Bone & Mechanics 7-11

• Diffusion


  Movement of molecules from a high concentration to a low concentration gradient
• Diffusion

  1. Movement within solids, liquids and gases

  2. Occurs spontaneously

  3. Continues until equal distribution of particles (equilibrium)
• Diffusion is crucial for movement of nutrients, waste and fluids across cell membranes for metabolism and excretion
• Osmosis


  Specific type of diffusion involving water molecules across a semi-permeable membrane

  
• Osmosis

  1. Water moves from lower to higher concentration until equilibrium is reached

  2. Allows certain particles to move across depending on size, charge and shape

  
• Cell membrane


  Divides body fluid volumes into intracellular and extracellular spaces
• Ways water moves across a membrane to balance tonicity

  • Isotonic solution
  • Hypertonic solution
  • Hypotonic solution

  
• Isotonic solution


  Concentration of water remains the same on both sides, no net movement
• Hypertonic solution


  Concentration of solutes is higher outside the cell, causes cell to shrink or undergo crenation

  
• Hypotonic solution


  Concentration of solutes is lower outside the cell, causes cell to swell and burst

  
• Chemical gradient


  Uneven distribution of molecules across the membrane, e.g. more Na+ ions outside than inside

  
• Chemical gradients drive the movement of ions through channels and transporters to achieve equilibrium
• The bigger the chemical gradient, the faster the movement
• Electrical gradient


  Difference in electrically charged ions across a membrane, determined by Na+ and K+ gradients

  
• The Na+ and K+ gradients are actively pumped using energy
• Resting membrane potential

  Electrical charge difference between inside and outside the cell when not actively sending signals, mostly negative inside

  
• Resting membrane potential is essential for cell functions like ion homeostasis, electrical signalling in nerves and muscles, and cell communication
• Depolarization


  Reversal of the membrane potential from its resting value to a more positive value, as positively charged ions rush through voltage-gated channels

  
• Repolarization


  Process when the cell membrane returns to its resting membrane potential after depolarization, involves movement of K+ ions out of the cell
• Repolarization prepares the cell for further depolarization events and maintains proper cell function
• Skeletal muscle


  • Applies force to bones to control posture and body movements, mostly under voluntary control
  • Develops force by contracting (shortening)
  • Offers support and protection for soft internal organs
  • Provides voluntary control over major openings
  • Converts energy into heat to maintain temperature
• Muscle structure

  • Individual muscle cells (fibres) gathered in bundles (fascicles) to form whole muscle
  • Connective tissue forms tendons that connect muscle to bone

  
• Muscle fibre

  Composed of myofibrils made of sarcomeres, the contractile units containing actin and myosin filaments

  
• Excitation-contraction coupling


  Pairing of a signalling event (voltage-gated sensor) with a mechanical event (calcium release and muscle contraction)
• Excitation-contraction coupling

  1. Voltage sensor activated by signal in T-tubules

  2. Interacts with ryanodine receptors to open and release calcium from sarcoplasmic reticulum

  3. Calcium diffuses through cell and activates contraction

  4. Relaxation occurs as calcium is pumped back into sarcoplasmic reticulum by ATP-powered pump
• Actin


  Thin filament that forms a structural scaffold in the myofilament
• Myosin


  Thick filament that acts as a motor molecule, attaching to actin and generating force to pull
• Cross-bridge cycling

  1. Myosin head binds to actin, pulling it and causing contraction

  2. ATP binds to myosin, causing it to release actin

  3. Myosin burns ATP to re-energize and prepare for next contraction
• Muscle force

  Regulated by number of muscle fibres recruited and rate of stimulation

  
• A small number of neurons activates a low force, increasing as more force is added
• Muscle contraction


  Sustained by a rapid sequence of action potentials causing repeated calcium release from sarcoplasmic reticulum
• Muscle types

  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle


  • Composed of long multinucleate fibres organized into bundles
  • Responsible for voluntary movement
  • Attached to bones by tendons

  
• Cardiac muscle is located only in the heart to generate force to pump blood, and is not under voluntary control
• Smooth muscle is found in hollow organs like the gut and blood vessels, and is not under voluntary control
• Recruited


  Activated by the regulation of how many neurons are active at one time
• Muscle force

  • A small number of neurons tends to produce low force
  • Increases as more force is added
  • Frequency of stimulation increases
• Muscle contraction

  1. Single action potential - pulse of Ca release into the cytoplasm

  2. Short period of many action potentials fired - Rapid sequence-sustained release of Ca from the SR

  3. Eventually reach maximal signaling and contraction capability (tetanus)
• Length-Tension Relationship


  Fast fibers fatigue quick, slow fibers are steady
• Skeletal muscle is composed of long multinucleate fibers organized into bundles (fascicles)