Lecture 11 - Skeletal Muscle Fiber Types, Tension, Summation (1)

Skeletal Muscle Fiber Types

Fast Twitch Muscle Fibers (Type II)

  • Anaerobic metabolism

  • Provides short bursts of energy rapidly

  • Fatigues quickly

  • Ideal for sprinters and explosive movements

Slow Twitch Muscle Fibers (Type I)

  • Utilizes oxygen for fuel

  • Provides continuous energy over extended periods

  • Fires slowly, offering high endurance

  • Best suited for marathoners

Length-Tension Relationship: Basics

  • The force of contraction is influenced by the length of the sarcomere before contraction.

Length-Tension Relationship: Filament Overlap

  • Ideal Length of Sarcomere: Depends on the overlap between myosin and actin filaments.

    • More cross-bridges can be formed when overlap is between 80-120%.

    • Sarcomeres produce maximal tension when conditions for power strokes are met.

Twitch Contraction Overview

  • A single action potential can trigger a contraction in muscle fibers (referred to as a twitch).

  • Twitch includes:

    • Latent Period: Delay before tension builds.

    • Contraction Period: Tension increases as the sarcomeres shorten.

    • Relaxation Period: Tension decreases as calcium is pumped back into storage.

Latent Period

  • Small delay occurs between receiving the nervous signal and tension buildup.

  • Action potential travels across the sarcolemma and into the sarcoplasmic reticulum.

Contraction Period

  • Calcium ions bind to troponin, leading to:

    • Tropomyosin moving away from actin-binding sites.

    • Formation of cross-bridges and power strokes, resulting in shortening of sarcomeres.

    • Peak tension is reached.

Relaxation Period

  • Calcium is pumped out of the sarcoplasm back into the sarcoplasmic reticulum.

  • Troponin and tropomyosin return to cover myosin binding sites, ceasing cross-bridge cycling.

  • Tension gradually decreases.

Refractory Period

  • A brief period (around 1 millisecond) where the muscle is unresponsive to new action potentials.

Twitch Summation

Overview

  • Occurs when a single twitch leads to increasing contractions due to successive action potentials.

Wave Summation

  • A second action potential occurs before muscle relaxation is complete, leading to increased tension.

  • Some relaxation occurs between twitches.

Incomplete Tetanus

  • Progressive increase in force output as contractions add together.

  • Characterized by partial relaxation before the next stimulation.

Tetanic Contraction

  • Complete/fused tetanus occurs when action potentials are sent rapidly, preventing muscle relaxation.

  • Leads to constant contraction with intensified force as calcium levels remain elevated.

  • Maximum tension achieved with limited available cross-bridges.

Clinical Note: Tetanus Disease

  • Caused by Clostridium tetani, affecting motor neuron communication.

  • Results in muscle spasms or sustained contractions; can be fatal if untreated.

Muscle Fiber Types Summary

Overview of Types

  • Type I (Slow oxidative): Fatigue resistant, low power.

  • Type IIa (Fast oxidative-glycolytic): Intermediate power and fatigue.

  • Type IIx (Fast glycolytic): High power but highly fatigable.

Muscle Tone

  • Even at rest, muscles maintain tension through involuntary activation of motor units.

  • Supported primarily by type I fibers, aiding posture and heat production.

Recruitment of Muscle Fibers

Motor Unit Activation

  • Each motor neuron innervates only one type of muscle fiber.

Strength Through Motor Units

  • Muscle contraction strength relies on the number of motor units activated.

  • More active motor units lead to a stronger contraction.

Strength by Motor Unit Size

  • Following Henneman’s size principle, motor units are recruited from small (Type I) to large (Type IIx).

Recruitment for Endurance

  • Alternating contractions of similar motor units sustain prolonged activity, ensuring endurance during tasks.