Adaptive Fibres

• Adaptive fibres:

  • the adaptive intrafusal fibres are the dynamic bag fibres
  • need to consider in detail what happens when a dynamic bag fibre is subjected to stretch - assume muscle is lengthened, stretching the spindle, so stretching the dynamic bag fibres

  
• Adaptive fibres:

  • when muscle is lengthened, the dynamic bag fibre is stretched, the axon's annulospiral endings membrane becomes tensioned and action potential will be generated
  • but if a muscle has been lengthened and been held at that longer length, there will be no further action potentials for the dynamic bag fibres as it'll alter its shape and the tension comes off the membrane, so permeability of the annulospiral membrane will go back to normal, so sodium stops flowing into the axon and therefore action potentials will no longer be generated as theres no depolarisation to generate them

  
• Adaptive fibres:

  • for dynamic bag fibres, the pattern of action potentials that are generated are time limited as there'll be a burst of action potentials that happen at the initial stretch - as soon as the stretch stops, the action potentials will stop
  • diagram on bottom right shows action potentials are only generated during the time limited event of the muscle lengthening, not before and not after its been lengthened as the dynamic bag has altered its shape to reduce tension on the membrane and stop depolarisation

  
• Adaptive fibres:

  • if muscle is lengthened rapidly, it would initially create more tension on the dynamic bag fibres before it adapts its shape
  • there would be a lot of tension on the annulospiral endings, so big increase in permeability, so big sodium influx and large degree of depolarisation, causing high frequency of action potentials
  • so rapid stretch would produce a higher frequency of action potentials compared to a slow stretch as slow stretch would produce less tension
  • so frequency of action potentials being generated during lengthening is directly proportional to speed of the stretch
• Adaptive fibres clinical relevance pt1:

  • there is clinical relevance of frequency of action potentials being generated during lengthening is directly proportional to speed of the stretch
  • if muscles lengthened at slow very speed, the dynamic bag fibre may alter shape quickly enough to prevent any tension occurring in the first place, so very slow stretch may not produce any tension on the dynamic bag fibres and the annulospiral endings, so no action potentials as the dynamic bag alters its shape fast enough
• Adaptive fibres clinical relevance pt2:

  • info about the muscles spindle's stretch, up the type 1A afferent is the info that supplies the stretch reflex - where stretch to a muscle leads to contraction of a muscle
  • if annulospiral endings that provide info for stretch reflex couldnt produce action potentials as the stretch was too slow, it can explain pts, such as strokes pts, that experience an impairment called spasticity - affects stretch reflex (hypersensitive stretch reflex)
  • you can lengthen pts muscles slowly, without setting off action potentials, so without setting off stretch reflexes