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HUBS191_Lect9_2025

HUBS 191 Overview

  • Lecture material not a substitute for private study; variations may occur.

Copyright Notice

  • All course materials including videos, audio, and notes are copyrighted. Authorized for private study only.

EC-Coupling and Muscle Relaxation

  • Example Exam Question: Identify which event contributes to muscle cell relaxation during excitatory-contraction coupling (EC-coupling).

    • Answer options include:

      • A. Signal arriving via t-tubules

      • B. SERCA pump moves calcium into SR

      • C. Activation of voltage-gated channel (DHPR)

      • D. Ryanodine receptor releases calcium from SR.

Objectives and Study Guide

  • After the lecture, you should be able to:

    • Describe the sequence of events in a cross-bridge cycle.

    • Explain determinants of skeletal muscle force generation.

    • Distinguish between fast and slow muscle fibres.

  • Related readings included.

Myofilament Composition

  • Composed primarily of actin and myosin.

Structure of Actin and Myosin

  • Actin: thin filament, structural scaffold, resembles a rope.

  • Myosin: thick filament, motor molecule, generates force to pull actin, resembles people with hands acting as myosin heads.

Cross-Bridge Formation

  • Calcium presence allows actin and myosin binding, forming cross-bridges for contraction.

Cross-Bridge Cycling Overview

  • Power stroke finishes: actin/myosin cross-bridges present.

  • ATP binds to myosin, prompting actin release.

  • Myosin uses ATP to change shape for another contraction cycle.

  • Energized myosin can bind to actin again if calcium is present.

  • Cross-bridge pulls actin, shortening sarcomere and causing muscle contraction.

Key Questions

  • Identify two key proteins in myofilament.

  • Order the five states in the cross-bridge cycle and when filament slides.

  • Events needed for cross-bridge formation and release.

Muscle Tension Factors

  • Depends on:

    1. Number of fibres recruited.

    2. Rate of muscle stimulation.

  • Recruitment: more neurons active increases force output.

Frequency of Stimulation

  • Single action potential leads to a twitch; multiple in rapid succession leads to sustained contraction (summation) and potential plateau (tetanus).

Length-Tension Relationship

  • Each muscle has an optimal length for strength; too stretched or compressed leads to weaker contraction due to filament overlap.

Muscle Fibre Types

  • Fast Fibres: fatigue quickly, large diameter, few capillaries, low mitochondria, quick peak tension.

  • Slow Fibres: steady force, small diameter, many capillaries, high mitochondria, high fatigue resistance.

Summary of Key Concepts

  • Cross-bridge formation relies on calcium presence; ATP powers the force generation through actin pulling.

  • Muscle force relates to fibre recruitment and stimulation frequency, culminating in maximal force (tetanus).

  • Fast fibres generate quick, powerful contractions but fatigue rapidly; slow fibres are more endurance-oriented.