Lecture Notes on Muscle Physiology

Overview of Muscle Types

• Skeletal, Cardiac, and Smooth Muscles
  • Can change membrane voltage.
  • Focus on cardiac and skeletal muscles.
  • Details on cardiac muscle to be covered in Unit 4.

Important Terms

• Muscle Fiber: Refers to a muscle cell, elongated and multinucleated due to myoblast fusion.
• Myoblasts: Embryonic progenitor cells that fuse to form muscle fibers.

Muscle Structure

• Cellular and Connective Tissue Structures
  • Not focusing on connective tissues like epimysium, paramysium, and endomysium.
  • Focus on muscle fiber physiology.
• Thick and Thin Filaments
  • Actin (thin filament) and Myosin (thick filament).
  • Myosin compared to intertwined golf club shafts.

Muscle Contraction

• Sliding Filament Theory
  • Sarcomere shortens, fibers do not change length.
  • Thick and thin filaments form myofibrils.
• Cross Bridges
  • Myosin heads bind to actin (myosin binding sites) causing contraction.

Regulatory and Contractile Proteins

• Actin: Forms helical "strings of beads".
• Tropomyosin and Troponin
  • Tropomyosin covers myosin binding sites.
  • Troponin: Calcium receptor regulating tropomyosin position.
  • Calcium binding causes tropomyosin to reveal myosin binding sites.

Muscle Fiber Anatomy

• Myofibrils and Myofilaments
  • Arranged in hexagonal patterns with myosin heads in spiral arrangements.
• T-tubules and Sarcoplasmic Reticulum (SR)
  • T-tubules bring action potentials deep into muscle fiber.
  • SR stores calcium ions.
  • Terminal cisterna in SR release calcium.

Motor Units and Muscle Control

• Motor Units
  • Single somatic motor neuron and all muscle fibers it innervates.
  • Larger motor units for gross motor control (e.g., posture muscles).
  • Smaller motor units for fine motor control (e.g., fingertips, eyes).
• Neuromuscular Junction (NMJ)
  • Synapse between motor neuron and muscle fiber.
  • Motor end plate with nicotinic receptors.
  • Acetylcholine as neurotransmitter.
  • Synaptic cleft between neuron and muscle cell.

Key Concepts

• Muscle Cells and Membrane Voltage
  • Action potentials spread via T-tubules.
  • Calcium release from SR crucial for contraction.
• Summary of Contraction Process
  • Myosin binds to actin, ATP provides energy.
  • Sarcomeres shorten, causing muscle contraction.

Review Points

• Ensure understanding of myosin and actin interactions.
• Importance of calcium in muscle contraction.
• T-tubule and SR relationship in excitation-contraction coupling.
• Differences in motor unit size and their functional implications.