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.