25% of fibers.
  • Type IIx: ~25% of fibers.

Type I Versus Type II (1 of 2)

• Variable speed of myosin ATPase:
  • Fast myosin ATPase yields fast contraction cycling; slower myosin ATPase results in slower contraction cycling.
• Muscle biopsy procedure:
  • A small piece (10-100 g) of muscle is removed, frozen, sliced, and examined under a microscope.
• Gel electrophoresis:
  • Different types of myosin in Type I and II fibers are separated by size.

Determination of Myosin Isoforms

• Myosin isoforms are determined by electrophoretic separation of proteins and staining.
• Identifying myosin isoforms aids in determining fiber type.

A Photomicrograph Showing Muscle Fibers

• Type I (black), type IIa (white), and type IIx (gray) muscle fibers.

Type I Versus Type II (2 of 2)

• Sarcoplasmic reticulum: Type II fibers possess a more developed SR, allowing faster Ca2+ release.
  • Speed of contraction (V_o) is 5-6 times faster in Type II fibers.
  • Calculated power (P_o) is 3-5 times greater in Type II fibers.
• Motor units:
  • Type I motor unit: smaller neuron, <300 fibers.
  • Type II motor unit: larger neuron, >300 fibers.

Distribution of Fiber Types (Type I:Type II Ratios)

• Unique ratios exist for each person.
• Similar ratios in arms and legs for individuals:
  • Type I predominates in endurance athletes.
  • Type II predominates in power athletes.
  • Soleus muscle is Type I in all individuals.

Type I Fibers During Exercise

• Type I fibers exhibit high aerobic endurance.
  • Can sustain exercise over long periods and require oxygen for ATP production.
  • Engaged in low-intensity aerobic exercise and daily activities.

Type II Fibers During Exercise

• Type II fibers generally fatigue quickly (poor aerobic endurance) and produce ATP anaerobically.
  • Type IIa: Produces more force; fatigues faster than Type I; utilized in short, intense endurance (e.g., 1,600m run).
  • Type IIx: Rarely engaged in everyday activities; used for short, explosive sprints (e.g., 100m).

Classification of Muscle Fiber Types (Table 1.1)

• System 1:
  • Type I
  • Type IIa
  • Type IIx
• System 2:
  • Slow twitch (ST)
  • Fast twitch a (FTa)
  • Fast twitch x (FTx)
• System 3:
  • Slow oxidative (SO)
  • Fast oxidative/glycolytic (FOG)
  • Fast glycolytic (FG)
• Characteristics of fiber types:
  • Oxidative capacity: High (Type I), Moderately High (Type IIa), Low (Type IIx)
  • Glycolytic capacity: Low (Type I), High (Type IIa), Highest (Type IIx)
  • Contractile speed: Slow (Type I), Fast (Type IIa & IIx)
  • Fatigue resistance: High (Type I), Moderate (Type IIa), Low (Type IIx)
  • Motor unit strength: Low (Type I), High (Type IIa & IIx)

Structural and Functional Characteristics of Muscle Fiber Types (Table 1.2)

Single Muscle Fiber Physiology

• Peak power varies between muscle fiber types, all reaching peak power at approximately 20% peak force.

Muscle Fiber Types Summary Key Points

• Skeletal muscles contain both Type I and Type II fibers.
• Different fiber types exhibit distinct myosin ATPase activities.
• Type II fibers have a more developed sarcoplasmic reticulum (SR), which provides a quicker release of Ca2+.
• Type II motor units are larger than Type I motor units, with more muscle fibers that produce greater force.
• Generally, proportions of Type I and Type II fibers are similar in individual arm and leg muscles.

Determination of Fiber Type

• Genetically determined: Twin studies indicate identical twins share identical proportions. Fraternal twins show variance in fiber type profiles.
• Fiber type determination is influenced by the α-motor neuron innervating the muscle fibers.
• Types of training or inactivity can induce shifts in myosin isoforms:
  • Endurance training reduces Type IIx while increasing Type IIa.
• Ageing may alter the distribution of Type I and Type II fibers, notably decreasing Type II while increasing Type I.

Athletes and Fiber Type

• Muscle fiber composition varies among athletes based on sport.
• Speed and strength events often constitute a higher percentage of Type II fibers, while endurance events typically feature a higher percentage of Type I fibers.

Percentages and Cross-Sectional Areas of Muscle Fibers in Athletes (Table 1.3)

Types of Muscle Contraction

• Concentric contraction (Dynamic): Force developed while the muscle shortens (thin filaments are pulled toward the center of the sarcomere).
• Isometric contraction (Static): Force generated without changing muscle length (cross-bridges form but external force prevents movement).
• Eccentric contraction (Dynamic): Force generated while the muscle lengthens (think stretching).

Muscle Force Generation

• Muscle contractions must be graded to match task requirements.
• Force production increases through the recruitment of more motor units (Henneman Size Principle) and increased stimulation frequency (Rate Coding).
• Factors affecting force production include:
  • Size of muscle fiber.
  • Maximal force production at optimal muscle length (Length-Tension Relation).
  • Contraction speed also impacts force output (Force-Velocity Relation).

Motor Unit Recruitment

• Principle of orderly recruitment: Motor units activate based on size order from small to large - type I → type IIa → type IIx. This process is informed by the Henneman Size Principle.

Motor Unit Recruitment Key Points

• Motor units exhibit an all-or-none response; activating more units increases produced force.
• Motor units recruit in an order to sustain prolonged force - type I → type IIa → type IIx.

Repetition Ranges and Associated Principles

• Kines: Repetition range affects muscle tissue recruitment and outcomes:
  • 1 to 4RM: Very high intensity with type II muscles primarily recruited.
  • 4 to 8RM: High intensity focusing on strength.
  • 8 to 12RM: Moderate intensity, hypertrophy and strength targeting.
  • 12 to 15RM: Low intensity, optimal hypertrophy.
  • 15 to 20RM: Very low loads with little muscle hypertrophy.
  • 20+RM: Light loads focus on endurance and foundational strength.

Variation in Force Production with Frequency of Stimulation (Rate Coding)

• Twitch: Minimal signal resulting in a small contractile response.
• Summation: Rapid signals before relaxation result in stronger contractions.
• Tetanus: Continuous stimulation causes peak force development.

Application of Rate Coding

• Force can increase without recruitment.
• Relevant to speed of contraction (Rate of Force Development - RFD).
• Training techniques such as plyometric jump squats enhance motor unit firing rates.

Generation of Force

• Length–tension relationship: Optimal muscle length encourages maximal force with ideal overlap. If excessively short/stretched, force falls off rapidly.
• Force-velocity relationship:
  • Concentric - Maximal force diminishes with increasing speed.
  • Eccentric - Maximal force increases with higher speeds.

Length-Tension Relationship

• Optimal overlap occurs at a sarcomere length of between 2.0 - 2.25 μm.
• Resting sarcomere lengths between 75-130% of optimal allow maximum cross-bridge formation, ensuring highest tension.
• Extreme sarcomere stretching beyond this range prevents effective overlap, disallowing muscle contraction due to reduced interactions between actin and myosin.

Force-Velocity Relationship Visualization

• Ability to generate force is contingent upon contraction speed:
  • Concentric - Force diminishes as speed rises.
  • Eccentric - Force also rises with slower contraction rates, as longer duration allows more cross-bridge formations.

Application: Training to Optimize Force Generation

• Surfing the F-V Curve:
  • Strength Cycle: High loads, low volumes.
  • Hypertrophy Cycle: Moderate high loads with higher volumes.
  • Conditioning Cycle: Moderate loads at moderate velocity.
  • Power Cycle: Moderate loads at fast speeds with recovery periods.
  • Metabolic/Pulse Cycle: Light to moderate loads at fast speeds with incomplete recovery.

Muscle Memory

• Muscle cells require multiple nuclei for synthesis and maintenance.
  • Satellite cells: Multiply during hypertrophy to fuse with existing fibers, providing additional nuclei for growth.
• During atrophy, unneeded nuclei may be removed through apoptosis, but few myonuclei display apoptosis markers, suggesting they may retain strength memories.

Research on Muscle Memory

• Challenges in understanding muscle memory arose when studies indicated that skeletal muscles could retain strength gains from initial training, despite no increase in myonuclear numbers, indicating potential neural components are involved. Further research is required to dissect the role of myonuclei vs neural mechanisms in memory retention.