Intrafusal Fibres

• Muscle Spindle - Intrafusal fibres - Dynamic Bag:

  • aka type 1 bag, D1 bag, adaptive bag
  • has a bag like swelling in the central portion that contains a viscous fluid
  • is dynamic as its able to adapt its shape in repone to mechanical perturbation and is able to do this due to the type of viscous fluid in the central portion - has a low level of viscosity (thin so can move easily)

  A) dynamic bag

  
• Muscle Spindle - Intrafusal fibres - Static Bag:

  • aka type 2 bag, D2 bag, nonadaptive bag
  • has a bag like swelling in the central portion that contains a viscous fluid
  • it is nonadaptive due to the viscous fluid being of higher viscosity (thicker), so is less likely to change its shape

  A) static bag

  
• Muscle Spindle - Intrafusal fibres - Chain Fibres:

  • do not contain a swelling like central section so do not contain any viscous fluid and so cant change their shape - mechanical perturbation on these will not cause them to change shape - so are also static/nonadaptive in nature as well
  • they are linear in nature and the nuclei are in a line, so not accumulated in a bunch

  A) chain

  
• Muscle Spindle - Intrafusal fibres:

  • There are 3 types of intrafusal fibres:
  • dynamic bag fibres
  • static bag fibres
  • chain fibres
  • the dynamic bag fibres are adaptive - can change shape in relation to mechanical perturbation
  • the static bag fibres and chain fibres are nonadaptive - can not change shape in relation to mechanical perturbation

  A) dynamic bag

  B) static bag

  C) chain

  
• Muscle Spindle - Intrafusal Fibres:

  • towards the ends of the intrafusal fibres, there will be a little bit of striated muscle
  • this is because intrafusal fibres are just specialised muscle fibres, with adaptations in the central portion, and normal striated muscle towards the end
  • this portion is contractile, and if it did contract, it would shorten and pull on the central portion and tension it up

  A) dynamic bag

  B) static bag

  C) chain

  
• Muscle Spindle - Intrafusal Fibres:

  • sensory receptors lay upon the intrafusal fibres

  A) dynamic bag

  B) static bag

  C) chain