Muscle Physiology L1

• Muscle Physiology


  The study of the structure and function of muscles
• Lecture topics


  • Structure and functions of muscles (molecular → cellular levels)
  • Excitation-contraction coupling (important for the lab)
  • Exercise training and muscle plasticity
• Learning Objectives


  • Appreciate why understanding muscle physiology is important
  • Understand the key differences between the 3 major muscle types
  • Be able to describe the sliding filament theory of muscle contraction, with reference to actin and myosin (contractile proteins) and troponin and tropomyosin (regulatory proteins)
• All animal phyla above Porifera (sponges) have muscle cells
• Muscles are the most important effectors of animal behaviour
• Functions of muscles in day to day life

  • Locomotion
  • Manipulation of objects
  • Circulation, digestion, and excretion
  • Production of heat as a metabolic by-product
  • Production of sound
• Understanding muscle physiology is important for optimising athletic performance, reducing muscle atrophy, and understanding/treating neuromuscular disorders
• A comparative approach (inter-species) is highly valuable for understanding muscle physiology
• Characteristics of muscle


  • Irritability - excitable and conductive
  • Contractility - ability to shorten and thicken
  • Extensibility - can also stretch
  • Elasticity - can revert to resting length following stretch
• Types of muscle


  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle


  • Attached to bones
  • Single, very long cells with obvious striations
  • Voluntary contractions
• Cardiac muscle


  • Walls of the heart
  • Branching chains of cells, less obvious striations
  • Involuntary contractions
• Smooth muscle


  • Walls of visceral organs
  • Single, fusiform shape, no striations
  • Involuntary contractions
• There are approximately 650 skeletal muscles in the human body
• Skeletal muscles generally work in pairs, called antagonistic muscle pairs
• Agonist


  The contracting muscle
• Antagonist


  The relaxing or lengthening muscle
• Muscles always pull, never push
• Muscle cell (fibre)


  • Many mitochondria
  • Many nuclei
  • T-tubules (transverse-tubules)
  • Sarcoplasmic reticulum
• Sarcomere


  The smallest component of a muscle fibre that can contract
• Sliding filament theory of muscle contraction

  1. Relaxed sarcomere

  2. Contracted sarcomere
• Myosin


  • Consists of 2 subunits shaped like golf clubs with two heads
  • Each head has 2 binding sites that are crucial to the contraction process
• Actin


  • Spherical proteins
• Tropomyosin


  • Threadlike molecules that lie end to end along the actin spiral, acting as a "blocking protein"
• Troponin


  • Consists of 3 polypeptide units (1 binds to actin, 1 binds to tropomyosin, and 1 binds to Ca2+)
• Relaxed muscle


  Troponin is not bound to Ca2+, tropomyosin remains in "blocking" position
• Excited muscle


  Troponin binds to Ca2+, tropomyosin slides away from its block position, myosin can bind to actin to form cross-bridges
• Sarcoplasmic reticulum (SR)


  Membranous network of tubules surrounding each myofibril, primary function is to store and release calcium ions
• Ryanodine receptors


  Large ion channels responsible for the release of Ca2+ from the sarcoplasmic reticulum
• Extreme exertion, struggle and stress during animal capture often leads to death due to capture myopathy
• Capture myopathy can cause damaged skeletal and cardiac muscles, swollen muscle fibres with striation loss, fragmentation of myofibrils, sarcolemma damage, blood acidosis, and haemolysis
• Prevention of capture myopathy

  1. Use of tranquilizers and immobilization drugs

  2. Use of blindfolds

  3. Well-trained staff who have in-depth knowledge of study species

  4. Minimizing stress before, during and after capture
• Treatment of capture myopathy

  1. Chemical inhibition of muscle contraction

  2. Dantrolene (drug): acts on the ryanodine receptor to prevent calcium release from the sarcoplasmic reticulum