Lecture Notes: Muscle Physiology and Contraction

Overview

• Muscles are used daily for various activities such as breathing, circulating blood, and movement.
• Types of muscle tissues:
  • Cardiac Muscles: Involuntary, control heart function.
  • Smooth Muscles: Involuntary, found in organs, control organ functions.
  • Skeletal Muscles: Voluntary, allow movement of the body.

Skeletal Muscle Structure

• Composed of bundles of muscle fibers.
• Muscle Fibers: Long cylindrical cells with multiple nuclei.
• Muscle contraction and relaxation are controlled by the nervous system.

Neuromuscular Junction

• The site where nerve signals are exchanged with muscle fibers.
• Connection point: Synaptic bulb of axon terminal and muscle fiber.

Composition of Muscle Fibers

• Composed of myofibrils which contain sarcomeres.
• Sarcomeres: Contractile units along the myofibril.
  • Consist of alternating thick (myosin) and thin (actin) protein filaments.
  • Responsible for the striated appearance of skeletal muscles.

Muscle Contraction Mechanism

• Sliding Filament Model:
  • Thick filaments (myosin) pull thin filaments (actin) causing contraction.
  • Myosin filaments are anchored at the M-line, actin filaments at Z-lines.
  • Sarcomeres shorten as actin filaments slide over myosin.
• Power Stroke:
  • ATP hydrolysis to ADP + Pi extends myosin heads, forming cross-bridges with actin.
  • Myosin pulls actin toward M-line, shortening the sarcomere.
  • ADP + Pi are released during the power stroke.

Control of Muscle Contraction

• Controlled by calcium ions and regulatory proteins (troponin and tropomyosin).
• Relaxed State: Tropomyosin blocks myosin binding sites on actin.
• Calcium Ion Role:
  • High calcium levels bind to troponin, displacing tropomyosin.
  • Exposes myosin binding sites on actin, allowing cross-bridge formation.
• Sarcoplasmic Reticulum: Stores calcium ions, releases them upon nerve signal.
  • Nerve signals cause neurotransmitter release, leading to muscle depolarization.
  • Electrical impulses open calcium stores, triggering contraction.

Contraction Process

• Actin and myosin slide past each other, shortening sarcomeres.
• As myofibrils contract, muscle fibers shorten, creating force for movement.
• Coordinated contraction of muscle fibers allows whole muscles to move the body.